P2 Physics

A fuse contains a thin wire which heats up, melts and cuts off the current if the current is too large

• The rate of a fuse is a maximum current that can pass through the fuse without melting the fuse wire
• If a fuse of the wrong rating is fitted the fuse might blow or it might let too much current through and cause a fire 
• If the earth connection of an appliance with a metal casing is broken and the live wire breaks and touches the case it is too dangerous to touch
• Even if the earth connection is mended it might still be dangerous because the increased current may cause the wires of the appliance to overheat
• Appliances with plastic cases dont need to be earthed because they are insulators 

Circuit breaker: An electromagnet switch that switches off (opens) when there is a fault

• they are better than fuses 1) work faster 2) can be reset more quickly 
• RCCB (Residual Current Circuit Breaker)
• It cuts of the current when the current in the live wire is different to the current in the neutral wire 

Power (watts) = energy transferred (joules)

                       time (seconds)

Power supplied (watts) = current (amperes) × potential difference (volts)

P = I x V

• Domestic electrical appliances usually fitted with 3A, 5A or 13A
• E.g, Which of these fuses would you use for a 920 W, 230 V appliance?
• Current = 920 W ÷ 230 V = 4 A  
• The current exceeds 3 A, and so a 5 A fuse should be used.

• Coulomb (C) is the unit of charge 
• Charge = current x time
• Q = I x t 

A resistor heats up when it is connected to a battery because when the resistor is connected, electrons are pushed through the resistor by the battery. As electrons repeatedly collide with the vibrating metal ions of the resistor, they transfer kinetic energy to them, which makes them vibrate more. This causes the resistor to heat up.

Two factors that affect the energy transferred by a resistor in a certain time:

1) The amount of charge passing through the resistor

2) The potential difference across the resistor

• Energy transferred (joules) = potential difference (volts) × charge (coulombs)
• The energy from a battery is equal to the sum of the energy transferred to the bulb (emits heat and light energy) and to the variable resisitor (warms it so heat energy to surroundings)

Electrical faults are dangerous because 1) electric shocks 2) cause fires

• Faults are caused by damaged sockets, plugs, cables or appliances
• Prevented by checking for damage regularly and getting a qualified electrician repair/replace damaged components
• Two core cable is used if the appliance is 'double-insulated' (doesnt need to be earthed)
• Electricians select wires of suitable thickness so that the heating effect of the current through the wires is insignificant

Factors that should be considered when choosing an electrical appliance:

1) cost  2) power rating 3) efficiency rating 

Two types of low energy light bulb:

1) Low-energy compact fluorescent lamp (CFL)  2) Low-energy light-emitting diode (LED)

• LEDs more efficient 
• CFLs more suited to room lighting and significantly cheaper

Greater resultant force = greater acceleration

Resultant force (N)  = mass (kg) x acceleration (m/s)

F= ma 

• The acceleration of an object is in the same direction as the resultant force acting on it 
• The velocity of an object increases if the resultant force is in the same direction
• The velocity of an object decreases if the resultant force acts in the opposite direction

• We say the acceleration is negative because it is in the opposite direction to the velocity

When an object is moved by a force, we say work is done.

• When a heavy object is lifted 10J of work is done on it which means 10J of energy has been transferred to the object ( Energy transferred = work done)

Work done (J) = force applied (N) x  distance moved in direction of force (m)

W = fd

• E.g  W= 6N x 2m = 12J when a box is pushed 2m across the ground with a force of 6N

• Your hands become warm if you rub them together because your muscles do work to overcome friction which transfers heat energy that warms your hands 
• When the brakes are applied on a vehicle the kinetic energy is transferred into heating the brake pads and wheel discs as well as the surrounding air

The acceleration of a falling object acted on only by gravity is 10 m/s (known as acceleration due to gravity)

• You can find the resultant force of an object falling through a fluid by subtracting the drag force from its weight 
• Terminal velocity of a falling object is the constant velocity the object reaches when the drag force is equal and opposite to its weight
• Resultant force on an object at terminal velocity is 0

• When you apply force to a tube of toothpaste the tube changes its shape and pushes toothpaste out of the tube 
• When you apply more force it comes out faster

A force can 1) change the shape 2) change state of rest 3) change motion of an object

When two objects push/pull on eachother they exert equal and opposite forces on one another

• If a kickboxer kicks his oponent with a force of 60N he will experience 60N of reverse force
• When a tractor is used to pull the car out of the mud the force of the ground on the tractor must be greater than the force of mud on the car 
• The forces are not nessacarily equal because the car and the tractor are not the same

Friction acts when the tyres are in contact with the ground.

