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Electric circuits

Physics (GCSE) Revision (Electricity)

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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