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Electric circuits
Physics (GCSE) Revision (Electricity)
| Statement | Response | Comment |
|---|---|---|
| A current will flow through an electrical component only if there is . . . | a VOLTAGE or POTENTIAL DIFFERENCE (p.d.) across its ends. | Potential difference makes charges flow |
| The bigger the potential difference across a component . . . | The bigger the CURRENT that flows through it. | Think of water flowing downhill - the steeper the hill the faster it flows (usually !) |
| Components resist a current flowing through them. The bigger their resistance . . . | The smaller the current produced by a particular voltage | This is like squeezing a garden hose - it restricts the flow of water |
| The p.d. across a component in a circuit is measured in . . . | VOLTS | 1 volt is equal to 1 joule of electrical energy for every 1 coulmob of charge |
| Voltage (p.d.) is measured using . . . | a VOLTMETER connected IN PARALLEL with the component. | Remember: you measure the potential difference between two points in a circuit |
| The CURRENT flowing through a component in a circuit is measured using . . . | an AMMETER connected IN SERIES with the component. | You need to put the ammeter in line with the current so it flows through the meter |
| The unit of current is . . . | the AMPERE (AMP) | It is a rate of flow of charge |
| A current of 1 A is equal to a charge flowing of . . . | 1 COULOMB per SECOND | 1 coulomb is a really BIG number of electrons |
| In metals, a current is a flow of charged particles called . . . | ELECTRONS | They are the negatively charged parts of atoms |
| In a circuit, we say that CURRENT always flows from . . . | The POSITIVE terminal of a cell, towards the NEGATIVE terminal. | Red is positive - black is negative |
| In metals, electricity is actually carried by NEGATIVELY charged electrons, which flow from . . . | the NEGATIVE terminal towards the POSITIVE terminal of a cell. | This was a mistake made over 100 years ago when they didn't know about electrons ! |
| The behaviour of a component in a circuit can best be studied by plotting . . . | a current-voltage graph. | Put voltage on the x-axis and current on the y-axis |
| A resistor at constant temperature has a constant ratio of . . . | current to voltage. Its graph is a straight line. | It should pass through the origin if voltage is directly proportional to current |
| The current-voltage graph for a filament lamp is NOT a straight line. The reason is . . . | Resistance increases as temperature increases. | The graph is a curve - current stops increasing at quickly because it gets harder for it to flow through the higher resistance |
| A diode is a device which . . . | only allows a current to flow one way through it. | The graph shows a sudden rapid rise in current when the p.d. exceeds 0.7 volt ( but in reverse the current is just zero ) |
| When components are connected in series . . . | the SAME CURRENT flows through each component. | There is nowhere else for it to go |
| The total resistance of components in series is . . . | equal to the SUM of their separate resistances. | If you keep adding components in series the current will just keep getting less and less |
| The total potential difference in a series circuit is . . . | shared between the individual compoents. | You only have so much pocket money to spend . . . |
| If a p.d. of 12 volts is shared between two equal resistances, each one will get . . . | 6 volts. | If the same current flows in each, the voltage is proportional to the resistance |
| If a p.d. of 12 volts is shared between two resistors of 6 ohms and 12 ohms, the bigger resistor will get . . . | 8 volts. ( The smaller one gets 4 volts because it has half the resistance of the other one.) | Voltage is directly proportional to resistance according to Ohm's Law |
| When components are connected IN PARALLEL . . . | There is the SAME p.d. across each component. | Think of the rungs on a ladder - they are all connected in parallel between the two uprights |
| The total current in a parallel circuit is equal to . . . | the SUM of the currents through each of the separate components. | Like cars joining (or leaving) a motorway |
| If one lamp is connected to a cell it gets a current of 1 amp. If two identical lamps are connected in parallel (to the same cell) they will get . . . | a current of 1 amp each - that makes the total current 2 amps. | You can put as many lamps in parallel as you like and they will all get 1 amp each |