• If you want to reduce your weight on earth go to the equator (mass will stay the same)
• The weight of an object is the force of gravity on the object (N) and the mass of an object is the quantity of matter in it (kg)
• Gravitational field strength on earth is about 10 N/kg (newton per kilogram)

Weight = mass x gravitational field strength 

W = mg 

• E.g Calculate the weight of a person in newtons with a mass of 60kg standing on earth
• Weight = 60 x 10 = 600N

Direct current: a current that is in one direction only 

Alternating current: a current that repeatedly reverses its direction

• The mains frequency in the UK is 50 cycles per second (50 Hz)
• A mains circuit always has a live wire and a neutral wire
• The live wire is dangerous because it reaches voltages of over 300V

We can use an oscilloscope to observe alternating pd

• We can use an oscilloscope to measure the frequency of an alternating current by measuring the time period of the waves (the time taken for one cycle) and then using the formula:
•                frequency (Hz) =                 1
•                                         time taken for one cycle (s)
• E.g, Frequency = 1/time taken for one cycle = 1/0.04 seconds = 25 Hz

The voltage of a mains live wire alternates between -325V and +325V which is equivalent to 230V of direct voltage

•  Henry Becequerel formed an image on film when uranium salts sent out radiation which passed around key on film

Marie Curie awarded 1911 Nobel Prize  for discovering polonium and radium

• She may have got leukaemia from radiation poisoning (from radioactive matierials worked with)
• A Geiger counter can detect radioactivity

Three types of radiation given out by radioactive substances:

1) alpha radiation, α   2) beta radiation, β   3) gamma radiation, γ

• Alpha radiation can be stopped by sheet of paper
• Unstable nuclei of radioactive substances emit radiation to become stable (radioactive decay)
• We say this is a random event, can't tell when an unstable nucleus will decay

Background radiation can come from the environment, from space or from devices such as X-ray machines

• Ernest Rutherford discovered the nucleus in 1913
• He used alpha particle scattering experiments to prove it
• He deduced the nuclei of atoms was positively charged because (positively charged) alpha particles were repelled by the nuclei, and like charges repel, therefore the nuclei must be positively charged.
• He deduced the nucleus was much smaller than the atom because most alpha particles pass through the atom without deflection

Why was it quickly accepted?

• Agreed with measurements that Geiger and Marsden had made in their experiments.
• Explained radioactivity in terms of the changes that happen to an unstable nucleus when radiation is emitted
• Predicted the existence of the neutron

'Plum pudding model' consisted of positively charged matter, evenly spread out, with electrons buried inside

• Disproved could'nt explain why some ap scattered at large angles by atoms

Two factors that need to be considered when choosing a radioactive substance for a particular job 1) The half-life of the substance 2) The type of radiation given out

• Automatic monitoring is used when making metal foil 
• It knows when the foil is too thick because the amount of β radiation penetrating the material drops and so the detector reading drops
• The detector then corrects the thickness by sending a signal to the rollers, which then increase the pressure on the sheet, making it thinner again.
• Radioactive tracers track the flow of a radioactive substance though a system
• Radioactive iodine is used as a radioactive tracer in patients with suspected blocked kidneys.
• Three reasons for choosing radioactive iodine.1) Its half-life is a few days , 2) It decays into a stable product,3) It emits gamma radiation, so it can be detected outside the body

Carbon dating finds the age of ancient rocks&uranium dating finds the age of igneous rocks 

