Speed and velocity
Speed and velocity are both measured in m/s. They both simply say how fast you're going, but there's a subtle difference between them which you need to know:
SPEED IS JUST HOW FAT YOU'RE GOING WITH NO REGARD TO THE DIRECTION. VELOCITY HOWEVER MUST ALSO HAVE THE DIRETION SPECIFIED.
Speed = distance / time. So the gradient of a distance-time graph tells you how fast your object is travelling. This is because the gradient is the change in the distance, divided by the change in time.
- Gradient =speed.
- Flat sections are where it's stationary.
- Straight uphill or downhill sections mean it is travelling at a steady speed.
- The steeper the graph, the faster it's going.
- Downhill section means mean it's going back toward its starting point.
- Curves represent acceleration or deceleration.
- A steepening curve means it's speeding up (increasing gradient).
- A levelling off curve means it's slowing down (decreasing gradiet).
1. Acceleration is how quickly the velocity is changing.
2. This change in velocity can be a change in speed or a change in direction or both.
ACCELERATION = CHAGE IN VELOCITY / TIME TAKEN
(V-U) / A x T
v = final velocity and u = initial velocity
- Gradient = acceleration.
- Flat sections represent steady speed.
- The steeper the graph, the greater the acceleration or deceleration.
- Uphill sections are acceleration.
- Downhill sections are deceleration.
- The area underany section of the graph is equal to the distanced travelled in that time interval.
- A curve means changing acceleration.
Gravity attracts all masses, but you only notice it when one of the masses is really really big. Anything near a planet or a star is attracted to it very strongly.
This has two important effects:
1. On the surface of a planet, it makes all things accelerate towards the ground (all with the same acceleration, g, which is about 10 m/s^2 on earth).
2. It gives everything a weight.
Weight and mass
1. Mass is just the amount of stuff in an object. For any given object this will have the same value anywhere in the universe.
2. Weight is caused by the pull of the gravitational force. In most questions the weight of an object is just the force of gravity pulling it towards the centre of the earth.
3. An object has the same mass whether it's on earth or the moon - but its weight will be different. A 1kg mass will weigh less on the moon than it does on the earth, simply because the gravitational force pulling on it is less.
4. Weight is a force measured in newtons. It is measured using a spring balance or newton meter. Mass is not a force. It's measured in kilograms with a mass balance.
Mass, weight and gravity
WEIGHT = MASS x GRAVITATIONAL FIELD STRENGTH
W = m x g
- Mass = kg
- Weight = newtons
- g represents the strength of gravity and its value is different for different planets. On earth, g = 20 N/kg. On the moon, g = 1.6 N/kg.
The notion of resultant force:
1. In most real situations there are at least two forces acting on an object along any direction.
2. The overall effect of these forces will decide the motion of the object - whether it will accelerate, decelerate or stay at a steady speed.
3. If you have a number of forces acting at a single point, you can replace them with a single force (so long as the single force has the same effect on the motion as the original forces acting all together.
4. If the forces all act along the same line (they're all parallel and act in the same or the opposite direction), the overall effect is found by just adding or subtracting them.
5. The overall force you get is called the resultant force.
A resultant force means a change in velocity
1. If there is a resultant acting on an object, then the object will change its state of rest or motion.
2. In other words it causes a change in the object's velocity.
An object needs a force to start moving
IF THE RESULTANT FORCE ON A STATIONARY OBJECT IS ZERO, THE OBJECT WILL REMAIN STATIONARY.
Things don't just start moving on their own, there has to be a resultant force to get them started.
No resultant means no change in velocity
IF THERE IS NO RESULTANT FORCE ON A MOVING OBJECT IT'LL JUST CRRY ON MOVING AT THE SAME VELOCITY.
1. When a train or car or bus or anything else is moving at a constant velocity then the forces on it must all be balanced.
2. Never let yourself entertain the ridiculous idea that things need a constant overall force to keep them moving.
3. To keep going at a steady speed, there must be zero resultant force.
A resultant force means acceleration
IF THERE IS A NON-ZERO RESULTANT FORCE, THEN THE OBJECT WILL ACCELERATE IN THE DIRECTION OF THE FORCE.
1. A non - zero resultant force will always produce acceleration (or deceleration).
2. This acceleration can take five different forms:
Starting, stopping, speeding up, slowing down and changin direction.
3. On a force diagram, the arrows will be unequal.
A non - zero resultant force
Any resultant force will produce acceleration, and this is the formula for it:
F = ma or a = F/m
m = mass in kilograms (kg)
a = acceleration in metres per second squared (m/s^2)
F is the resultant force in newtons (N)
Reaction forces are equal and opposite
WHEN TWO OBJECTS INTERACT, THE FORCES THEY EXERT ON EACH OTHER ARE EQUAL AND OPPOSITE.
1. That means if you push something, say a shopping trolley, the trolley will push back against you, just as hard.
2. And as soon as you stop pushing, so does the trolley.
3. So far so good. The slightly tricky thing to get your head round is this - if the forces are always equal, how does anything ever go anywhere? The important thing to remember is that the two forces are acting on different objects. Think about a pair of ice skaters:
When skater A pushes on skater B (the action force), she feels an equal and opposite force from skater B's hand (the rection force). Both skaters feel the same sized force, in opposite directions, and so accelerate away from eachother. Skater A will be accelerated more than skater B, though because she has a smaller mass - remember a = F/m.
