- Created by: Krissie
- Created on: 26-04-11 13:39
Static Electricity: Atomic Structure
Atoms are made up of subatomic particles, called Protons, Neutrons and Electrons. Protons and Neutrons are held tightly together in the neucleus of the atom, whilst electrons circle around the outside of the atom. Unlike protons, they are not bound tightly to the atom so they can move away from the atom.
Protons and Electrons have charges. Protons carry a positive charge and Electrons carry a negative charge. If there are more electrons than protons in an atom, the atom would be negatively charged, and if there were more protons than electrons in the atom, the atom would be positively charged.
Ions are atoms that have gained a charge by either gaining or losing electrons. Positive ions are atoms which have lost electrons, and negative ions are atoms which have gained electrons.
Conductors must be insulated from the ground in order to charge them, otherwise electrons lost will be replaced and electrons gained will be lost into the ground.
Static Electricity: Charging by Friction
Uncharged materials have equal amound of protons and electrons. When 2 materials are rubbed together, electrons from one material may be transferred to the other material. The material which loses the electrons would be positively charged and the material which gains the electrons would be negatively charged.
Because no electrons are lost during the process, the materials will be equally charged, but with opposite charges, and because oppositve charges attract, the 2 materials would be attracted together.
Some materials are more likely to lose electrons than gain electrons, or visa versa.
If you rubbed a woolen cloth on a polythene rod, the wool would ***** electrons from the rod and become negatively charged, where the polythene rod would become positively charged. If the polythene rod lost 3 electrons, it would have a charge of 3+ and the wool would have a charge of 3-.
Perspex, however, is more likely to become positively charged and the woolen cloth would become negatively charged.
Static Electricity: Induced Charges
An induced charge is created when a charged object is moved near to an uncharged object. Although electrons aren't transferred between the objects, an induced charge is created in the uncharged object because the electrons inside it move.
If a positively charged object is brought near to an uncharged object, the electrons from the uncharged object will be attracted to the surface nearest to the positively charged object, creating an induced charge.
If a negatively charged object is brough near to an uncharged object, the electrons from the uncharged object will be repelled from the surface nearest to the negatively charged object, creating an induced charge.
REMEMBER: PROTONS DON'T MOVE.
Induced charges are used in the gold leaf electroscope.
Static Electricity: Gold Leaf Electroscope
When a charged object is brought near to the metal disk at the top of the gold leaf electroscope, an induced charge is created at the bottom of the metal rod, causing the gold leaf and metal rod to repel eachother so the gold leaf would rise.
For example, if a positively charged object is brought to the metal disk, the electrons from the metal rod and gold leaf would move towards the object, leaving a positively induced charge at the bottom of the stem. Because both the gold leaf and the metal rod are now the same charge, they would repel eachother, causing the gold leaf to rise, showing that the object was definately charged.
Static Electricity: Uses
Electrostatics are used in spray painting cars. The nozzle of the paint spray gun is given a charge, which in turn charges the paint droplets as they leave the paint can. The object being sprayed is given the opposite charge, so the paint droplets are attracted to the object. Because all of the droplets are the same charge, they repel eachother, and the paint distributes itself evenly across the object.
Old photocopiers used electrostatics to create copies of images and text. When an original is copied, an invisible electrostatic image is made on a large drum. The light parts of the image have no charge because light can get rid of charges, and the dark parts have a positive charge. Black toner powder is blown over the drum, which is attracted to the positively charged dark parts of the drum. The toner is transferred to a new piece of paper, which is then heated to melt the toner and create the final image.
Lots of smoke is often produced by factories, and electrostatics can be used to stop this smoke from polluting the atmosphere. A smoke precipitator consists of 2 metal plates and a metal grid, placed in a chimney. The grid is in the middle and plates are at the edges. The grid and plates are given oppositve charges. When the smoke passes up the chimney, they rub against the grid and gain the grid's charge. They repel the grid because they have like charges to it, and are attracted to the oppositely charged plates, which are then knocked so the smoke falls and can be collected.