• The nucleus of a fissionable substance splits into two smaller ‘fragment’ nuclei during nuclear fission
• 'Chain reaction' -  a nuclear fission event causes other fissionable nuclei to split, which causes yet more fission events, etc.
• Electromagnetic radiation and kinetic energy is released during fission
• In a nuclear fission reactor there are several fission neutrons from each fission event but on average only one neutron from each event will go on to produce further fission so energy is released at a steady rate
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• Most nuclear reactors use enriched uranium (the isotope Uranium-235 is fissionable)
• U-238 can change into Plutonium-239 in a reactor 

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• Fission neutrons need to be slowed, fast neutrons don’t cause further fission of U-235
• Control rods absorb surplus neutrons to keep the chain reaction under control
• The reactor core is made from thick steel to withstand the high temperature and pressure in the core

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• During nuclear fusion two small nuclei are fused together to form a single, larger nucleus, releasing energy
• The Suns core consists of a plasma of bare hydrogen and helium nuclei with no electrons
• They fuse together when they collide 
• When two protons fuse togther a heavy hydrogen nucleus is formed - 
• The two heavier nuclei collide to form the helium nucleus- 
• The energy released is carried away as kinetic energy of the product nucleus and other emitted particles

Two nuclei approching eachother repel eachother because they are positively charged

• The repulsion can be overcome by getting the nuclei to move very fast towards eachother

 ‘Activity’ of a radioactive isotope- The number of atoms that decay per second

• We measure the count rate due to a sample by using a Geiger counter to measure the number of counts per second
• The average time taken for a count rate to fall by a half is always the same

Half-life : The average time it takes for the number of nuclei of a radioactive isotope in a sample to half

• In 1 half-life the count rate would have halved 
•  We say that radioactivity is a random process because we can’t predict when an individual atom will decay
• We predict how many atoms will decay in a certain time because there are (usually) a very large number of atoms in a sample

• Beta particle is a fast moving electron
•  α particles deflected in the opposite direction to β particles because α radiation consists of positively charged particles while β radiation consists of negatively charged particles
• α particles harder to deflect because they have a much greater mass than β particles
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A magnetic field can be used to deflect radiation

• When an alpha particle is passing through a field the force deflecting it is twice as great as the force deflecting the beta particle (charge of alpha particle is twice that of a beta particle)

Ionisation is when radiation from a radioactive substance knocks electrons out of other atoms, causing them to become charged.

• Ionisation in a living cell can damage or kill the cell. Damage to the genes inside a cell can be passed on if the cell generates more cells. (alpha radiation most ionising)

Isotopes: Atoms of the same element with different numbers of neutrons

• Alpha particle consists of 2 protons and 2 neutrons 

• When the nucleus emits an alpha particle atom number goes down by 2, mass number down by 4

When a beta particle is created in a nucleus a neutron in the nucleus changes into a proton and a β particle (electron). The β particle is instantly emitted at high speed.

• Realtive mass is 0, relative charge is -1

• When an unstable nucleus emits a beta particle the atomic number goes up by 1, and the mass number stays the same

Gamma radiation takes place in a nucleus after a alpha/beta has been emitted 

• It is uncharged and has no mass
• The number of protons/neutrons in the nucleus stays the same when gamma radiation is emitted

How to use a Geiger counter to find count rate of radioactive source:

• 1) Measure the background count rate
• 2) Measure the count rate with the source in place, and subtract the background count rate from this to find the source count rate

We can test the range of a source by moving the Geiger tube away from the source until the count rate due to the source drops to zero.

Radiation type                    Absorber material        Thickness needed

alpha                                sheet of paper              very thin

beta                                  metal sheet                 a few mm

gamma                             lead or concrete           several cm of lead, or more than 1 m concrete

• Gamma radiation spreads out without being absorbed (electromagnetic radiation)

The same current passes through each component when they are in series with one another

• This is because the same number of electrons pass through each component every second
• The total potential difference of the voltage supply is shared between the components
• We can find the pd of several cells in series by adding the potential differences of each cell

When a series circuit is made of several components we find the total resistance of the components by adding the resistances of each component 

You calculate the total current in a parallel circuit by finding the sum of the currents through the separate components