1. If an object has no force propelling it along it will always slow down and stop because of friction (unless you are in space where there's nothing to rub against).
2. Friction always acts in the opposite direction to movement.
3. To travel at a steady speed, the driving force needs to balance the frictional forces.
4. You get friction between two surfaces in contact, or when an object passes through a fluid (drag).
RESISTANCE OR DRAG FROM FLUIDS (air or liquid)
Most of the resistive forces are cause by air resistance or drag. The most important factor by far in reducing drag in fluids is keeping the shape of the object streamlined. The opposite extreme is a parachute which is about as high drag as you can get - which is, of course, the whole idea.
Frictional forces from fluids always increase with speed. A car has much more friction to work against when travelling at 70 mph compared to 30 mph. SO at 70 mph the engine has to work much harder just to maintain a steady speed.
When falling objects first set off, the force of gravity is much more than the frictional force slowing them down, so they accelerate. As the speed increases the friction builds up. This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force and then it won't accelerate any more. It will have reached its maximum speed or terminal velocity and will fall at a steady speed.
Terminal velocity depends on shape and area
The accelerating force acting on all falling objects is gravity and it would make them all fall at the same rate, if it wasn't for air resistance. This means that on the moon, where there's no air, hamsters and feathers dropped simultaneously will hit the ground together. However, on earth, air resistance causes things to fall at different speeds, and the terminal velocity of any object is at different speeds, and the terminal velocity of any object is determined by its drag in comparison to its weight. The frictional force depends on its shape and area.
The most important exaple is the human skydiver. Without his parachute open he has quite small area and force of W=mg pulling him down. He reaches a terminal velocity of about 120mph. But with the parachute open, there's much more air resistance (at any given speed) and still only the same force W=mg pulling him down. This means his terminal velocity comes right down to about 15mph, which is a safe speed to hit the ground at.
Factors affecting stopping distance
1. Looking at things simply - if you need to stop in a given distance, then the faster a vehicle's going, the bigger braking force it'll need.
2. Likewise, for any given braking force, the faster you're going, the greater your stoppping distance. But in real life it's not quite that simple - if your maximum braking force isn't enough, you'll go further before you stop.
3. The total stopping distance of a vehicle is the distance covered in the time between the driver first spotting a hazard and the vehicle coming to a complete stop.
4. The stopping distance is the sum of the thinking distance and the braking distance.
It's affected by two main factors:
A. How fast you're going - Obviously, whatever your reaction time, the faster you're going, the further you'll go.
B. How dopey you are - This is affected by tiredness, drugs, alcohol and a careless blase attitude.
Bad visibility and distractions can also be a major factor in accidents - lashing rain, messing about with the radio, bright oncoming lights, etc. might mean that a driver doesn't notice a hazard until they're quite close to it. It doesn't affect your thinking distance, but you start thinking about stopping nearer to the hazard, and so you're more likely to crash.
It's affected by four main factors:
A. How fast you're going - The faster you're going, the further it takes to stop.
B. How good the brakes are - All brakes must be checked and maintained regularly. Worn or faulty brakes will let you done catastrophically just when you need them the most.
C. How good the tyres are - Tyres should have a minimum tread depth of 1.6mm in order to be able to get rid of the water in wet conditions. Leaves, diesel spills and muck on the road can greatly increase the braking distance, and cause the car to skid too.
D. How good the grip is - This depends on three things:
1. Road surface 2. weather conditions 3. tyres
Work and potential energy
WHEN A FORCE MOVES AN OBJECT THROUGH A DISTANCE, ENERGY IS TRANSFERRED AND WORK IS DONE.
This statement sounds far more complicated than it needs to. Try this:
1. Whenever something moves, something else is providing some sort of effort to move it.
2. The thing putting the effort in needs a supply of energy.
3. It then does work by moving the object - and one way or another it transfers the energy it receives into other forms.
4. Whether this energy is transferred usefully or is wasted, you can still say that work is done. Just like Batman and Bruce Wayne, work done and energy transferred are indeed one and the same. (And they're both given in joules)
WORK DONE = FORCE x DISTANCE
W / F x d
Whether the force is friction or weight or tension in a rope, it's always the same. To find how much energy has been transferred (in joules), you just multiply the force in N by the distance moved in m.
Gravitational potential energy
GRAVITATIONAL POTENTIAL ENERGY = MASS x G x HEIGHT
Ep / m x g x h
Gravitational potential energy (measured in joules) is the energy that an object has by virtue of (because of) its vertical position in a gravitational field. When an object is raised vertically, work is done against the force of gravity (it takes effort to lift it up) and the object gains gravitational potential energy. On earth the gravitational field strength is approximately 10 N/kg.
Anything that's moving has kinetic energy. There's a slightly tricky formula for it, so you have to concentrate a little bit harder for this one.
KINETIC ENERGY = 1/2 x MASS x SPEED^2
Ek / 1/2 x m x v^2
Remember, the kinetic energy of something depends both on mass and speed. The more it weighs and the faster it's going, the bigger its kinetic energy will be.