Static Electricity: Reducing Danger
A charged conductor can be discharged by connecting it to Earth with another conductor (e.g. wire). The charge can then flow safely to or from Earth without causing a spark. Alternately, the connecting lead can be connected to the source of the charge so it carries the charge back to wehre it came from.
When oil from pumps or tanks is transferred to a vehicle, friction between the oil and the tubing can cause a static charge to occur. Sparks and oil together are very dangerous, so the pump must be earthed so the charge doesn't build up and cause a spark.
Lightning is caused by sparks within a storm cloud reaching the Earth to discharge. Lightning causes induced charges at the highest object, so the spark can reach it easily. Lightnight conductors are metal strips attached to the side of high buildings so the charge can run safely down to Earth without damaging the building.
Static Electricity: Currents
When a static electric charge flows, it makes an electrical current, because currents are created by the flow of electrons.
Van de Graaff generators: The dome charges up when the generator is switched on because the belt rubs against a felt pad and becomes charged. The belt then carried the charge onto an insulated metal dome. Sparks are produced when the dome can no longer hold any more charge.
SEE REVERSE FOR DIAGRAMS
If you connected the dome to a metal plate, which was next to another metal plate with a small ball coated in conducting material between them, with the second metal plate connected to the place at which the charge was taken, an electric current could be observed. The ball is attracted to the first plate where it collects an electric charge and is then attracted to the other plate. It touches the other plate which collects the charge and the ball returns to the charged plate to collect more charge. The ball rattles backwards and forwards between the plates. When this happens, an ammeter could be connected to show that an electric current can be prouduced.
Van de Graaff Generator diagrams
Energy comes in many different forms:
- Chemical energy is energy stored in fuel (including food), which is released when chemical reactions take place
- Kinetic energy is the energy of a moving object
- Gravitation Potention Energy (GPE) is the energy of an object due to its position GPE (j) = Mass (kj) X Gravity (m/s2)X Height (m)
- Elastic (strain) energy is the energy stored in a springy object when we stretch or squash it
- Electrical energy is energy transferred by an electric current
- Thermal (heat) energy of an object is energy due to its temeperature. This is partly because of the random kinetic energy of the particles of the object
- Light energy is energy transferred by visible light rays
- Heat energy is energy transferred by infra-red rays
- Sound energy is energy transferred by the vibrations in the air
Laws of thermodynamics:
- Energy can neither be created nor destroyed, merely changed (transferred/transformed) from one form to another.
- Energy flows from hot to cold. The bigger the temerapture difference, the fast the energy flows.
Heat Transfer: Efficiency
When we use energy, we usually transform it from a less useful form to a more useful form. For example, in a car, chemical energy from the fuel is turned into kinetic energy to move the car, light energy to power the headlight, sound energy for the radio etc. Energy is measured in Joules.
But Energy is often 'wasted' as heat which dissipates into the atmosphere. The better a device is at converting energy into a useful form, the more efficient it is.
Efficiency = useful energy transferred / total energy input X 100. (Percentage)
Energy transfer diagrams show the amount of energy input and what it's converted to, to scale. They look like this:
Heat Transfer: Design
Sometimes we need to control heat transfer.
In a car, the engine has to be kept cool to stop it going up in flames, so the radiator is given a large surface area to transmit heat and a fan is there for when it overheats so more air would pass over the radiator.
Vacuum flasks are specially designed to stop hot drinks getting cold and cold drinks getting hot. The inside surfaces are silver to stop radiation, a vacuum between a double-walled plastic container to stop conduction and convection and something holding up the inner flask to stop heat from being lost/gained from the bottom.
In the home, walls and rooves are insulated to reduce heat loss from the house. Double glazed windows have a layer of vacuum between the 2 sheets of glass to prevent conduction and convection, and aluminiun foil between a radiation panel and the wall reflects heat radiation away from the wall.
