3.5.1.4
Current is the same through each resistor in series
Circuits & Potential Dividers — AQA A-Level Physics
- 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). is the same through each resistor in seriesComponents connected end-to-end in a single path, so current flows through each in turn..
- P.d. across each resistor: $V = IR$.
- P.d. splits in proportion to 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)..
- The largest resistor gets the largest share of the p.d.
$$R_{\text{total}} = R_1 + R_2 + R_3$$
- Total resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). in series is always greater than the largest individual resistor.
- Adding a resistor in series always increases the total resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω)..
Worked Example
Two resistors (20 ohm and 30 ohm) are connected in series to a 10 V supply. Calculate the p.d. across each resistor.
Show Solution
1
$R_total = 20 + 30 = 50 \;\Omega.$
2
Total current
$I =\;\text{V} / R_total = 10 / 50 = 0.20\;\text{A}.$
3
V across 20 ohm
$V_1 = IR_1 = 0.20 \times 20 = 4.0\;\text{V}.$
4
V across 30 ohm
$V_2 = IR_2 = 0.20 \times 30 = 6.0\;\text{V}.$
5
Check
4.0 + 6.0 = 10 V. Correct.
Answer
4.0 V across the 20 ohm resistor, 6.0 V across the 30 ohm resistor.
Examiner Tips and Tricks
- Always check your answer: p.d.s across series resistors must add up to the supply voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). Informal term for potential difference..
- If they do not, you have made an error.
Series resistor circuit
Two resistors R_1 and R_2 in series with a cell, showing current I through both and V_1 + V_2 = V_total.