Electrical charge and current — AQA GCSE Physics
Test yourself on Electrical charge and current with AQA GCSE practice questions.
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Electrical charge and current explained
Charge does not flow on its own; something has to push it.
Read the full explanation
That push is a potential difference, supplied by a cell, a battery or a mains supply. A cell or battery transfers energy from its chemical store to the charges and drives them round the circuit. Two conditions must both hold for charge to flow. There must be a source of potential difference, and the circuit must be complete, forming a closed loop of conductors with no gaps. Open a switch, blow a fuse or break a wire in a series circuit and the current everywhere stops instantly. However, in a parallel circuit, a break in one branch does not stop current flowing in other intact branches. Adding more cells in series raises the potential difference, so the current increases; closing a switch that adds a parallel branch lowers the total resistance, so the current from the supply increases.
Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation: charge f low = current × time Q = I t charge flow, Q, in coulombs, C current, I, in amperes, A (amp is acceptable for ampere) time, t, in seconds, s A current has the same value at any point in a single closed loop.
An electric current is a flow of electrical charge, and its size is the rate of that flow: how much charge passes a point each second. That is written Q = I t, with charge flow Q in coulombs, current I in amperes and time t in seconds, so a current of 3 A for 20 s moves 60 C. Rearranged, I = Q ÷ t and t = Q ÷ I. The other half of this statement is just as heavily examined: in a single closed loop the current has the same value at every point, because charge is not created or used up on the way round. So an ammeter placed anywhere in a series loop reads the same, and a lamp nearer the battery does not take a bigger share. It also explains why one blown lamp in a series loop stops the current everywhere at once.
Your focus
- State the two conditions needed for charge to flow round a circuit.
- Explain why opening a switch stops the current everywhere in a single loop.
- Predict the effect on the current of adding an extra cell in series with the supply.
Show all 8 objectives
- Explain why closing a switch that adds a parallel branch increases the current from the battery.
- Explain what a current of 2 A means in terms of the coulombs passing a point each second.
- Calculate the charge that passes a point from a given current and time in seconds.
- Rearrange Q = I t to find the current or the time from the other two quantities.
- Explain why an ammeter reads the same value wherever it is placed in a single closed loop.
Electrical charge and current exam tips
Marking Points
- one mark for stating that a source of potential difference is needed to make charge flow
- one mark for stating that the circuit must be closed or complete
- one mark for explaining that an open switch or break stops the current throughout a series circuit
- one mark for linking a change in supply potential difference or total resistance to a change in current
- one mark for writing charge flow = current × time, or Q = I t
- one mark for a correct substitution with the time converted to seconds
- one mark for the answer in coulombs, or in amperes when current is the unknown
- one mark for stating that the current is the same at every point in a single closed loop
Examiner Tips
- 💡Name both conditions, the source and the complete circuit, because a one-condition answer only scores half.
- 💡Use the phrase charge flows rather than electricity flows, which is too vague to credit.
- 💡When a switch is closed or opened, check if it is in a series loop or a parallel branch before deciding if the whole current stops.
- 💡Where the question says write down the equation, give it in words or symbols; both are accepted.
- 💡Convert minutes to seconds on the first line of working so the conversion is visible for method marks.
- 💡In a series circuit question, state that the current is the same everywhere before doing any calculation.
Common Mistakes
- saying the battery supplies the charges, when the charges are already in the wires and the battery supplies the potential difference
- claiming current is used up as it goes round the circuit
- thinking a break in a series circuit only stops the current after the break, rather than everywhere instantly
- assuming a break in one branch of a parallel circuit stops the current in all other branches
- leaving the time in minutes or hours, so the charge is 60 or 3600 times too small
- writing the unit of charge as amperes rather than coulombs
- claiming current is used up by each component, so less returns to the battery
- confusing charge flow with potential difference when both are given in a question