Kinetic energy transferred is work done
WHEN A CAR IS MOVING IT HAS KINETIC ENERGY
1. A moving car can have a lot of kinetic energy. To slow a car down this kinetic energy needs to be converted into other types of energy.
2. To stop a car, the kinetic energy (1/2mv^2) has to be converted to heat energy as friction between the wheels and the brake pads, causing the temperature of the brakes to increase:
KINETIC ENERGY TRANSFERRED = WORK DONE BY BRAKES
1/2mv^2 = F x d
m = mass of car and passengers (kg) v = speed of car (m/s) F = maximum braking force (N)
d = braking distance (m)
FALLING OBJECT COVERT Ep INTO Ek
KINETIC ENERGY GAINED = POTENTIAL ENERGY LOST
Ek is transferred into heat and sound
When meteors and space shuttles enter the atmosphere, they have a very high kinetic energy. Friction due to collisions with particles in the atmosphere transfers some of their kinetic energy to heat energy and work is done. The temperatures can become so extreme that most meteors burn up completely and never hit the earth. Only the biggest meteors make it through to the Earth's surface - these are called meteorites.
Space shuttles have heat shelds made from special materials which lose heat quickly, allowing the shuttle to re-enter the atmosphere without burning up.
Work done to an elastic object
1. When you apply a force to an object you may cause it to stretch and change in shape.
2. Any object that can go back to its original shape after the force has been removed is an elastic object.
3. Work is done to an elastic object to change its shape. This energy is not lost but is stored by the object as elastic potential energy.
4. The elastic potential energy is then converted to kinetic energy when the force is removed and the object returns to its original shape.
Extension of an elastic object
1. The extension, e, of a stretched spring (or other elastic object) is directly proportional to the load or force applied, F. The extension is measured in metres, and the force is measured in newtons.
2. This is the equation:
F = k x e
3. k is the spring constant. Its value depends on the material that you are stretching and it's measured in newtons per metre (N/m).
Limit of extension to an elastic object
There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally.
1. The graph shows force against extension for an elastic object.
2. For small forces, force and extension are proportional.
3. There is a maximumm force that the elastic object can take and still extend proportionally. This is known as the limit of proportionality.
4. If you increase, the force past the limit of proportionality, the material will be permanently stretched. When the force is removed, the material will be longer than at the start.
Power is the rate of doing work
Power is not the same thing as force, nor energy A powerful machine is not necessarily one which can exert a strong force (though it usually ends up that way). A powerful machine is one which transfers a lot of energy in a short space of time. This is the very easy formula for power:
POWER = WORK DONE (ENERGY TRANSFERRED) / TIME TAKEN
P = E/t E/P x t
The proper unit of power is the watt. One watt = 1 joule of energy transferred per second. Power means how much energy per second, so watts are the same as joules per second.
Momentum = mass x velocity
1. Momentum (p) is a property of moving objects.
2. The greater the mass of an object and the greater the velocity the more momentum the object has.
3. Momentum is a vector quantity - it has size and direction.
p / m x v
MOMENTUM (kg m/s) = MASS (kg) x VELOCITY (m/s)
MOMENTUM BEOFRE = MOMENTUM AFTER
In a closed system, the total momentum before an event is the same as after the event. This is called conservation of momentum.
Forces cause changes in momentum
1. When a force acts on an object, it causes a change in momentum.
2. A larger force means a faster change of momentum (and so greater acceleration).
3. Likewise, if someone's momentum changes very quickly (like in a car crash), the forces on the body will be very large, and more likely to cause injury.
4. This is why cars are designed with safety features that slow people down over a longer time when they have a crash - the longer it takes for a change in momentum, the smaller the force.
When you apply the brakes to slow down a car, work is done. The brakes reduce the kinetic energy of the car by transferring it into heat (and sound) energy. In traditional braking systems that would be the end of the story, but new regenerative braking sustems used in some electric or hybrid cars make use of the energy, instead of converting it al into heat during braking.
1. Regenerative brakes use the system that drives the vehicle to do the majority of braking.
2. Rather than converting the kinetic energy of the vehicle into heat energy, the brakes put the vehicle's motor into reverse. With the motor running backwards, the wheels are slowed.
3. At the same time, the motor acts as an electric generator, converting kinetic energy into electrical energy that is stored as chemical energy in the vehicle's battery. This is the advantage of regenerative brakes - they store the energy of braking rather than wasting it. It's a nifty chain of energy transfer.
1. If a car crashes it will slow down very quickly - this means that a lot of kinetic energy is converted into other forms of energy in a short amount of time, which can be dangerous for the people inside.
2. In a crash, there'll be a big change in momentum over a very short time, so the people inside the car experience huge forces that could be fatal.
3. Cars are designed to convert the kinetic energy of the car and its passengers in a way that is safest for the car's occupants. They often do this by increasing the time over which momentum changes happen, which lessens the forces on the passangers.
- CRUMPLE ZONES - at the front and back of the car crumple up on impact.
- SIDE IMPACT BARS - are strong metal tubes fitted in to car door panels.
- SEAT BELTS - stretch slightly, increasing the time taken for wearer to stop.