Heat Transfer: Conduction, Convection & Radiation
Conduction is the transfer of heat through touching objects. Metals are very good conductors. This is because energy is transfered by moving particles which pass energy along to those next to them, and metals have 'free electrons' (see chemistry, metallic bonding). All objects are the same temperature in a room. They feel different because some materials are better conductors than others. Conduction happens best in solids, less well in liquids and worse in gases. Conduction doesn't happen at all in a vaccum because it needs particles.
Convection currents are caused by the movement of particles in a fluid (liquid or gas). As a fluid is heated, the particles gain energy, move around more and spread out. This causes the density of that part of the fluid to decrease. Less dense things rise to the top of more dense things, so hot air or hot water rises.
Thermal Radiation is the transfer of heat without particles (e.g. the way the heat from the sun gets to Earth). It's a form of electromagnetic wave, also called 'infra-red' Every object emits thermal radiation. Different characteristics of materials are better at emmiting thermal radiation than others. The best surfaces for emitting and absorbing radiation are dark, matt surfaces.
Electrical Energy- Electrical Power
Power is the energy supplied to a machine per second, or the rate of the transfer of energy. The more powerful a machine is, the faster it is able to transform energy. We measure power in watts (W) or kilowatts (kW). For any device, the input power is the energy per second supplied to it, and the output energy is the useful energy per second transferred by it.
Power (in W) = energy transferred (in J) / time taken to transfer the energy (in s)
For example, if a motor transfers 10,000J of energy in 25s, its power is 400 W because 10,000 / 25 = 400 W
The energy supplied to a device in 1 hour is measured in kilowatt hours (kW h). We use the kilowatt-hour as the unit of energy supplied by mains electricity. You can work out the energy, in kilowatt-hours used by a mains device in a certain time using this equations:
Energy tranferred (kW h) = power of device (kW) X time in use (hours)
For example, a 0.5kW heater switched on for 6 hours uses 3 kW h of electrical energy.
Electrical Energy- Electrical Power- continued
The electricity meter in a home measures the amount of electrical energy a family uses. It recorrds the total energy supplied, no mater how many devices the family uses. It gives a reading of the number of kilowatt-hours (kW h) of energy supplied by the mains. We use the kilowatt-hour to work out the cost of electricity. For example, a cost of 7p per kilowatt-hour (or 7p per unit) means that each kilowatt-hour of electrical energy cost 7p. So,
total cost = number of kW h used x cost per kW h.
The national grid is a network of cables which connects homes and power stations so homes can recieve electricity. Transformers are used at each end to minimise the amount of energy lost through the journey along the wires. Step-up transformers are used at the power stations to increase the voltage, step-down transformers are used at the home to decrease the voltage. The National Grid's volatage is 132,000 volts at least, because transmitting electricity at a high bvoltage reduces power loss, making the system more effictient. (V = I x R) Power lines are above ground because it's less expensive and easier to repair than if they were underground.
Generating Electricity- Fuel
Most electricity we use is generated in power stations.
- In a coal or oil-fired power station, the burning fuel heats water in a boiler to produce steam. The steam drives a turbine that turns an electricity generator.
- In a gas-fired power station, we burn natural gas directly in a gas turbine enegine. This produces a powerful jet of hot gases and air that drives the turnbine. A gas-fired turbine can be switched on very quickly.
- In a nuclear power station, unstable atoms (uranium) split in a reactor, which realeases thermal energy, which is used to heat a 'coolent' (a fluid) which flows to a 'geat exchanger', then back to the reactor core. The heat is used to heat water to produce steam, which drives turbines which turn electricity generators.
Nuclear power stations produce a lot more energy and don't release greenhouse gases, but they do produce dangerous radioactive waste, which is almost impossible to dispose of.
Generating Electricity- Renewable energy
A wind turbine is an electricity generator at the top of a narrow tower. The force of the wind drives the turbine's blades around. This turns a generator.
