Electricity lessons
What is electricity?
A battery, a switch and a lamp: close the switch and change the voltage, with live V, I and P meters.
Electricity is energy carried by moving charge. A battery pushes charge round a closed loop called a circuit.
Open the switch and the loop is broken, so the charge stops. Close it and the lamp lights at once.
This course covers charge, current, voltage, resistance and power. It ends with how mains electricity is kept safe.
Electric charge
Atoms contain protons, which are positive, and electrons, which are negative. An atom with equal numbers of each is neutral.
Two charges push apart or pull together as their signs change.
Like charges repel and opposite charges attract. The closer the charges are, the stronger the push or pull.
Charge is measured in coulombs (C). One electron carries a tiny charge of -1.6 \times 10^{-19}\ \text{C}.
Static electricity
Rubbing two materials together can move electrons from one to the other. Only electrons move, because protons stay locked inside each atom.
A balloon rubbed on a wool jumper picks up electrons, one rub at a time.
The object that gains electrons becomes negative. The object that loses them is left positive by the same amount.
On an insulator like a balloon the charge stays where it is, so it is called static. A spark or a small shock is that charge suddenly jumping away.
Electric fields
Every charge is surrounded by an electric field. Any other charge inside that field feels a force.
Field lines around a positive charge, a negative charge and a pair, with a test charge following them.
Field lines show which way a positive charge would be pushed. They point away from positive charges and towards negative ones.
Where the lines crowd together, the field is stronger. That is why the force grows as charges get closer.
Practice: Charges and fields
Questions on forces between charges, static charging and field lines.
Conductors and insulators
A conductor lets charge flow through it easily. Metals conduct because some of their electrons are free to move between atoms.
A circuit with a gap: each material placed across it lights the lamp or not.
An insulator holds on to its electrons, so charge cannot flow. Plastic, rubber, glass and dry wood are insulators.
A wire uses both. Its copper core carries the current and its plastic coat keeps the current away from you.
Practice: Conductor or insulator
Sort materials into conductors and insulators.
Electric current
Current is how much charge flows past a point each second. It is measured in amps (A), and one amp is one coulomb per second.
Charge and current: Q = It
Charge flowing past a point is counted against a timer to give the current.
Here Q is the charge in coulombs, I is the current in amps and t is the time in seconds.
Conventional current is drawn from the positive terminal round to the negative one. Electrons are negative, so they really drift the other way.
Potential difference
A battery gives energy to every coulomb of charge that passes through it. The charge then hands that energy over to the lamp.
Charges pick up energy at the battery and give it to the lamp; the voltage sets how much.
Potential difference, or voltage, is the energy transferred per coulomb. It is measured in volts (V), and one volt is one joule per coulomb.
Energy transferred: E = QV
Here E is the energy in joules, Q is the charge in coulombs and V is the potential difference in volts.
Practice: Charge, current and energy
Calculate current, charge, energy or voltage with Q = It and E = QV.
Measuring current and voltage
An ammeter measures current. It goes in the loop, in series, so the same current flows through it.
A voltmeter measures potential difference. It connects across a part, in parallel, on its own side branch.
Drag the ammeter and the voltmeter into the right gaps in a circuit.
Resistance
Resistance is how strongly something opposes the current. It is measured in ohms (\Omega).
A wire whose length, thickness and metal set its resistance.
A longer wire has more resistance and a thicker wire has less. The metal matters too, and most metals resist more when they get hot.
Moving electrons keep bumping into the atoms of the wire. Each bump turns some electrical energy into heat.
Ohm's law
For a resistor at a steady temperature, the current is proportional to the voltage. Double the voltage and the current doubles too.
Ohm's law: V = IR
Current against voltage for a resistor and a filament lamp.
So a graph of current against voltage is a straight line through the origin. At any point on it, R = \frac{V}{I}.
A steeper line means a smaller resistance. A filament lamp curves instead, because its resistance rises as it heats up.
Practice: Resistance from a graph
Read a resistor's I-V graph to find its resistance or a current.
Electrical power
Power is how fast energy is transferred. It is measured in watts (W), and one watt is one joule per second.
Charges carry energy into a lamp; current times voltage gives joules per second.
Each second, I coulombs pass through the lamp and each one gives up V joules. So the lamp gets I \times V joules every second.
Electrical power: P = IV
Paying for electricity
Energy used is power multiplied by time, E = Pt. A joule is tiny, so bills use the kilowatt-hour (kWh) instead.
One kilowatt-hour is the energy a 1 kW appliance uses in one hour. Multiply the kilowatt-hours by the price of each one to get the cost.
E = P \times t = 2\ \text{kW} \times 3\ \text{h}(a 2 kW heater for 3 hours)E = 6\ \text{kWh}(the energy used)\text{cost} = 6 \times 30\text{p} = 180\text{p}(at 30p per kWh)
Energy and cost for household appliances over a chosen time.
Practice: Power and cost
Calculate power, current, kilowatt-hours or cost.
AC and DC
A battery gives direct current (DC). The charge always flows the same way round the circuit.
Voltage against time for a battery and for the mains, with the charges moving in step.
Mains sockets give alternating current (AC). The current keeps reversing its direction.
UK mains is about 230 V with a frequency of 50 hertz (Hz). That is 50 full cycles each second, so the current changes direction 100 times a second.
Practice: AC or DC
Read voltage traces and pick out AC and DC supplies.
Mains plugs
A UK plug has three wires, each with its own colour and job. The live wire is brown and carries the 230 V alternating voltage.
Inside a UK plug: live, neutral, earth and the fuse.
The neutral wire is blue and completes the circuit, staying close to 0 V. The earth wire is green and yellow and is there for safety.
The live wire is the dangerous one. Touching it can drive a current through your body to the ground.
Practice: Wire a plug
Match each plug wire to its colour and its job.
Fuses and earthing
The earth wire joins a metal case to the ground. If a loose live wire touches the case, a huge current rushes down the earth wire.
A fault in a kettle: the current surges down the earth wire and the fuse melts.
A fuse sits in the live wire. Too much current melts its thin wire, which cuts off the supply.
Pick the fuse rated just above the normal current, found with I = \frac{P}{V}. UK plug fuses are usually 3 A, 5 A or 13 A.
Practice: Pick the fuse
Work out an appliance's current and choose its fuse.
Where next
You now know what charge, current, voltage, resistance and power are. You can also explain how mains electricity is kept safe.
The Circuits course builds on this. It covers series and parallel circuits, voltage dividers, capacitors and logic gates.