Electricity and magnetism, Year 10: circuits, charge and resistance

Each lamp is on its own branch between the same two junctions

HintTrace the wire from the cell with a finger and see whether it ever splits.

The whyIn series, the components follow one another on a single loop with no junctions. In parallel, the wire divides, each lamp sits on a separate path, and the paths join up again. Reading this off a diagram is the first step in interpreting any circuit.

Lamp B only

HintFollow each complete path from one end of the cell back to the other, and see which is broken.

The whyAn open switch breaks only the branch it is in. Branch 2 still makes a complete loop with the cell, so lamp B stays on, while no current can flow through lamp A. Interpreting a diagram means tracing every loop like this.

A battery

HintIn everyday speech people use this word for a single AA cell as well.

The whyIn physics a single source of potential difference is a cell. Joining several end to end adds their potential differences, which is why a torch that needs 3 V holds two 1.5 V cells. Circuit diagrams use different symbols for the two.

An ammeter must be in series with the lamp

HintFor the meter to count the charge passing a component, where does that charge have to go?

The whyAn ammeter measures the current flowing through itself, so it has to sit on the same loop as the component, with all of that component's current passing through it. Joined across the lamp it would offer the current an easy path around the lamp instead. Checking a diagram for this error is part of drawing circuits correctly.

The potential difference across the second lamp

HintLook at which component lies between the two points where the meter's leads touch the loop.

The whyA voltmeter compares two points in a circuit, so it is always connected in parallel with the component of interest. Whatever sits between its two connection points is what it measures. Here that is one lamp only, not the cell and not both lamps.

The coulomb

HintIts symbol is a capital C, and it is named after an eighteenth-century French physicist.

The whyCharge flow has the symbol Q. One of these units is the charge that passes a point when a current of 1 ampere flows for 1 second. An ampere is therefore one of them per second.

charge

HintIt is the property that electrons carry, measured in coulombs.

The whyA bigger current means more of it passes each point every second. This is what the equation Q = I t says: the amount that flows is the current multiplied by the time. In a single closed loop the current has the same value at every point.

60

HintEach second, 2 C goes past.

The whyCharge flow = current × time, or Q = I t, so Q = 2 × 30. AQA expects this equation to be recalled. The time must be in seconds.

2 A

HintBefore dividing, check the unit that the time has been given in.

The whyRearranging Q = I t gives I = Q ÷ t. The time must be in seconds: 5 minutes is 300 s, so I = 600 ÷ 300. The equation is one AQA expects pupils to recall.

There is no source of potential difference

HintThe charges are free to move, but nothing is pushing them.

The whyA closed circuit is not enough on its own: the loop must also include something, such as a cell, battery or power supply, that provides a potential difference to drive the charge round. Take that away and the charges in the wire have no reason to drift one way more than the other.

potential difference = current × resistance

HintThe two quantities on the right multiply; neither is divided.

The whyIn symbols this is V = I R, with V in volts, I in amperes and R in ohms. AQA expects it to be recalled and rearranged for any of the three quantities. It shows that, for a fixed potential difference, a larger resistance means a smaller current.

3

HintThe current is not yet in the unit the equation needs.

The whyFirst convert: 20 mA = 0.020 A. Then V = I × R = 0.020 × 150. Currents in electronic circuits are often given in milliamps, and forgetting the conversion makes the answer a thousand times too large.

length

HintIt is the factor a pupil changes by sliding a crocodile clip along the wire.

The whyIn required practical 15, readings of potential difference and current are taken for different amounts of the same wire, and resistance is calculated each time. A graph of resistance against this quantity is a straight line through the origin, so doubling it doubles the resistance. The charge has to pass more of the metal on its way through.

To stop the wire heating up, which would change its resistance

HintA current has a side effect you can feel with your fingers.

The whyThe resistance of a metal wire rises as it gets hotter. If the wire warmed during the experiment, the readings would reflect temperature as well as length, and the test would not be fair. AQA specifies that this investigation is done at constant temperature.

A systematic error (a zero error)

HintEvery reading seems to be too large by the same small amount.

The whyA zero length of wire should have zero resistance. A constant extra amount on every reading points to a fault in the set-up: usually the resistance of the contacts at the crocodile clips, or a clip not sitting exactly at the start of the ruler. Random errors would scatter the points instead of shifting the whole line.