A wave generator uses the waves to make a floating section move up and down. This motion drives a turbine which turns a generator. A cable between the generator and the shore delivers electricity to the grid system. Wave generators need to withstand sotrms and don't supply electricity all the time. Lots of cables and buildings would be needed alond the coast to connect the wave generators to the electricity grid. The would spoid areas of coastline. Tid flow patterns might be changed, affecting the habitats of marine life and birds.
Hydroelectricity is generated when rainwater collected in a reservoir flows downhill. The flowing water drives turbines that turn electricity generators at the foot of the hill.
Tidal power stations trap water from each high tide behind a barrage. We can then release the high tide into the sea through turbines. The turbines drive generators in the barrage. One of the most promising sites in Britain is the Severn estuary, because the estuary becomes narrower as you move up-river. away from the open sea, so it 'funnels' the incoming tide and makes it high than elsewhere.
Generating Electricity- Renewable energy
Solar power is transfered from the sun, and we can use this energy to generate electricity using solar cells. Solar cells now convert less than 10% of the solar energy they absorb into electrical energy. We connect them together to make solar cell panels, which are useful when we only need small amounts of electricity (e.g. in watches or calculators), or in remote places (e.g. on small islands in the middle of the ocean), but are very expensive to buy, even though they cost nothing to run. We need lots of them to generate enough power to be useful, and they need a lot of sun to work efficiently.
A solar heating panel heats water that flows through it. Even on a cloudy day, they can heat lots of water in Britain.
Geothermal energy fromes from energy released by radioactive substances, deep within the Earth. The energy released by these readioactive substances heat the surrounding rock, and as a result, heat is transferred towards the Earth's surface. We can build geothermal power stations in colcanit areas or where there are hot rocks deep below the surface. Water gets pumped down to these rocks to produce steam. Then, the steam produced drives electricity turbines at ground level.
Generating Electricity- Energy & The Environment
Problems with Fossil Fuels:
- They cause greenhouse gases
- Impurities may cause harmful gases, such as sulphur dioxide
- It'll never run out
- They don't produce greenhouse gases or acid rain
- They don't create radioactive waste products
- Wind turbines are unsightly and create a whining noise that can upset people nearby
- Tidal barrages affect river estuaries and the habitats of creatures and plants there
- Hydroelectric schemes need large reservoirs of water, which can affect nearby plant and animal life. Habitats are often flooded to create dams
- Solar cells would need to cover large areads to generate large amounds of power
The Electromagnetic Spectrum
Electromagnetic waves are electric and magnetic disturbances that transfer energy from one place to another.
The Electromagnetic Spectrum divides different types of waves by wavelength, (the distance between one wave peak to the next), so each type of wave has different effects to the next. These wavelengths range from 1000m (long-wave radio waves) to less than a millionth of a millionth of a metre (gamma rays).
The frequency of electromagnetic waves of a certain wavelength is the number of complete waves passing a point each second, (measured in herts ( Hz , 1 Hz = 1 complete wave per second)),
In a vacuum (e.g. space) all electromagnetic waves travel at the same speed (the speed of light.) This is 300 million m/s.
We can link the speed of the waves to their frequency and wavelength using this equation:
wave speed (in m/s) = frequency (in Hz) X wavelength (in m)
Electromagnetic Waves- Gamma
Gamma rays have the shortest wavelength (typical Wavelength: 10-12m (a million-millionth of a metre)) and the highest frequency. This makes them the most penetrating of the electromagnetic waves, so they're very difficult to block. They can be absorbed/blocked by several centimetres of lead.
Gamma rays come from radioactive substances, such as uranium. The radioactivity means that they can be detected by a Geiger-Müller tube.
They're very dangerous because they can kill and harm cells in the human body, and even cause mutations in cells so that they cause cancer. This is because they are ionizing, which means that they can alter the atomic structure of cells in the body, causing mutations in the DNA.
They're used to
- Kill bacteria in food
- Sterilise surgical instruments
- Kill cancer cells.