• If you increase the resistance of a component the current through the comp decreases
• The potential difference across each component is the same 

Current (amperes) = potential difference (volts)

                             resistance (ohms)

For example : Current = potential difference ÷ resistance

                                = 12 V / 6 Ω = 2 A

                     Total current = sum of current through each resistor = 2 A + 2 A = 4 A

• The gradient line of a distance-time graph represents speed
• When the speed is increased the line gets steeper
• If a car was stationary the line would be horizontal

Speed= metres per second (m/s)

Speed (m/s) = distance (m) / time (s) (at a constant speed) 

• On a distance time graph we divide the distance the lorry has travelled by the time taken
• Long distance vehicles are fitted with tachographs to check that their drivers don't drive for too long 

Seat belts protect people by 1) stopping people from continuing forwards when a car stops 

                                         2) increase the impact force (reduce deaccelerating force)

                                         3) by spreading force out across the chest

Air bags are better than seat belts because they spread the force of an impact across the upper part of the body and so the force is lessened compared with a seat belt.

• Child car seat law applies up to 12 Yrs old and 1.35 m to reduce deaths of children in cars 
• The driver is responsible for making sure the kids have seatbelts on
• Manufacturers always include them because it is required by law
• They usually chose not to add side impact bars as they are not required by law and add to the cost of manufacture 

• An appliance with a metal casing needs to be earthed to stop the metal casing becoming ‘live’ if the live wire touches the case
• Sockets and plugs are made from plastic matierials because very good electrical insulators
• There is a live pin, neutral pin and earth pin 
• Pins are made out of brass because it is a good electrical conductor, it does not rust, and it is harder than copper (does not bend easily)
• The live wire is brown, the neutral wire is blue, and the earth wire is striped green and yellow
• The main cables consist of two or three insulated copper wires encased by a layer of flexible plastic matierial
• Cables that join wall sockets are thicker because more current passes through the socket cables than through the lighting cables, and so the wires must be thicker to prevent them becoming too hot.

The momentum of a moving object is calculated by:

Momentum = mass × velocity

p (kg m/s) = m (kg) x v (m/s) 

• E.g, calculate the momentum of a person of mass 60 kg walking at a velocity of 4 m/s.
• Momentum = mass × velocity
•  60 kg × 4 m/s = 240 kg m/s

Law of conservation of momentum: In a closed system, the total momentum before an event equals the total momentum after the event.

When a vehicle crashes into the back of a line of cars the momentum is transferred along the line of cars, to the car at the front.

• If you rub a baloon against your clothes it becomes electrically charged with static electricity
• It will stick to the ceiling because the charge of the baloon attracts it to the ceiling 

A Van de Graaf generator charges up because a rubber belt rubs against a felt pad and charges up. The belt transfers the charge onto the metal dome.

• When the charge on the dome builds up too much charge massive sparks are produced 

A charged atom is an ion - when you add more electons it becomes negative 

                                    - it becomes positive when you remove electrons because there are more                                       protons 

• By rubbing a polythene rod with a dry cloth you make the rod negatively charged because the dry cloth transfers electrons to the surface of the rod. 
• By rubbing a perspex rod with a dry cloth you make th rod postively charged surface electrons are transferred to from the rod to the cloth

• An ammeter can measure the current across the bulb in an electrical circuits (amps)
• A voltmeter can measure the potential difference across the bulb (volts)

Potential difference across the component (volts) = work done (joules)

                                                                         charge (coulombs)

Resistance of an electrical component = potential difference (volts)

                                                         current (amps)

• A circuit diagram shows how the components in a circuit are connected together (each component has its own symbol) 

• Electric current: flow of charge, Amperes (A)
• Electric charge: Coulombs (C)
• The size of an electric current depends on the rate of flow of charge
• Electic current = charge flow/ time taken 
• I= Q/t 

• When a skateboarder jumps off his skateboard the skateboard flies away because the momentum of the skateboad is equal and opposite to the skateboarders momentum
• The mass of the skateboard is lower than the person so it moves away faster 
• Momentum has a direction

If the total momentum before an explosion is 0 it will be 0 afterwards (conservation of momentum)

• When the barrel of the gun is fired it recoils backwards
• This is minimised in artillery guns by a spring which slows it down 

• During a front end impact the momentum of a car is reduced 
• The crumple zone at the front of the car reduces the force of impact by increasing the impact time
• Impact force = mass x acceleration

If the deceleration of the ball is 100 m/s2 when it hits the racket and the mass of the ball is 0.1 kg, calculate the impact force on the racket.