| If one lamp gets a current of 1A from one cell, two lamps joined in series will get . . . | approximately half as much current ( 0.5A) because they have twice as much resistance. | Three lamps get 1/3 amp, etc. |
| The p.d. provided by cells connected in series is equal to . . . | the SUM of the p.ds. of each cell separately bearing in mind the direction in which they are connected. | If you connect a cell backwards you have to subtract its p.d. from the total |
| How many different p.ds. can you get from 4 cells, each of 1.5volts, connected in series? | THREE ( 6v, 3v, 0v) | What will 5 cells give ? |
| The resistance of a component is measured in | OHMS | 1 ohm is 1 volt per amp |
| Potential difference, current and resistance are related by Ohm's Law which states | P.d. = current x resistance | V = IR (if you want to save ink) |
| The resistance of a component is calculated by applying Ohm's law as follows: | R = p.d. / current | R = V / I (always put V on top) |
| If a current of 2A flows in a resistor of resistance 10 ohms, the p.d. across the resistor is . . . | 20 volts | V= IR = 2x10 = 20 v |
| What is the current when a p.d. of 12v is applied across a 10 ohm resistor? | 1.2 A | I = V/R = 12/10 |
| The current through a resistor at constant temperature is . . . | directly proportional to the p.d. across the resistor. | A graph of current against voltage would be a straight line which passes through the origin |
| The resistance of a light dependent resistor . . . | decreases as the light intensity increases. | They are used to switch on street lights at dusk |
| The resistance of a thermistor with a negative temperature coefficient will . . . | decrease as the temperature increases. | Can be used as a thermostat to control central heating |
| As an electric current flows through a circuit . . . | energy is transferred from the battery or power supply to the components in the circuit. | A battery is a source of electrical energy which can easily be converted into more useful forms such as light, heat and sound |
| A lamp converts electrical energy into . . . | light and heat. | More light than heat - especially if energy efficient lamps are used |
| When electric charge flows through a resistor, electrical energy is transferred as . . . | HEAT | Joule showed that heat is always produced when energy is transferred |
| The rate of energy transfer is called . . . | POWER | Power has a special meaning in Physics (see above) |
| Power is measured in watts. 1W is equal to . . . | 1 joule per second | 1 watt means that 1 joule of energy is transferred EVERY second ! |
| In an electric circuit, POWER = | current x potential difference | P = I V ( Pretty Impressive Victory ) |
| Energy transferred = | Power x Time | E = P x t ( Eeeeh, Poor Thing ! ) |
| CHARGE = | Current x Time | Q = It ( Quit - while you're ahead ) |
| The higher the p.d. the greater the energy transferred for a given amount of charge which flows. Energy transferred = | charge x potential difference | E = Q V ( Eastenders Queen Victoria ) |
| The letters D.C. stand for . . . | Direct current | What else ? |
| The letters A.C. stand for . . . | Alternating current | It goes back and forth 50 times a second |
| A direct current can be obtained from . . . | a cell or battery ( or dc power supply) | An ac supply can be made into dc by using diodes |
| A direct current is one in which . . . | charge carriers flow in one direction continuously. | For example, in a solution of copper sulphate |
| An alternating current is one which flows . . . | first in one direction then in the opposite direction alternately. It is constantly changing. | It is produced using a dynamo or a generator which has a coil spinning in a magnetic field |
| The frequency of alternating current in the UK is . . . | 50 Hertz ( or cycles per second) | It is 60Hz in the USA - which makes it difficult to watch imported DVDs or videos |
| The voltage of the UK mains supply is . . . | About 230 volts (AC) | It used to be 240v but what with inflation . . . |
| In Europe, the colour of the LIVE cable in a 13 amp plug is . . . | BROWN | You'll know what this means if you touch it ! |
| In Europe, the earth cable is coloured . . . | GREEN & YELLOW | The colours of springtime . . . |
| In Europe,the neutral wire in a 13A plug is coloured . . . | BLUE | The colour of the summer sky - I'm just wishing I was on holiday ! |
| If a fault occurs in an electrical circuit the current is interrupted by a . . . | FUSE (or circuit breaker) | Circuit breakers can be reset - fuses need to be replaced |
| The fuse in a plug is designed to MELT when . . . | the current exceeds the value of the fuse e.g. 5amp. | Don't say fuse 'blows' - it's rude ! |
