Electric current and Ohm's law

Electricity lights our homes and runs our phones. To understand it, we need three ideas: current, potential difference and resistance. A simple picture helps: think of water flowing through a pipe from a tank on the roof.

Electric current

An electric current is a flow of electric charge. In a metal wire the charges that move are electrons. Current is the amount of charge passing a point in one second.

Current = Charge ÷ Time, or I = Q ÷ t

Charge is measured in coulombs (C) and current in amperes (A). One ampere is one coulomb of charge flowing per second. Current is measured with an ammeter, connected in series in the circuit. By convention, the direction of current is taken from the positive terminal of the cell to the negative, opposite to the flow of electrons.

Potential difference

Water flows in a pipe only if there is a difference in pressure between its ends. In the same way, charge flows in a wire only if there is an electrical "pressure difference" between its ends. This is called the potential difference, or voltage. A cell or battery provides it.

Potential difference is measured in volts (V) with a voltmeter, connected in parallel across the two points. A torch cell gives 1.5 V. The mains supply to Indian homes is 220 V, which is why it is so dangerous.

Resistance

A narrow pipe lets less water through than a wide one. A wire also opposes the flow of current. This opposition is called resistance, and it is measured in ohms (Ω). The resistance of a wire

• increases with its length;
• decreases as it gets thicker;
• depends on its material: copper and aluminium have low resistance, while alloys such as nichrome have high resistance and are used in heaters;
• generally increases with temperature, for metals.

Ohm's law

In 1827 the German physicist Georg Simon Ohm found a simple relationship: the current through a conductor is directly proportional to the potential difference across its ends, provided its temperature stays the same.

V = I × R

So I = V ÷ R and R = V ÷ I. If we plot V against I for a metal wire, we get a straight line through the origin.

Worked example: a bulb of resistance 440 Ω is connected to a 220 V supply. I = V ÷ R = 220 ÷ 440 = 0.5 A.

Resistors in series

When resistors are joined end to end, the same current flows through each, and the total resistance is the sum of the individual resistances:

R = R₁ + R₂ + R₃

The total is greater than any single resistance. If one part breaks, the whole circuit stops. Old-style decorative lights were wired this way: one bulb fused and the whole string went dark.

Resistors in parallel

When resistors are connected side by side between the same two points, each has the same potential difference across it, and the current divides among them.

1/R = 1/R₁ + 1/R₂ + 1/R₃

The total is smaller than the smallest single resistance. Household appliances are connected in parallel, so that each gets the full 220 V and each can be switched on or off without affecting the others.

Heating and power

When current flows through a resistance, electrical energy changes into heat. Electric irons, heaters and geysers use this heating effect, and so does a fuse, a thin wire that melts and breaks the circuit when the current becomes too large. Electric power is given by P = V × I, measured in watts.