People who work with gamma and x-rays have to wear a special badge which shows whether the person is absorbing too much radiation and are at risk of harming themselves.
Gamma rays can be aborbed by lead or concrete, transmitted by most materials, but they're very difficult to reflect. A gamma ray mirror has been invented, but they're not 100% effective.
Electromagnetic Waves- X-rays
X-rays have the 2nd shortest wavelength (Typical Wavelength: 10-10m (a ten-thousand-millionth of a metre)) and the 2nd highest frequency in the electromagnetic spectrum.
They are typically used for taking special photos (radiograph) of bones in hospitals, and are recently used in airports to detect weapons or other possible dangers at customs.
Like gamma rays, they can be very dangerous because they are a form of ionizing radiation, and over-exposure to x-rays can cause damage to living tissue.
They can be detected by photographic film and absorbed by a lead shield or concrete. They are transmitted by most substances, including human skin and tissue. They can be reflected by gamma ray mirrors when aimed at a 'grazing' angle.
Electromagnetic Waves- Ultra Violet Rays
UV waves have the 3rd shortest wavelength (Typical Wavelength: 10-8m (a hundred-millionth of a metre)) and 3rd highest frequency in the electromagnetic spectrum. They come from very hot objects, such as the sun, sparks, or mercury lamps.
They are typically used for detecting security markers on items to check that they're not forged, and are used in tanning beds to give users a skin tan. They can also kill microbes.
They can be detected by photographic film and skin and they make flourescent substances glow. They are reflected by snow and ice, transmitted by ultraviolet ray glass and absorbed by normal glass, avobenzone (used in sun cream) and ozone.
They are dangerous, but an optimum intake of ultra violet light is a good source of Vitamin D. They aren't as penetrating as gamma or x-rays, but they can harm the cells near the surface of the body, causing skin cancer and skin burns. They can also damage the retina of eyes. Dark skin contains more melanin than light skin, which absorbs UV light and protects against the harmful rays.
Electromagnetic Waves- Visible Light Rays
Visible light rays have the middle wavelength (Typical Wavelength: 5 x 10-7m (a two-millionth of a metre)) and the middle frequency in the electromagnetic spectrum.
They're typically used for seeing things.
They come from hot objects, like the sun, flourescent substances, lasers and LEDs.
They can be detected by the eyes, photographics film and an LDR (light-dependent resistor). They are reflected by shiny objects, like mirrors, and transmitted by translucent and transparent materials, like glass. They are absorbed by filters (visible spectrum; different filters absorb different colours).
They're not dangerous unless it's very bright, which is why you shouldn't stare at the sun.
Electromagnetic Waves- Infra-Red Rays
Infra red rays have the 3rd longest wavelength (Typical Wavelength: 10-5m (a hundred-thousandth of a metre)) and 3rd lowest frequency in the electromagnetic spectrum. The hotter an object, the more infra-red radiation it emits.
They're used in optical fibre and communication and are used by emergency services to detect life in earthquakes and fires, because special cameras can detect the heat a body gives off. They are also used in medicine to detect circulation problems, arthritis and cancer, and in remote controls for television, as well as in heating and cooking.
They come from anything with a temperature about 0 degrees, such as the sun, Earth, fires, grills etc., and can be detected by the skin (feels warm), a blackened thermometer and a thermistor. They're reflected by chlorophyll in plants and absorbed by the human body and the Earth. They're not transmitted by many things.
They're not dangerous unless you touch something hot directly, whereby you can get burnt.
Electromagnetic Waves- Microwaves
Microwaves have the 2nd longest wavelength (Typical Wavelength: 10-3m-103m (a 1mm to 1km)) and 2nd lowest frequency in the electromagnetic spectrum.
They're used in microwave ovens, mobile phones, communicating with satellites and detecting the echoes from objects for radar.
They come from magnetrons, and can be detected by satelite dishes, mobile phone and other things.