| Fuses and circuit breakers should always be fitted in the . . . | LIVE wire | So that the current is stopped before it enters the appliance |
| An appliance with a metal casing should always be EARTHED because . . . | the current can flow to earth preventing the user from receiving a shock. | If you touch a LIVE metal case, the current will flow through YOU to earth - not nice ! |
| It is safe to use an appliance fitted with a 2-pin plug because . . . | there are no metal parts which can become LIVE | e.g. a hairdryer with a plastic case and handle |
| A loudspeaker converts electrical energy into . . . | sound energy. | Don't abbreviate loudspeaker to 'speaker' |
| A microphone converts sound energy into . . . | electrical energy. | Testing, testing . . . 1, 2, 1, 2, etc. |
| A motor converts electrical energy into . . . | kinetic energy. | e.g. an electric drill |
| A dynamo converts kinetic energy into . . . | electrical energy. | On a bicycle |
| The amount of electrical energy transferred from the mains is measured in units called . . . | kilowatt hours | That's kilowatts x hours |
| 1 kWh = | 1kW x 1hour | 1000 W x 3600 seconds = 3 600 000 joules |
| Cost of electricity used = | Number of units (kWh) x cost per unit | Roughly 8p per unit (kWh) |
| If 1 unit of electricity costs 8pence, what would it cost to run a 500W lamp for 6 hours? | Cost = 0.5 x 6 x 8 = 24p | Convert to kilowatts and hours first |
| Metals are good conductors of electricity because . . . | some of their electrons can move freely throughout the metal. | They are 'delocalised' or mobile and not firmly attached to any one atom |
| When a current flows through a solution the process is called . . . | electrolysis | It uses a device called a voltammeter - bad name really as it isn't really either |
| In an electrolyte, current is carried by charged particles called . . . | IONS | An atom which has either gained or lost electrons is called an ion |
| Positve ions move towards the . . . | CATHODE | cat means 'down' in Greek (apparently) |
| Negatively charged ions move towards the . . . | ANODE | an means 'up' in Greek |
| During electrolysis the mass or volume of the substance deposited or released at the cathode depends on . . . | the current and the time for which it flows. | This is Faraday's Law |
| A capacitor is a device which is used to store . . . | electric charge (energy) | Rechargeable batteries derived from electrolytic capacitors |
| A capacitor takes time to charge up or to discharge. This can be used in . . . | a timing circuit | e.g. courtesy or security lights |
| When two different materials are rubbed together . . | electrons are transferred from one to the other | Friction removes electrons |
| Certain materials can be used as electrical insulators. This is because . . . | they do not conduct electricity | Their electrons are tightly bound to the atoms |
| When a substance loses electrons, it becomes . . . | POSITIVELY charged. | Double negative : minus minus equals plus |
| Opposite electric charges will . . . | ATTRACT each other | Life used to be so simple |
| Like charges . . . | REPEL each other | They just can't stand competition ! |
| A charged object can be discharged by . . . | connecting it to EARTH with a conductor | The Earth acts like a bank - you can pay in or take out whatever you want |
| In a photocopier, electric charge is used to create an image on . . . | a copying plate which conducts when light shines on it | You need to read up on this |
| Smoke particles can be extracted from the emissions from power stations by means of . . . | an electrostatic precipitator | This is a great idea - make sure you know how it works |
| When walking along a nylon carpet with plastic-soled shoes, you may pick up a static charge. This happens because . . . | electrons are transferred by friction from the shoes to the carpet | It could be the other way round - anyway you get a big charge |
| When you touch a metal door handle you sometimes receive an electric shock because . . . | a charge passes from your body to earth through the door handle | Let's suppose electrons flow from you to Earth - it could be the other way round but it still hurts ! |
| When filling a car with petrol, the nozzle should make good electrical contact with the neck of the tank. This is to prevent . . . | a spark which could ignite the fuel. | Aircraft need to be connected to the tanker by a metal bonding line for the same reason |
Created by:
J Thomson
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