Kirchhoff's first law
Charge & Current - OCR A-Level Physics
Key Definition
Kirchhoff's first law
The total current entering a junction equals the total current leaving the junction.
The total current entering a junction equals the total current leaving the junction.
$$\sum I_{\text{in}} = \sum I_{\text{out}}$$
- Based on the conservation of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). cannot be created or destroyed.
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). does not get used up as it flows around a circuit.
- CurrentThe rate of flow of charge. Measured in amperes (A). entering a component equals currentThe rate of flow of charge. Measured in amperes (A). leaving that component.
- Components in series carry the same currentThe rate of flow of charge. Measured in amperes (A)..
Junction diagram showing I_1 entering and splitting into I_2 and I_3, with the equation I_1 = I_2 + I_3 labelled clearly.
Common Mistake
MEDIUM
Wrong: Current is used up by components in a circuit.
Right: Current is NOT used up. The same current enters and leaves every component. EnergyThe capacity to do work. Measured in joules (J). is transferred, not charge.
Right: Current is NOT used up. The same current enters and leaves every component. EnergyThe capacity to do work. Measured in joules (J). is transferred, not charge.