They're reflected by metal, transmitted by glass and absorbed strongly by water molecules. When the water molecules absorb the waves, they are given more energy, so they vibrate food, which heats it.
Dangerous because living cells contain water. If that water is heated by microwaves, the cell can be damaged.
Electromagnetic Waves- Radio Waves
Radiowaves have the longest wavelength (Typical Wavelength: 1000m+) and lowest frequency in the electromagnetic spectrum.
They have no real danger. They're used in television and radio signals. They come from aerials when the voltage is applied to them, and are detected by aerials, such as that in a TV set or radio set.
They can be absorbed by water and transmitted by antennas. High energy radio waves can be transmitted through the atmosphere. They are reflected by metal places and low-energy radio waves are reflected by the ionosphere, which makes them useful for transmitting radio waves over long distances. The ionosphere is a layer around the Earth. It's stronger in the summer than in the winter.
They're divided into different wavebands, ranging from microwaves, which have a frequency range of other 3000MHz, to low frequency, or LF which have a frequency range of less than 300kHz. In between are ultra-high frequency (UHF), very high frequency (VHF), high frquency (HF) and medium frquency (MF). They're properties make them useful for different things. Their uses depend on how much energy they can carry, how long their range is, and how much they spread out.
Analogue and digital signals
TV stations transmit analogue and digital signals, but soon they will only transmit digital signals.
Digital signals are a sequence of pulses. The volatage level of each pulse is either a 1 or a 0, which are called 'bits.' An analogue signal is a wave that varies continuously in amplitude or frequency between zero and a maximum value. For example, a microphone generates electrical waves when it detects sound waves.
The waves we use to carry a signal are called carrier waves. They could be radio waves, microwaves, infra-red radiation or light. To send a digital signal, pulses are used to switch the carrier waves on and off repeatedly. Digital radio transmitters, scanners and fax machines convert analogue signals into digital signals which are then sent. Carrier waves and the signal are fed into a tranmitter which produces either FM, AM or Digital waves.
To send an analogue signal, the signal waves are used to vary or modulate the carrier waves. The amplitude modulation (AM) or the frequency modulation (FM) of the carrier waves is modulated in this process.
Digital signals suffer less interference than analogue signals. Interference causes a hissing noise when you listen to analogue radio. It doesn't happen with digital signals because regenerator circuits are used to clean 'noisy' pulses, so a digital signal has a higher quality than an analogue signal.
Much more information can be sent using digital signals instead of analogue signals. Digital pulses can be made very short so more pulses can be carried each second.
Analogue and digital signals
The motor effect is the force exerted on a wire when the wire is cutting through a magnetic field. This doesn't work if the wire is parallel to the magnetic field lines or if both the magnets and the wire are stationary.
- The greater the current through the wire, the greater the force
- The stronger the magnet, the greater the force
- The more perpendicular to the magnetic field lines the wire is, the greater the force (zero when parallel)
An electric motor uses the motor effect to produce electricity. We can control the speed of an electric motor by changing the current, and we can control the turning direction by reversing the current. A simple motor consists of a rectangular coil of insulated wire (the armature coil) that is forced to rotate, because when a current is passed through the coil, a force acts on each side of the coil due to the moto effect, and the force on one side is in the opposite direction to the force on the other side. The coil is connected via 2 metal or graphite 'brushes' to the battery. The brushes press onto a metal 'split-ring' commutator fixed to the coil. The split-ring commutator reverses the current round the coil every half turn of the coil. Because the sides swap over each half-turn, the coil is pushed in the same direction every half-turn.
Electromagnetism- Left Hand Rule
The direction of the force is always at right angles to the wire and the field lines. Also, the direction of the force is reversed if the direction of the current or the magnetic field is reversed. The direction of the current, field lines and force can be worked out using the Left Hand Rule, F= Force / Movement, B = Magnetic field lines, I = Current.
