Kirchhoff's Laws
Electrical Circuits - OCR A-Level Physics
Key Definition
kirchhoff's first law
The sum of currents entering a junction equals the sum of currents leaving it. This is conservation of charge.
The sum of currents entering a junction equals the sum of currents leaving it. This is conservation of charge.
Key Definition
kirchhoff's second law
The sum of the EMFs around any closed loop equals the sum of the p.d.s around that loop. This is conservation of energy.
The sum of the EMFs around any closed loop equals the sum of the p.d.s around that loop. This is conservation of energy.
- Kirchhoff's first lawThe sum of currents entering a junction equals the sum of currents leaving. A consequence of conservation of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. applies at junctions (nodes) in a circuit.
- 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). $in = currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). out. No charge$ is created or destroyed.
- Kirchhoff's second lawThe sum of EMFs around any closed loop equals the sum of the products of currentThe rate of flow of charge. Measured in amperes (A). and resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). (IR). A consequence of conservation of energyThe capacity to do work. Measured in joules (J).Energy cannot be created or destroyed, only transferred from one form to another. The total energyThe capacity to do work. Measured in joules (J). of a closed system remains constant.. applies around any closed loop.
- Total $EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). = total p.d. dropped across all components$ in the loop.
- These two laws are enough to solve any circuit problem.
Junction showing currents in = currents out (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)
A nodeA point on a stationary wave where the displacement is always zero. Located at half-wavelength intervals. with I_1 entering and I_2 + I_3 leaving, with I_1 = I_2 + I_3 labelled.
Examiner Tips and Tricks
- When stating Kirchhoff's second lawThe sum of EMFs around any closed loop equals the sum of the products of current and resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). (IR). A consequence of conservation of energyEnergy cannot be created or destroyed, only transferred from one form to another. The total energy of a closed system remains constant.., always say 'closed loop' not just 'circuit'.
- OCR mark schemes require this phrase for full marks.