Current in Circuits (Edexcel GCSE Physics)
Revision Note
Current in Circuits
A current will flow in a circuit if:
The circuit includes a source of potential difference
The circuit is closed - i.e. there are no gaps in the circuit
Sources of potential difference include:
A cell
Batteries (multiple cells)
Electrical generator
Charge Conservation
In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point
This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
This means that all components in a closed-loop have the same current
The current is the same at each point in a closed-loop
Current at a Junction
At a junction in a circuit (where two or more wires meet) the current is conserved
This means the amount of current flowing into the junction is equal to the amount of current flowing out of it
This is because charge is conserved
The current coming into a junction must equal the current coming out of a junction
In electrical wires, the current is a flow of electrons
Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood
By definition, conventional current always goes from positive to negative (even through electrons go the other way)
Worked Example
In the circuit below, ammeter A0 shows a reading of 10 A, and ammeter A1 shows a reading of 6 A.
What is the reading on ammeter A2?
Answer:
Step 1: Recall that at a junction, the current is conserved
This means that the total amount of current flowing into a junction is equal to the total amount flowing out
Step 2: Consider the first junction in the circuit where current splits
The diagram below shows the first junction in the circuit
Step 3: Calculate the missing amount of current
Since 10 A flows into the junction (the total current from the battery), 10 A must flow out of the junction
The question says that 6 A flows through ammeter A1 so the remaining current flowing through ammeter A2 must be:
10 A − 6 A = 4 A
Therefore, 4 A flows through ammeter A2
Examiner Tips and Tricks
The direction of current flow is super important when considering junctions in a circuit, you should remember that current flows from the positive terminal to the negative terminal of a cell / battery
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