Starter Physics - Electricity

Electricity

Electricity uses electrons as charge carriers.

For an electrical circuit to work we need charge to flow from one area to another.
Electrons carry a negative charge, and move very easily through metals.
This is why metals make good materials for electric circuits and wires.
The flow of charge is measured in Amperes (or Amps).
A mobile phone's battery is charged by flowing electrons and using their electrical energy to drive a chemical reaction in the battery, producing a voltage difference at the battery's + and - terminals, ready for use in the phone's circuits.
An electric kettle heats water by flowing electrons through a heater which gets hot as the electrons flow through it.

Current

Electric current is a measure of the flow of charge.
It measures the number of Coulombs of charge flowing past a certain point per second.
The SI unit is the Ampere (usually shortened to Amp).
1 Amp is 1 Coulomb/sec.
This is over 6 billion billion electrons per second!
A mobile phone is usually charged at 1 or 2 Amps.
Your electric kettle might take up to 13 Amps.

Voltage

Voltage is what drives the electrons to move.
If there is a voltage difference between two ends of a wire then electrons will move along the wire.
It's like the pressure that pushes the electrons.
A battery provides a known voltage to drive current around a electric circuit, acting like an 'electron pump'.
Voltage is also known as 'potential' or 'potential difference'.

Electrical Energy

Electrical energy is measured as the amount of charge, q, separated by a potential difference (a voltage, V): E = q.V
This is the potential energy that can be delivered by an electrical circuit.
A more common expression might be E = I.V.t, where I is electrical current, V is voltage and t is time.
So an electrical heater operating at 240 Volts and 10 Amps for 100secs would deliver 240000 Joules of energy (or 240kJoules).

Electrical Power

We have seen previously that Power is Energy delivered per second.
Electrical power is measured as the amount of current, I, flowing across a potential difference (a voltage, V): P = I.V
This is the power that can be delivered by an electrical circuit.
It has exacty the same units and dimensions as other forms of Power.
An electrical heater operating at 240 Volts and 10 Amps would deliver a power of 2400 Watts (or 2.4kWatts).

Electrical Circuits

When we say 'electric circuit' we mean that the charge must flow as if 'in a circle'.
For example, if we are using a battery to drive a circuit, this means that wire must go from one end of the battery, through the circuit, and then back to the other end of the battery, completing the circuit loop.
For current to flow, it must return to the battery (or whatever voltage source is used).
This is why batteries have two ends - a positive end and a negative end.
And why voltage sources have two connections - a positive and a negative terminal.
When current flows it can be used to do many things: to light an LED, to power your phone, to run a computer, to drive an electric motor, to heat a heater, etc.

A simple electric circuit. Electric current flows from the positive end of the battery, around the circuit, then back to negative end of the battery. On the way it passes though the LED making it light up. A resistor is included to control how much current goes through the LED.

Electric Fields

Opposite electric charges (positive and negative) will be pulled towards each other.
Charges that are of the same sign will push each other apart.
This electric force comes from the electric field.
The force is calculated as F = ke.q1q2/r2 where q1 and q2 are the two charges and r is the gap between them. ke is the Coulomb constant (8.99x109 kg.m3.s−2.C−2).
This follows an 'inverse square law' meaning that the force depends on the inverse square of the separation, so falls off quickly with distance.