What is electricity?
Electricity is a form of energy caused by the movement of tiny charged particles called electrons. When electrons flow through a wire, they form an electric current. Current only flows when there is a complete loop, called a circuit, with a source such as a battery or dry cell.
Three important quantities
- Electric current (I) is the rate of flow of charge. It is measured in amperes (A) using an ammeter, which is connected in series.
- Voltage (V), also called potential difference, is the energy that pushes the current. It is measured in volts (V) using a voltmeter, connected in parallel.
- Resistance (R) is how much a component opposes the current. It is measured in ohms (Ω).
Key idea
Ohm's law: V = I × R. From it we also get I = V ÷ R and R = V ÷ I. When resistance is higher, less current flows for the same voltage.
Ohm's law in action
Ohm's law tells us that, at constant temperature, the current through a conductor is directly proportional to the voltage across it. Doubling the voltage doubles the current. Thicker and shorter wires have lower resistance, while thin and long wires have higher resistance.
Example
A bulb of resistance 4 Ω is connected to a 12 V battery. The current is I = V ÷ R = 12 ÷ 4 = 3 A.
Remember
- Ammeter connects in series; voltmeter connects in parallel.
- Unit of current is ampere (A); voltage is volt (V); resistance is ohm (Ω).
- V = IR is true only at constant temperature.