A generator contains coils of wire that spin in a magnetic field. A voltage is created or induced in the wire when it cuts across the magnetic field lines. If the wire is part of a complete circuit, the induced voltage makes an electric current pass round the circuit.
A bike has its own mini-generator called a cycle dynamo. A magnet spins in accordance with the wheel, and a voltage is induced in a coil inside as the magnetic field lines cut across the wires of the coil, which powers the bicycle lamp.
A simple alternating current generator consists of a rectangular coil which is forced to spin in a magnetic field. The coil is connected to a centre reading meter via metal 'brushes' that press on two metal slip-rings. When the coil turns steadily in one direction, the meter pointer deflects first one way, then the opposite way, then back again. This carries on as long as the coil is rotating. The currect through the meter is an alternating current. The faster the coil rotates, the larger the peak value of the alternating current is, and the greater the frequency iss the alternating current is.
A loudspeaker is designed to make a diaphragm attached to a coil bibrate when alternating current passes through the coil. When a current passes through the coil, a force, due to the motor effect, makes the coil move. Each time the current changes its direction, the force reverses its direction, so the coil is repeatedly forced backwards and forwards, so it vibrates.
At a power station, the voltage is set very high so that resistance is kept to a minimum and energy isn't lost along the power lines of the National Grid. When it gets to a home, the voltage must be 'stepped down', to make it a suitable volatge to use for appliances. To do this, we use transformers.
A transformer has 2 coils of insulated wire, both wound around the same iron core, which is shaped like an 'O'. When alternating current passes through the primary coil, an alternating voltage is induced in the secondary coil, because alternating current passing through the primary coil produces an alternating magnetic field, and the lines of the alternating magnetic field pass through the seconday coil , inducin gan alternating voltage in it.
If a lamp is connected across the secondary coil, the induced voltage cases a current in the secondary circuit, so the lamp lights up. Electrical energy is therefore transferred from the primary to the seconday coil. This happens eeven though they are not electrically connected in the same circuit.
Transformers only work with an alternating current because with a direct current, there is no changing magnetic field so the secondary voltage is zero.
100V / 400V = 5 turns / 20 turns
The secondary voltage of a transrformer depends on the primary voltage and the number of turns on each coil. We can use this equation to calculate any one of these factors if we know the others ones:
Voltage across primary (Vp) / voltage across secondary (Vs) = number of turns on primary (Np) / number of turns on seconday (Ns).
For a step-up transformer, the number of secondary turns, Ns, is greater than the number of primary turns, Np). Therefore Vs is greater than Vp. This is the opposite for a step-down transformer.
Current Electricity- Cicuit Symbols
- When components are connected in series, their current is the same but the voltage of the battery is shared between them. The total resistance of components in series is equal to the sum of their seperate resistances
- When components are connected in parallel, their current is shared, but their voltage is the same. The current through each componenet depends on the resistance of that component. The bigger the resistance, the smaller the current. I = V / R
When electrons flow through wires and components, they have to push through lots of vibrating atoms. We call this resistance, which is measured in ohms. Ohms law says that resistance is constant. It means that current and voltage are directly proportional to eachother. It's defined by this equation:
R (resistance) = V (voltage) / I (current)
In a wire, the larger the cross-section of the wire, the lower the resistance, and the longer the wire, the higher the resistance.
Current-Voltage Graphs- Filament Lamp
In a filament lamp resistance graph, the line curves away from the current azis., so the current is not directly proportional to the voltage, making the filament lamp a non-ohming conductor.
The resistance increases as the current increases, so the resistance increases as the temperature of the filament lamp increases. This is because the atoms are vibrating more, making it harder for the electrons to flow.
Reversing the potential difference makes no difference to the shape of the curve. This resistance is the same for the same current, regardless of its direction.
Current-Voltage Graphs- Diode
With a diode in the 'forward' direction, the line curves towards the current axis, so the current is not directly propertional to the voltage, making it a non-ohming conductor.
In the reverse direction, the current is negligible, so its resistance in the reverse direction is much higher than in the forward direction.
