Electricity in a nutshell

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  • Created on: 14-04-13 13:29
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IGCSE Electricity in a nutshell
1. Current is rate of flow of charge in Coulombs per second I=Q/t . In metals the charges flowing
are electrons but in electrolytes they are ions
2. Voltage between A & B is the energy changed as each Coulomb of charge flows between A &
B. V=E/Q
3. dc flows in one direction only and ac continually changes direction. Current against time curves
show if current is dc or ac. Note that dc may be varying or constant.
4. Batteries pass steady dc, generators supply ac (although they can be adapted to provide varying
dc
5. Diodes convert ac into varying dc
6. The current in a circuit can be increased by increasing the voltage of the supply or by reducing
the total resistance in a circuit
7. A variable resistor operates by changing the length of wire in a circuit a longer wire means
more resistance so less current.
8. Calculate resistance using R= V/I
9. Circuit to measure current and voltage. The shape of a current against voltage graphs can be
used to identify which component the graph is for: metals at constant temperature, (straight
line as current is proportional to voltageOhms law) bulbs (curved graph), diodes(zero current
when voltage is negative).
10. The resistance of an ldr decreases in the light, and of a thermistor decreases when it is hot.
11. Series resistors share the voltage of the supply but have the same current, higher resistors have
a higher voltage
12. Parallel resistors have the same voltage as each other as they share current. Higher resistors
have a lower current.
13. The colour code for domestic wiring is : brown live, blue neutral, yellow green earth. The fuse
is in the live wire. Choose the correct fuse by calculating the usual current from I=P/V
14. Live current = neutral current , even though the neutral is at 0V and the live is at 230V. These
wires are needed to pass current through the appliance but the earth wire is joined to the metal
case and has no current unless there is a fault. It prevents electric shock. An alternative to
earthing is double insulation
15. Fuses prevent overheating by melting when the current is more than the fuse rating whereas
circuit breakers disable the circuit when the live current is more than the neutral current ( this
only happens when there is a fault such as the live wire coming loose and touching the
appliance's metal case)
16. Hazards of electricity include frayed wires, long cables, damaged plugs, water around sockets,
pushing metal objects in to plugs
17. Energy saving light bulbs are more environmentally friendly as they produce less wasted heat.
18. Appliances that use the heating effect of a current are as irons, kettles, toasters.
19. Calculate electrical power from P=IV and electrical energy transferred as E = Ivt
IGCSE Static Electricity in a nutshell
1. When insulators are charged by friction, electrons are transferred onto them making them
negative or from them making them positive
2. Like charges repel, unlike charges attract, charged and neutral materials attract
3. Excess charge on a metal case is removed through an earth connection
4. Applications of static electricity include fuelling aircraft, photocopiers, ink jet printers,
cleaning smoke from chimneys, spray painting cars or crops

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