IGCSE Physics 0625: Electric Circuits, Current, Voltage and Resistance - Study Guide PDF with Answers

Separate charge flow from energy transfer

Electric current is the rate of charge flow: I = Q/t, so 1 A means 1 C passes a point each second. Conventional current is drawn from the positive terminal around the external circuit to the negative terminal, although electrons in a metal move the other way. Potential difference is energy transferred per unit charge: V = E/Q. A 6 V supply transfers 6 J to each coulomb of charge across its terminals in an idealised account. Charge is not used up in a lamp.

Measure the right thing

An ammeter measures current through a branch and is placed in series with it. A voltmeter measures potential difference across a component and is placed in parallel with that component. Do not place a standard ammeter directly across a cell. For a fixed resistor at constant temperature, V = IR links potential difference, current and resistance. The ratio V/I need not stay constant for every component: a filament lamp changes temperature and resistance as it warms.

Series circuits have one path

In a series loop, current is the same at every point. The supply potential difference is shared among components, so component voltages add to the supply voltage. For resistors in series, total resistance is the sum of their resistances. Increasing series resistance with the same ideal supply generally reduces the loop current. A broken component opens the only path, so the whole series circuit stops carrying current.

Parallel circuits have branches

In a parallel circuit, each branch across the same two supply nodes has the same potential difference. The current entering a junction equals the sum of currents leaving it. Adding a branch can increase the total current drawn from an ideal fixed-voltage supply, while the current through an unchanged existing branch stays the same in the ideal model. A break in one branch need not stop current in other intact branches. State any ideal-supply assumption when applying these rules.

1. Current from charge

A charge of 24 C passes a point in 8 s. Find current.

1. Current means charge per unit time: I = Q/t.
2. I = 24/8 = 3 A.
3. This says 3 C passes that point each second on average.

2. A fixed resistor

A fixed resistor has 12 V across it and carries 0.5 A. Find its resistance.

1. Use R = V/I for these simultaneous readings.
2. R = 12/0.5 = 24 ohm.
3. The conclusion that this ratio stays 24 ohm assumes the resistor's conditions, especially temperature, remain suitable.

3. A junction and branches

A supply provides 2.0 A to two parallel branches. One branch carries 0.7 A. Find current in the other.

1. At the junction, incoming current equals total outgoing current.
2. I-other = 2.0 - 0.7 = 1.3 A.
3. Both branches have the same potential difference if they connect across the same two nodes.