- AIR BAGS - also slow you down more gradually and prevent you from hitting the hard surfaces in the car.
Cars have different power ratings
1. The size and design of car engines determine how powerful they are.
2. The more powerful an engine is, the more energy it transfer from its fuel every second, and so the faster its top speed can be.
3. E.g. the power output of a typical small car will be around 50 kW and a sports car will be about 100 kW (some are much higher).
4. Cars are also designed to be aerodynamic. This means that they are shaped in such a way that air flows very easily and smoothly past them, so minimising their air resistance.
5. Cars reach their top speed when the resistive force equals the driving force provided by the engine. So, with less air resistance to overcome, the car can reach a higher speed before this happens. Aerodynamic cars therefore have higher top speeds.
1. When certain insulating materials are rubed together, negatively charged electrons will be scraped off one and dumped on the other.
2. This'll leave a positive static charge on one and a negative static charge on the other.
3. Which way the electrons are transferred depends on the two materials involved.
4. Electrically charged objects attract small objects placed near to them.
5. The classic exaples are polyethene and acetate rods being rubbed with a cloth duster. With the polyethene rod, electrons move from the duster to the rod. With the acetate rod, electrons move from the rod to the duster.
6. Both +ve and -ve electrostatic charges are only ever produced by movemet of electrons. The positive charges definetely do not move!
7. A positive static charge is always caused by electrons moving away elsewhere. The material that loses the electrons loses some negative charge, and is left with an equal positive charge.
1. Electrical charges can move easily through some materials. These materials are called conductors.
2. Metals are known to be good conductors.
1. CURRENT - is the flow of electric charge round the circuit. Current will only flow through a component if there is a potential difference across that component. Unit: ampere, A.
2. POTENTIAL DIFFERENCE - is the driving force that pushes the current round. Unit: volt, V.
3. RESISTANCE - is anything in the circuit which slows the flow down. Unit: ohm, Ω.
THE GREATER THE RESISTANCE ACROSS A COMPONENT, THE SMALLER THE CURRENT THAT FLOWS (FOR A GIVEN POTENTIAL DIFFERENCE ACROSS THE COMPONENT)
1. Current is the rate of flow of charge. When current (I) flows past a point in a circuit for a length of time (t) then the charge (Q) that has passed is given by this formula:
CURRENT = CHARGE / TIME
I = Q / t
Q / (I x t)
2. Current is measured in amperes (A), charge is measured in coulombs (c), time is measured in seconds (s).
3. More charge passes around the circuit when a bigger current flows.
1. The potential difference (or voltage) is the work done (the enrgy transferred, measured in joules, J) per coulomb of charge that passes between two points in an electrical circuit. It's given by this formula:
POTENTIAL DIFFERENCE = WORK DONE / CHARGE
W / (V x Q)
2. So, the potential difference across an electrical component is the amount of energy that is transferred by that electrical component (e.g. to light and heat energy by a bulb) per unit of charge.
3. Voltage and potential difference mean the same thing.
The standard test circuit
This is thr circuit you use if you want to know the resistance of a component. You find the resistance by measuring the current through and the potential difference across the component. It is absolutely the most bog standard circuit you could know. So know it.
1. THE AMMETER
- Measures the current (in amps) flowing through the component.
- Must be placed in series.
- Can be put anywhere in series in the main circuit, but never in parallel like the voltmeter.
2. THE VOLTMETER
- Measures the potential difference (in volts) across the component.
- Must be placed in parallel around the component under test.
1. This very basic circuit is used for testing components, and for getting V-I graphs from them.
2. The component, the ammeter and the variable resistor are all in series, which means they can be put in any order in the main circuit. The voltmeter, on the other hand, can only be placed in parallel around the component under test.
3. As you vary the variable resistor it alters the current flowing through the circuit.
4. This allows you to take several pairs of readings from the ammeter and voltmeter.
5. You can then plot these values for current and voltage on a V-I graph and find the resistance.
Potential difference - current graphs
DIFFERENT RESISTORS FILAMENT LAMP DIODE
The current through a resistor As the temperature of the Current will only flow through (at constant temperature) is filament increases, the a diode in one direction as directly proportional to P.D resistance increases, shown. The diode has very Different resistors have hence the curve. high resistance in the different resistances, hence opposite direction. the different slopes.
Resistance increases with temperature
1. When an electrical charge flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot.
2. This heat energy causes the ions in the conductor to vibrate more. With the ions jiggling around it's more difficult for the charge-carrying electrons to get through the resistor - thecurrent can't flow as easily and the resistance increases.
3. For most resistors there is a limit to the amount of current that can flow. More current means an increase in temperature, which means an increase in resistance, which means the current decreases again.
4. This is why the graph for the filament lamp levels off at high currents.
Resistance, P.D. and current
POTENTIAL DIFFERENCE = CURRENT x RESISTANCE
V / (I x R)
For the straight-line graphs above, the resistance of the component is steady and is equal to the inverse of the gradient of the line, or I/gradient. In other words, the steeper the graph the lower the resistance.
If the graph curves, it means the resistance is changing. In that case R can be found for any point by taking the pair of values (V, I) from the graph and sticking them in the formula R = V/I.
