Series circuits · Grade 10
In Grade 10 you learn: adding resistances in series and sharing p.d. between resistors in series.
In a series circuit everything is in one loop. The same current flows everywhere, the supply's p.d. is shared between the components, and the resistances add up. If one part breaks, the whole loop stops.
Key facts
- Series: same current everywhere; p.d.s add up to the supply; R_total = R₁ + R₂ + …
- In series, the bigger resistor gets the bigger share of the p.d.
- If one bulb breaks in series, the loop is broken and all the bulbs go out.
- Cells in series, facing the same way, add their p.d.s: two 1.5 V cells give 3 V.
Worked examples
Example 1
Three lamps are in series with a cell. The ammeter next to the cell reads 0.2 A. What do ammeters placed between the lamps read?
Answer: 0.2 A everywhere
Example 2
One bulb breaks in a series circuit of three bulbs. What happens to the other two?
Answer: They go out too.
Example 3
A switch is opened in a series circuit with two lamps. Which lamps go off?
Answer: Both lamps go off.
Example 4
Two 1.5 V cells are joined in series, facing the same way. What is their total p.d.?
Answer: 3 V
Example 5
Two lamps are in series with a 9 V battery. The voltmeter across one lamp reads 5 V. What does the voltmeter across the other lamp read?
Answer: 4 V
Example 6
A second lamp is added in series with the first. What happens to the brightness, and why?
Answer: Both lamps are dimmer.
Example 7
Resistors of 3 Ω, 5 Ω and 4 Ω are in series. What is the total resistance?
Answer: 12 Ω
Example 8
A 6 V battery is connected to 10 Ω and 20 Ω resistors in series. What current flows?
Answer: 0.2 A
Example 9
A 12 V battery is connected to 4 Ω and 2 Ω resistors in series. Find the current and the p.d. across each resistor.
Answer: 2 A; 8 V and 4 V
Example 10
A 12 V supply is connected across 3 kΩ and 1 kΩ resistors in series. Find the p.d. across the 3 kΩ resistor without working out the current.
Answer: 9 V