• Impact force = mass × acceleration = 0.1 kg × -100 m/s2 = -10 N

When two vehicles collide the total momentum stays the same

• We assume that no external forces act on the cars 
• If a heavy lorry crashes into the back of a small car it gains momentum and accelerates forwards 

Resultant force: A single force that has the same effect as all the forces acting on the object

• If the resultant force on an stationary object is 0 it remains at rest
• If the object was in motion with a RF of 0 it would continue to move at the same speed in the same direction
• When a heavy box is pushed across the floor at a constant velocity the RF is 0

You can find the resultant force on a jet during take off by finding the difference between the thrust force and the air resistance on it.

• When a driver brakes the braking force is greater than the engine force 

• You could find the resultant force on the car by finding the difference between the engine force and braking force
• When two forces are acting in the same direction you calculate the resultant force by finding the sum of two forces

• When an object is accelerating the line on the graph goes up because its velocity is increasing with time
• The line is straight when the object is constant
• The gradient represents acceleration

A car moving at a constant speed: straight and horizontal

A car that is slowing down: line goes down 

The area under the line on a velocity-time graph represents distance travelled. 

• Vehicles use more fuel at higher speeds because an increased air resistance (from a lower speed) means more power is need to maintain the speed 
• Lorries can be fitted with wind deflectors which decrease air resistance and decrease fuel used

Driving at high speeds is dangerous to pedestrians 1) Driver is less likely to stop safely

                                                                         2) Force on person struck would be greater

• Speeding motorist might be discouraged by speed cameras because if they are exceeding the speed limit they can be fined (possibly license taken off them)
• Average speed cameras are linked to the measure of the average speed of a vehicle

Anti-skid surfaces are rougher than normal surfaces to increase friction between the road and the tyres, reduces chance of skidding when car brakes

Skidding usually happens when the brakes are applied too harshly

Velocity is speed in a given direction

• When a plane does the 'loop the loop' the velocity constantly changes because direction of motion changes constantly 

Acceleration is the change of velocity per second

• Scientific unit is metre per second squared (m/s2)

Acceleration = change in velocity/ time taken 

• In the alternative equation a=(v-u)/t  u is the initial velocity and v is the final velocity 

Deceleration is the change in velocity per second when an object slows down 

• This is also known as 'negative acceleration' 

Resistive forces on a car are friction and air resisitance 

The braking force needed to stop a vehicle is:

Resultant force (braking force) = mass x acceleration

• The greater the speed, the greater the breaking force needed
• The mass of the vehicle affects the breaking force needed

Stopping distance: The shortest distance that the car can safely stop in 

• Stopping distance = thinking distance + braking distance
• The thinking distance depends on the drivers reaction time 
• The thinking distance can be calculated by multiplying cars speed by drivers reaction time 
• The braking distance can be calculated by multiplying the average speed of the car during braking by the braking time 

Two things that increase braking/thinking distance 1) Driving in poor weather conditions 

                                                                        2) Driving a vehicle more quickly 

Elastic object - regains its original shape after forces deforming it are removed

• E.g, rubber band and a squash ball
• Extension of a spring = length of spring - original length
• A graph line is straight and goes through the origin which means the extension on the spring is directly propotional to the weight hung on it

Elastic potential energy is stored in a stretched spring

• When the energy is released it is transferred into kinetic and heat energy 

Hookes law : The extension of a spring is directly proportional to the force applied to it, provided its limit of proportionality is not exceeded.