Current through a diode
1. A diode is a special device made from semiconductor material such as silicon.
2. It ise used to regulate the potential difference in circuits.
3. It lets current flow freely through it in one direction, but not in the other (i.e. there's a very high resistance in the reverse direction).
4. This turns out to be real useful in various electronic circuits.
1. A light-emitting diode (LED) emits light when a current flows through it in the forward direction.
2. LEDs are being used more and more as lighting, as they use a much smaller current than other firms of lighting.
3. LEDs indicate the presence of current in a circuit. They're often used in appliances (e.g. TVs) to show that they are switched on.
4. They're also used for the numbers on digital clocks, in traffic lights and in remote controls.
A light-dependent resistor
1. An LDR is a resistor that is dependent on the intensity of light.
2. In bright light, the resistance falls.
3. In darkness, the resistance is highest.
4. They have lots of applications including automatic night lights, outdoor lighting and burgular detectors.
Resistance of a thermistor
1. A thermistor is a temperature dependent resistor.
2. In hot conditions, the resistance drops.
3. In cool conditions, the resistance goes up.
4. Thermistors make useful temperature detectors, e.g. car engine temperature sensors are electronic thermostats.
1. In series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply (except for voltmeters, which are always connected in parallel, but they don't count as part of the circuit).
2. If you remove or diconnect one component, the circuit is broken and they all stop.
3. This is generally not very handy, and in practice very few things are connected in series.
1. POTENTIAL DIFFERENCE IS SHARED - In series circuits the total P.D. of the supply is shared between the various components. So the voltages round a series circuit always add up to equal the source voltage: V = V1 + V2 + ...
2. CURRENT IS THE SAME EVERYWHERE - In series circuits the same curent flows through, all parts of the circuit, ie. A1 = A2
The size of the current is determined by the total P.D, of the cells and th total resistance of the circuit.
3. RESISTANCE ADDS UP - In series circuits the total resistance is just the sum of all the resistances: R = R1 + R2 + R3
The bigger the resistance of a component, the bigger its share of the total P.D.
4. CELL VOLTAGES ADD UP - There is a bigger potential difference when more cells are in series, provided the cells are all connected the same way. For example, when two batteries of voltage 1.5V are connected in series thay supplu 3V between them.
1. In parallel circuits, each component is seperately conne ted to the +ve and -ve of the supply.
2. If you more or disconnect one of them, it will hardly affect the others at all.
3. This is obviously how most things must be connected, for example in cars and in household electrics. You have to be able to switch everything on and off seperately.
1. P.D. IS THE SAME ACROSS ALL COMPONENTS - In parallel circuits all components get the full source P.D, so the volatage is the same across all components: V1 = V2 = V3
This means that identical bulbs connected in parallel will all be at the same brightness.
2. CURRENT IS SHARED BETWEEN BRANCHES - In parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the seperate components. A = A1 +A2 + ...
In a parallel circuit, there are junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving.
If two identical components are connected in parallel then the same current will flow through each component.
Voltmeters and ammeters
1. Ammeters and voltmeters are exceptions to the series and parallel rules.
2. Ammeters are always connected in series even in a parallel circuit.
3. Voltmeters are always connected in parallel with a component even in a series circuit.
Mains supply is AC, battery supply is AC
1. The UK mains supply is approximately 230 volts.
2. It is an AC supply (alternating current), which means the current is constantly changing direction.
3. The frequency of the AC mains supply is 50 cycles per second or 50 Hz (hertz).
4. By contrast, cells and batteries supply direct current (DC). This just means that the current always keeps flowing in the same direction.
1. A cathode ray oscilloscope is basically a snazzy voltmeter.
2. If you plug an AC supply into an oscilloscope, you get a trace on the screen that shows how the voltage of the supply changes with time. The trace goes up and down in a regular pattern - some of the time it's positive and some of the time it's negative.
3. If you plug in a DC supply, the trace you get is just a straight line.
4. The vertical height of the AC trace at any point shows the input voltage at that point. By measuring the height of the trace you can find the potential difference of the AC supply.
5. For DC it's a lot simpiler - the voltage is just the distance from the straight line trace to the centre line.
Hazards in the home
1. Long cables.
2. Frayed cables.
3. Cables in contact with something hot or wet.
4. Water near sockets.
5. Shoving things into sockets.
6. Damaged plugs.
7. Too many plugs into one socket.
8. Lighting sockets without bulbs in.
9. Appliances without their covers on.
Most cables have three wires
1. Most electrical appliances are connected to the mains supply by three-core cables. This means that they have three wires inside them, each with a core of copper and a coloured plastic coating.
2. The brown LIVE WIRE in a mains supply alternates between a HIGH +VE AND -VE VOLTAGE.
3. The blue NEUTRAL WIRE is always at OV. Electricity normally flows in and out through the live and neutral wires only.
4. The green and yellow EARTH WIRE is for protecting the wiring, and for safety - it works together with a fuse to prevent fire and shocks. It is attached to the metal casing of the plug and carries the electricity to earth (and away from you) should something go wrong and the live or neutral wires touch the metal case.
Three-pin plugs and cables
GET THE WIRING RIGHT - 1. The right coloured wire is connected to each pin, and firmly screwed in. 2. No bare wires showing inside the plug. 3. Cable grip tightly fastened over the cable outer layer. 4. Different appliances need different amounts of electrical energy. Thicker cables have less resistance, so they carry more current.
