Cells in Series & Parallel
Electrical Circuits - OCR A-Level Physics
Key Definition
electromotive force
The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).
The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).
$$\varepsilon = \frac{E}{Q}$$
- Total $EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). = sum$ of individual EMFs.
- Total internal $resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). = sum$ of individual internal resistances.
- Greater EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). means greater 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). through the external circuit.
- Total $EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). = difference between$ the EMFs.
- The cell with larger EMF 'wins' and determines current direction.
- This is how a flat cell can be recharged.
- Total EMF stays the same as one cell.
- Total internal resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).The resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). within the source of EMF itself, which causes energyThe capacity to do work. Measured in joules (J). to be dissipated inside the source. halves (for two identical cells).
- The circuit lasts longer: greater total energyThe capacity to do work. Measured in joules (J). store.
- Current does not increase (same EMF, similar total R).
Common Mistake
MEDIUM
Wrong: Thinking cells in parallel doubles the EMF.
Right: Parallel cells keep the same EMF. Only series cells add EMFs together.
Right: Parallel cells keep the same EMF. Only series cells add EMFs together.
Two cells in series (same direction) and two cells in parallel
Left: two cells in line, total EMF = 1.5 + 1.5 = 3.0 V. Right: two cells side by side, total EMF = 1.5 V, total r = r/2.