• When a spring stretches beyond its limit (proportionality) the spring stretches more than predicted, as the the extension is no longer directly proportional to the force applied to it
• Force applied  (N) = spring constant (N/m) x extension (m)
• Spring constant is the force per unit extension needed to stretch an object

Kinetic energy depends on 1) mass and 2) speed of object

• Kinetic energy = ½ × mass × speed
•      (J)                       (kg)      (m/s )

KERS are kinetic energy recovery systems store kinetic energy when cars brake down so that it can be used later on 

• EPE - The energy stored when work is done on an object.
• When firing an arrow from a bow the EPE is transferred into kinetic energy

GPE - The energy stored in an object due to its position in the Earth’s gravitational field

• If 500J of work is done lifting an object it gains 500J of GPE
• When it is put down again the GPE decreases 

Work done on an object moving up or down 1) Change of height 

                                                               2) Weight of an object

Change of gravitational potential energy (J) = weight of the object (N) × change of height (m)

or 

Change of gravitational= mass of object (kg) x gravitational field strength (N/kg) x change of height

potential energy (J)              EP = m × g × h

On the moon it is easier to lift objects because the GFS is weaker 

In a fusion reactor:

• the plasma is heated by by passing a large electric current through it
• then contained by a magnetic field so it doesn't touch the reactor walls (if i did plasma would go cold and fusion stops)

Technical difficulty:plasma of light nuclei must be heated to very high temps before they will fuse

• Important area of research as it could meet all our energy needs

Nuclear Fusion is better at generating electricity than Nuclear Fission because:

1) Fuel (heavy hydrogen) is readily available

2) The reaction product is helium, which is harmless compared to nuclear waste.

• A person on average gets 5 units/year from nuclear weapons testing
• A person on average gets 1,190 units/year from natrual radioactivity in air
• Medical sources of radiation include X rays because they have an ionising effect

Radon gas in the air has seeped up through the ground from radioactive substances deep underground

• It is dangerous as it emits alpha particles which can endanger peoples health if breathed in

Fuel rods from nuclear reactors are stored after removal in large water tanks for up to a year

• once the waste has cooled it is stored securely for many years after unused plutonium and uranium have been removed 

The Chernobyl accident happened in 1986

• It could have been avoided bby installing high-speed shutdown systems, following safety regulations and using a better reactor design

• Alpha radiation is the most dangerous when inside the body (most ionising)

Workers at risk of ionising radiation can minimise their exposure by:

1) Keep as far as possible from sources of radiation.

2) Spend as little time as possible inside ‘at risk’ areas.

3) Use radiation shields or stay behind thick concrete barriers.

• The new reactors that will replace our reactors today are called third generation nuclear reactors

• the universe was created by the Big Bang
• As the universe expanded it cooled down
• When the universe was 100 seconds old protons and neutrons were formed
• There are billions of galaxies in the universe

During the 'dark age' the universe was completely dark (a patchy cloud of hydrogen and helium)

• The formation of stars and galaxies was at the end of the dark age
• Gravity pulled matter together to form dense clumps, which eventually turned into stars and galaxies to end the dark age

• Stars are formed from clouds of dust and gas
• Protostar: concentrated cloud that can go on to form a star
• When a protostar heats up the nuclei of light elements fuse together and energy is released, causing the core to get hotter and hotter, eventually forming a star.

Main sequence star: a star in the main stage of its life 

• Force of gravity (inwards) and force of radiation (outwards) have to be balanced for the star to be stable
• These force stop being blanced when most of the hydrogen nuclei in the core have been fused together

Main stages of life for low mass star:

Protostar ---> main sequence star---> red giant ----> white dwarf ---->black dwarf

• Light chemical elements are formed in stars during fusion reactions
• Iron is the heaviest element that can be formed by fusion
• The debris from a supernova contains all the known elements
• This debris will eventually be pulled together to form new stars and planets 

The heaviest known natrual element is uranium

• the prescence of uranium suggests that the Solar System formed from the debris of a supernova
• scientists can make elements heavier than uranoum by bombarding heavy elements with high speed neutrons
• plutonium (heavier than uranium)  isnt natrually present in the earth because it decayed after the supernova before the formation of the earth