PLUG FEATURES - 1. The metal parts are made of copper or brass because these are very good conductor. 2. The case, cable grip and cable insulation are made of rubber or plastic because they're really good insulators, and flexible too. 3. This all keeps the electricity flowing where it should.
Earthing and fuses
The earth wire and fuse (or circuit breaker) are included in electrical appliances for safety and work together like this:
1. If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, too great a current flows in through the live wire, through the case and out down the earth wire.
2. This surge in current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating. This cuts off the live supply and breaks the circuit.
3. This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current.
4. As well as people, fuses and earthing are there to protect the circuits and wiring in your appliances from getting fried if there is a current surge.
5. Fuses should be rated as near as possible but just higher than the normal operating current.
All appliances with metal cases are usually earthed to reduce the danger of electric shock. Earthing just means the case must be attached to an earth wire. An earthed conductor can never become live. If the appliance has a plastic casing and no metal parts showing then it's said to be double insulated.
Anything with double insulation like that doesn't need an earth wire - just a live and a neutral. Cables that only carry the live and neutral wires are known as two - core cables.
1. Circuit breakers are an electrical safety device used in some circuits. Like fuses, they protect the circuit from damage if too much current flows.
2. When circuit breakers detect a surge in current in a circuit, they break the circuit by opening a switch.
3. A circuit breaker (and the circuit they're in) can easily be reset by flicking a switch on the device. This makes them more convenient than fuses - which have to be replaced once they've melted.
4. They are, however, a lot more expensive to buy than fuses.
Energy and power in circuits
Anything which supplies electricity is also supplying energy. So cells, batteries, generators, etc. all transfer energy to components in the circuit:
- Motion: motors
- Light: light bulbs
- Heat: hair dryers/kettles
- Sound: speakers
1. Whenever a current flows through anything with electrical resistance (which is pretty much everything) then electrical energy is converted into heat energy.
2. The more current that flows, the more heat is produced.
3. A bigger voltage means more heating because it pushes more current through.
4. Filament bulbs work by passing a current through a very thin wire, heating it up so much that it glows. Rather obviously, they waste a lot of energy as heat.
All this energy wasted as heat can get a little depressing - but there is a solution.
1. When you buy electrical appliances you can choose to buy ones that are more energy efficient.
2. These appliances transfer more of their total electrical energy output to useful energy.
3. For example, less energy is wasted as heat in power-saving lamps such as compact fluorescent lamps and light emitting diodes than in ordinary filament bulbs.
4. Unfortunately, they do cost more to buy, but over time the money you save on your electricity bills pays you back for the initial investment.
The toal energy transferred by an appliance depends on how long the appliance is on and its power rating. The power of an appliance is the energy that it uses per second.
ENERGY TRANSFERRED = POWER RATING x TIME
E / P x t
Electrical power and fuse ratings
The formula for electrical power is:
POWER = CURRENT x POTENTIAL DIFFERENCE
P = I x V
P / I x V
Most electrical goods show their power rating and voltage rating. To work out the size of the fuse needed, you need to work out the current that the item will normally use.
1. When an electrical charge (Q) goes through a change in potential difference (V), then energy (E) is transferred.
2. Energy is supplied to the charge at the power source to raise it through a potential.
3. The charge gives up this energy when it falls through any potential drop in components elsewhere in the circuit.
ENERGY TANSFORMED = CHARGE x POTENTIAL DIFFERENCE
E / Q x V
4. The bigger the change in P.D. (or voltage), the more energy is transferred for a given amount of charge passing through the circuit.
5. That means that a battery with a bigger voltage will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up higher at the start.
- The nucleus is tiny but it mkes up most of the mass of the atom. It contains protons and neutrons. Which gives it an overall positive charge,
- The rest of the atom is mostly empty space. The negative electrons are round the outside of the nucleus. They give the atom it's overall size - the radius of the atom's nucleus is about 10 000 times smaller than the radius of the atom.
- NUMBER OF PROTONS EQUALS NUMBER OF ELECTRONS: Atoms have no charge overall. The charge on an electron is the same size as the charge on a proton - but opposite. This means the number of protons always equals the number of electrons in a neutral atom. If some electrons are added or removed, the atom becomes a charged particle called an ion.
1. Isotopes are atoms with the same number of protons but a different number of neutrons.
2. Hence they have the same atomic number, but different mass numbers.
3. Atomic number is the number of protons in an atom.
4. Mass number is the number of protons + the number of neutrons in an atom.
5. Carbon - 12 and carbon 14 are good examples of isotopes.
6. Most elements have different isotopes, but there's usually only one or two stable ones.
7. The other isotopes tend to be radioactive, which means they decay into other elements and give out radiation.
1. Radioactive substances give out radiation from the nuclei of their atoms - no matter what is done to them.
2. This process is entirely random. This means if you have 1000 unstable nuclei, you can't say when any one of them is going to decay, and neither can you do anything at all to make a decay happen.
