3.5.1.4
Conservation of charge
Circuits & Potential Dividers — AQA A-Level Physics
Key Definition
Conservation of charge — In a closed system, charge cannot be created or destroyed.
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). is never used up as it flows around a circuit.
- The same chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). passes through every component each second.
- CurrentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). entering a component equals currentThe rate of flow of charge. Measured in amperes (A). leaving it.
- Components in seriesComponents connected end-to-end in a single path, so current flows through each in turn. carry the same currentThe rate of flow of charge. Measured in amperes (A)..
Key Definition
Kirchhoff's first law — At any junction, the total current entering equals the total current leaving. This is a consequence of conservation of chargeAt any junction in a circuit, the total current flowing in equals the total current flowing out (Kirchhoff's first law)..
$$\sum I_{\text{in}} = \sum I_{\text{out}}$$
Junction current diagram
A junction splitting current I_1 into branches I_2 and I_3, with I_1 = I_2 + I_3 labelled.
Worked Example
A wire carrying 0.50 A splits at a junction into two branches. One branch carries 0.30 A. What is the current in the other branch?
Show Solution
1
Apply KirchhoffKirchhoff's laws: (1) Conservation of charge at junctions. (2) Conservation of energyThe capacity to do work. Measured in joules (J). around closed loops. Sum of EMFs = sum of IR drops.'s first law
$I_in = I_out1 + I_out2.$
2
0.50 = 0.30 + I_out2.
3
$I_out2 = 0.50 - 0.30 = 0.20\;\text{A}.$
Answer
0.20 A
Examiner Tips and Tricks
- AQA often draws junctions with three or four branches.
- Write out the equation $I_{\text{in}} = I_{\text{out}} before substituting$.
- Do not assume branches carry equal current.
Common Mistake
MEDIUM
Students often: Thinking current gets 'used up' by components.
Instead: Current is the same at every point in a series circuitA circuit in which components are connected end-to-end so the same current flows through each component.. EnergyThe capacity to do work. Measured in joules (J). is transferred, not current.
Instead: Current is the same at every point in a series circuitA circuit in which components are connected end-to-end so the same current flows through each component.. EnergyThe capacity to do work. Measured in joules (J). is transferred, not current.