Parallel Circuit Rules
Electrical Circuits - OCR A-Level Physics
- P.d. is the same across each branch in parallel.
- 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). splits at junctions (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)..).
- More 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). flows through the branch with lower resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω)..
- Total resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). is always less than the smallest individual resistor.
$$\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \ldots$$
For two resistors in parallel, a quicker formula is often useful:
$$R_{\text{total}} = \frac{R_1 \times R_2}{R_1 + R_2}$$
Two resistors in parallel connected to a cell
Cell with two branches, each containing a resistor. Same V across both. I_total = I_1 + I_2.
Common Mistake
MEDIUM
Wrong: Forgetting to take the reciprocal at the end of the parallel resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). calculation. Getting 1/R instead of R.
Right: After adding the reciprocals, flip the result. If 1/$R = 0.05$, then $R = 20 ohms$.
Right: After adding the reciprocals, flip the result. If 1/$R = 0.05$, then $R = 20 ohms$.
Examiner Tips and Tricks
- Always check: your parallel resistance answer must be smaller than the smallest resistor in the combination.
- If it is not, you have made an error.