Current-Voltage Graphs- Thermistor & LDR
With a thermistor, resistance decreases with temperature. This means that on a current-voltage graph, a line would be steeper at a high temperature than at a lower temperature. At a constant temperature, the line is straight, so its resistance is constant.
Similarly, with an LDR, resistance decreases with light intesity. On a current-voltage graph, a line would be steeper with bright light than at dim light. At a constant light intensity, the line is straight, so its resistance is constant.
- 'Direct Current' or D.C. means that the electrons in a circuit are only flowing one way.
- 'Alternating Current' or A.C. means that the electrons in a circuit are repeatedly changing direction. Mains electricity is an a.c. supply. The frequency of alternating current is the number of cycles it passese through each second, measured in Hz. In the UK, the mains frequency is Hz.
Mains cicuits have 3 wires: Live, Neutral and Earth. The current through a mains appliance alternates because the mains supply provides an alternating voltage between the Live and Neutral wires.
The neutral wire is Earthed at a local sub-station. The live wire is dangerous because its voltage repeatedly changed from + to - every cycle. It reaches over 300V in each direction.
Oscilloscopes measure the voltage and frequency of a low voltage a.c. supply. Oscilloscopes have centimeter grids which can be adjusted to a suitable size. Frequency is measured along the bottom and voltage is the height of the waves.
Mains Electricity- Plugs
Inside a cable for a plug, there are 3 wires; the Earth (green & yellow), Live (brown) and Neutral (blue) wires, fixed to 3 brass pins inside the plug. The Earth wire is a safety wire. It means that if the live wire were to break and touch the outer casing of a metal appliance, you wouldn't get an electric shock if you touched it. The Earth pin is the longest so that the appliance is already earthed by the time it is switched on. Plugs, Sockets and cables are all made of hard-wearing electrical insulators so you can't touch the dangerous wires inside. The metal pins of plugs are made of brass because it's a good electrical conductor and it does not oxidise or rust.
Mains Electricity- Fuses
The fuse in a plug is a ***** of wire which melts (blows) when too high a current is passed through it. This stops the appliance from working with a dangerous voltage and catching fire.
Different fuses have different ratings. The rating of a fuse is the maximum current that can pass through it without melting the fuse wire. An plug for an electrical appliance must have the right fuse in, otherwise it will either wont blow when it should, leaving the appliance at risk of catching fire, or will blow as soon as the appliance is switched on.
A circuit breaker is an electromganetic switch that opens (trips) and cuts the current off if the current is greater than a certain value. It can then be reset once the fault that made it trip has been put right. Circuit breakers are sometimes fitted in 'fuse boxes' in place of fuses. They work faster than fuses and can be reset quicker.
When an electrical appliance is used, it transforms electrical energy into other forms of energy. The power of the appliance is the energy it transforms per second.
Power (in watts) = energy transformed (in joules) / time (in seconds)
For an electrical appliance, the current through it is a measure of the number of electrons passing through it each second, the voltage across it is a measure of how much energy each electron passing through it transfers to it, and the power supplied to it is the energy transferred to it each second. This is the electrical energy it tranforms every second.
Power supplied (in watts) = current (in amps) X voltage (in volts)
'Charge', in coulombs (C), is the amount of charge flowing through a wire or composnent in 1 second when the current is 1A. This depends on the current and the time.
Charge flow (in coulombs) = current (in amps) X time (in seconds)
When charge flows through a resistor, electrical energy is transformed nito heat energy. The energy tranformed in a certain time in a resistor depends on the amound of charge that passes through it and the voltage across the resistor.
Energy transformed (in joules) = voltage (in volts) X charge flow (in coulombs)
Charge (Q) = Current (I) X Time (s)
Voltage (V) = Current (I) X Resistance (R)
Energy (E) = Charge (Q) X Voltage (V)
Energy (E) = Power (P) X Time (s)
Power (P) = Current (I) X Time (s)
Frequency = Current / time