3. It's completely unaffected by physical conditions like temperature or by any sort of chemical bonding etc.
4. Radioactive substances spit out one or more of the three types of radiation, alpha, beta or gamma.
Background radiation is radiation that is present at all times, all around us, wherever you go. The background radiation we receive comes from:
- Radioactivity of naturally occuring unstable isotopes which are all around us - in the air, in food, in building materials and in the rocks under our feet.
- Radiation from space, which is known as cosmic rays. These come mostly from the sun.
- Radiation due to man-made sources, e.g. fallout from nuclear weapon tests, nuclear accidents or dumped nuclear waste.
1. An alpha particle is two neutrons and two protons - the same a a helium nucleus.
2. They are relatively big and heavy and slow moving.
3. They therefore don't penetrate very far into materials and are stopped quickly, even when travelling through air.
4. Because of their size they are strongly ionising, which just means they bash into a lot of atoms and knock electrons off them before they slow down, which creates lots of ions - hence the term ionising.
1. Beta particles are in between alpha and gamma in terms of their properties.
2. They move quite fast and they are quite small (they're electrons).
3. They penetrate moderately into materials before colliding, have a long range in air, and are moderately ionising too.
4. For every beta- particle emitted, a neutron turn to a proton in the nucleus.
5. A beta-particle is simply an electron, which virtually no mass and a charge of -1.
1. Gamma rays are the opposite of alpha particles in a way.
2. They penetrate far into materials without being stopped and pass straight through air.
3. This means they are weakly ionising because they tend to pass through rather than collide with atoms. Eventually they hit something and do damage.
4. Gamma rays have no mass and no charge.
1. THE DAMAGE CAUSED BY RADIATION DEPENDS ON THE RADIATION DOSE
- Radiation dose depends on the type and amount of radiation you've been exposed to
- The higher the radiation dose, the more at risk you are of developing cancer.
2. RADIATION DOSE DEPENDS ON LOCATION AND OCCUPATION
- Certain underground rocks can cause higher levels at the surface, especially if they release radioactive radon gas, which gets trapped inide people's houses.
- Nuclear industry workers and uranium minors are exposed to 10 times the normal amount of radiation. They wear protective clothing and face masks to stop them from touching or inhaling that radioactive material, and monitor their radiation doses with special radiation badges and regular check ups.
- Radiographers work in hospitals using ionising radiation and so have a higher risk of radiation exposure. They were lead aprons and stand behind lead screens to protect them from prolonged exposure to radiation.
- Underground it increases because of the rocks all around, posing a risk to minors.
Alpha and beta particles
1. Alpha particles have a positive charge, beta particles have a negative charge.
2. When travelling through a magnetic or electric field, both alpha and beta particles will be deflected.
3. They're deflected in opposite directions because of their opposite charge.
4. Alpha particles have a larger charge that beta particles, and feel a greater force in magnetic and electric fields. But they're deflected less because they have a much greater mass.
5. Gamma radiation is an electromagnetic wave and has no charge, so it doesn't get wave and has no charge, so it doesn't get deflected by electric or magnetic fields.
Radioactivity decreases over time
1. Each time a decay happens and an alpha, beta or gamma is given out, it means one more radioactive nucleus has disappeared.
2. Obviously, as the unstable nuclei all steadily disappear, the activity will decrease. So the older a sample becomes, the less radiation it will emit.
3. How quickly the activity drops off varies a lot. For some substances it takes just a few microseconds before nearly all the unstable nuclei have decayed (others take years).
4. The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off. HALF LIFE IS THE AVERAGE TIME IT TAKES FOR THE NUMBER OF NUCLEI IN A RADIOACTIVE ISOTOP SAMPLE TO HALVE.
5. A short half-life means the activity falls quickly, because lots of the nuclei decay quickly.
6. A long half-life means the activity falls more slowly because most of the nuclei don't decay for a long time.
1. A weak source of alpha radiation is placed in the detector, close to two electrodes.
2. The source causes ionisation, and a current flows between the electrodes.
3. If there is a fire then smoke will absorb the radiation - so the current stops and the alarm sounds.
Tracers in medicine
1. Certain radioactive isotopes can be injected into people (or they can just swallow them) and their progress around the body can be followed using an external detector. A computer converts the reading to a display showing where the strongest reading is coming from.
2. A well-known example is the use of iodine-131, which is absorbed by the thyroid gland just like normal iodine-127, but it gives out radiation which can be detected to indicate whether the thyroid gland is taking in iodine as it should.
3. All isotopes which are taken into the body must be GAMMA or BETA emitters (never alpha), so that the radiation passes out of the body - and they should only last a few hours, so that the radioactivity inside the patient quickly disappears.
1. Since high doses of gamma rays will kill all living cells, they can be used to treat cancers.
2. The gamma rays have to be directed carefully and at just the right dosage so as to kill the cancer cells without damaging too many normal cells.
3. However, a fair bit of damage is inevitably done to normal cells, which makes the patient feel very ill. But if the cancer is successfully killed off in the end, then it's worth it.
1. Food can be exposed to a high dose of gamma rays which will kill all microbes, keeping the food fresh for longer.
2. Medical instruments can be sterilised in just the same way, rather than by boilig them.
3. The great advantage of irradiation over boiling is that it doesn't involve high temperatures, so things like fresh apples or plastic instruments can be totally sterilised without damaging them.
