- Electricity is caused by the movement of charged particles (usually electrons) in an electrical circuit.
- Current (I) is a measure of the rate at which charge (Q) flows through the circuit.
- I = Q/t .
- Electrical current is measured in units called Amperes or Amps (A).
- Quantity of charge is measured in units called Coulombs (C).
- One coulomb is equal to the total charge carried by 6.25 x 10
^{18}electrons. - Power supplies contain electrical cells that produce an electro motive force (e.m.f.).
- An e.m.f. is what causes current to flow in an electrical circuit.
- The e.m.f. supplies electrical potential to the charge carriers (usually electrons).
- Electrical potential is measured in volts (V).
- Electrons flow from the negative terminal of a power supply, to its positive terminal, (this is called electron flow).
- Conventional current flow assumes (incorrectly), that electric current flows from the positive to the negative terminal. (This is due to a historical misconception about the true nature of electricity).
- Conventional current flow is still used to show the direction of current flow, in electrical circuit diagrams.
- Electrical circuits also possess a property called resistance (R). Resistance limits the flow of current through a circuit and converts electrical energy into heat.
- Resistance is measured in Ohms (Ω).
- A resistor is a component which has been designed to possess a specific amount of resistance.
- The electrical potential of charged particles is reduced, when they flow through a resistor. This causes a potential drop (or voltage drop) across the resistor.
- The relationship between the current, voltage and resistance in a circuit is described by Ohm's law, R=V/I.

After completing this section you should be able to:

- Recall the three basic circuit laws, - Ohm's law and Kirchhoff's current and voltage laws.
- Use these circuit laws, to find unknown values of current, voltage and resistance, in simple electrical circuits.
- Add resistors in series and parallel, to find total resistance.
- Label currents, e.m.f.s, voltage drops and resistors on a circuit diagram.