Electricity

Voltage

Energy per unit charge

Emf

the electrical work done by a source in moving a unit charge around a complete circuit

Pd

the work done by a unit charge passing through a component

Electric current

the charge passing a point per unit time

(rate of flow of charge)

Current formula

I = Q/t

Conventional current

From positive to negative

Flow of free electrons

From negative to positive

Ohm’s law

Voltage is proportional to current when resistance is constant (when temp is constant)

Power equations [4]

P=IV

P=I²R

P=V²/R

P=E/t

Energy equation

E = VIt

Voltage equations [2]

V = E / Q

V = W / Q [W: work done]

Ohm’s Law equation

V = IR

Thermistor

When temperature increases, resistance decreases

Filament Lamp

When temperature increases, resistance increases

Light Dependent resistor

When light intensity increases, resistance decreases

Diode

Allows current to flow in only one direction

A.C. current

Current flows both directions alternately

D.C. current

Current flows in only one direction

Filament lamp IV graph

V increases more rapidly than current

Voltage increases, increasing brightness and therefore temperature and resistance.

Thermistor IV graph

Current increases more rapidly than voltage

Diode IV graph

Forward bias: same as thermistor

Reverse bias: very high resistance (I=0)

Series circuit

• same current

• shares p.d./voltage

Parallel circuit

• same p.d./voltage

• shares current

Advantage of parallel circuit

• when one equipment breaks, the rest still works

• maximum voltage/p.d. for all equipment

• a switch can be used to control each equipment separately

Advantage of series circuit

• uses less wires

• can be switched on/off all at once

• easy to set up

Live, Neutral and Earth wire

Live: Brown

Neutral: Blue

Earth: yellow-green

Earth wire

Connects to the metal case of the appliance

• provides a low resistance path to earth

• causes a surge of current in earth wire and live wire to melt the fuse —> cuts of supply of electricity to appliance

KiloWatt Hours

The electrical energy consumed by 1000W of power in 1 hour

Potential divider

R1 / R2 = V1 / V2

Hazards of damaged insulation

If someone touches an exposed piece of wire, they could be subject to lethal shock

Hazards of overheating of cables

Too much current can overheat a wire which could cause a fire or melt insulations

Hazards of damp conditions

If moisture comes in contact with live wires, it could conduct electricity which can cause a short circuit (could cause fire) or an electrocution risk

Hazard of excess current from overloading plugs, extension leads, single and multiple sockets when using a mains supply

Overload can cause fires from the heat created

Main circuit components

• live wire

• neutral wire

• earth wire