4. The food is not radioactive afterwards, so it's perfectly safe to eat.
5. The isotope used for this needs to be a very strong emitter of gamma rays with a reasonably long half-time (at least several months) so that it doesn't need replacing too often.
Radiation harms living cells
1. Alpha, beta and gamma radiation will cheerfully enter living cells and collide with molecules.
2. These collisions cause ionisation, which damages or destroys the molecules.
3. Lower doses tend to cause minor damage without killing the cell.
4. This can give rise to mutant cells which divide uncontrollably. This is cancer.
5. Higher dose tend to kill cells completely, which causes radiation sickness if a lot of body cells all get blatted at once.
6. The extent of the harmful effects depends on two things:
- How much exposure you have to the radiation.
- The energy and penetration of the radiation, since some types are more hazardous than others, of course.
OUTSIDE THE BODY, BETA AND GAMMA SOURCES ARE THE MOST DANGEROUS - This is because beta and gamma can bet inside to the delicate organs, whereas alpha is much less dangerous because it can't penetrate the skin. INSIDE THE BODY, AN ALPHA SOURCE IS THE MOST DANGEROUS - Inside the body alpha sources do all their damage in a very localised area. Beta and gama sources on the other hand are less dangerous inside the body because they mostly pass straight out without doing much damage. SAFETY PRECAUTIONS:
- When conducting expriments, use radioactive sources for as short a time as possible so your exposure is kept to a minimum.
- Never allow skin contact with a source. Always handle with tongs.
- Hold the source at arm's length to keep it as far from your body as possible.
- Keep the source pointing away from the body and avoid looking directly at it.
- Lead absorbs all three types of radiation.
- When someone needs an X-ray or radiotherapy, only the area of the body that needs to be treated is exposed to radiation.
Nucelar power stations generate electricity using nuclear reactors. In a nuclear reactor, a controlled chain reaction takes place in which atomic nuclei split up and release energy in the form of heat. This heat is then simply used to heat water to make steam, which is used to drive a steam turbine connected to an electricity generator. The fuel that's split is usually uranium-235, though sometimes it's plutonium-239 (or both).
- For nuclear fission to happen, a slow moving neutron must be absorbed into uranium or plutonium nucleus. This addition of a of a neutron makes the nucleus unstable, causing it to split.
- Each time a uranium or plutonium nucleus splits up, it spits out two or three neutrons, one of which might hit another nucleus, causin it to split also, and thus keeping the chain reaction going.
- When a large atom splits in two it will form two new smaller nuclei. These new nuclei are usually radioactive because they have the wrong number of neutrons in them.
- A nucleus splitting gives out a lot of energy - lots more energy than you get from any chemical reaction. Nuclear processes release much more energy than chemical processes do. That's why nuclear bombs are so much more powerful than normal.
- The main problem with nuclear power is with the disposal of waste. The products left over after nuclear fission are highly radioactive, so they can't just be thrown away. They're very difficult and expensive to dispose of safely.
- Nuclear fuel is cheap but the overall cost of nuclear power is high. Dismantling a nuclear plant can take decades. Nuclear power also carries the risk of radiation leaks from the plant or a major catastrophe like Chernobyl.
1. Two light nuclei can join to create a larger nucleus - this is called nuclear fusion.
2. Fusions releases a lot of energy (more that fission for a given mass) - all the enegy released in stars comes from fusion. So people are trying to develop fusion reactors to generate electricity.
3. Fusion doesn't leave behind a lot of radioactive waste like fission, and there's plenty of hydrogen knocking about to use as fuel.
4. The big problem is that fusion can only happen at really high temperatures.
5. You can't hold the hydrogen at the high temperatures and pressures required for fusion in an ordinary container - you need extremely strong magnetic field.
6. There are a few experimental reactors around, but none of them are generating electricity yet. At the moment it takes more power to get up to temperature than the reactor can produce.
Life cycle of stars
1. Stars initially form from clouds of DUST AND GAS. The force of gravity makes the gas and dust spiral in together to form a protostar.
2. Gravitational energy is converted into heat energy,so the temp rises. When the temp gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of heat and light. A star is born. Smaller masses of gas and dust may also pull together to make planets that orbit the star.
3. The star immediately enters a long stable period, where the heat created by the nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards. The star maintains its energy output for millions of years due to the massive amounts of hydrogen it consumes. In this stable period it's called a MAIN SEQUENCE STAR and it lasts several billion years.
4. Eventually the hydrogen begins to run out. Heavier elements such as iron are made by nuclear fusion of helium. The star then swells into a RED GIANT, if it's a small star, or a RED SUPER GIANT if it's a big star. It becomes red because the surface cools.
5. A small-to-medium-sized star like the sun then becomes unstable and ejects its outer layer of dust and gas as a PLANETARY NEBULA.
6. This leaves behind a hot, dense solid core - a WHITE DWARF, which just cools down to a BLACK DWARF and eventually disappears.
7. Big stars, however, start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. Eventually they explode in a SUPERNOVA, forming elements heavier than iron and ejecting them into the universe to form new planets and stars.
8. The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a NEUTRON STAR. If the star is big enough this will become a BLACK HOLE.