Sec 4 Physics D.C. Circuits topical questions

State the three series-circuit rules.

State the three parallel-circuit rules.

A 4.5 V supply feeds two identical lamps in series. State the p.d. across each lamp and the comparison between the current in lamp 1 and lamp 2.

Name the component: (a) protects a circuit by melting; (b) changes resistance with light; (c) gives a continuously adjustable output voltage.

Two 12 Ω lamps are connected in parallel. Calculate their combined resistance.

A 9.0 V battery drives a series chain of a 2.0 Ω and a 7.0 Ω resistor.

1. (a)Calculate the total resistance and current.[2]
2. (b)Calculate the p.d. across each resistor.[2]
3. (c)The 7.0 Ω resistor is replaced by another 2.0 Ω. State the effect on the battery current, with a reason.[2]

A 4.0 V supply feeds a 6.0 Ω and a 3.0 Ω resistor in parallel.

1. (a)Calculate the branch currents.[2]
2. (b)Calculate the supply current and the effective resistance.[2]
3. (c)Explain why the effective resistance is smaller than 3.0 Ω.[2]

A 12 V supply is shared between two series resistors, 4.0 kΩ and 8.0 kΩ, with the output tapped across the second one.

1. (a)Calculate the output voltage.[2]
2. (b)The 8.0 kΩ resistor is replaced by a 4.0 kΩ one. Calculate the new output.[2]
3. (c)Explain why the output cannot exceed 12 V in this arrangement.[2]

Two lamps rated 6.0 V, 3.0 W are connected across a 6.0 V battery: first in parallel, then in series.

1. (a)Show that one lamp’s resistance is 12 Ω.[2]
2. (b)Calculate the battery current in the parallel arrangement.[2]
3. (c)Calculate the battery current in the series arrangement and state which lamps glow brighter.[2]

A battery of negligible internal resistance supplies 12 V to a network: a 4.0 Ω series element followed by a split into 6.0 Ω and 3.0 Ω branches.

1. (a)Determine the single resistance that could replace the entire circuit.[2]
2. (b)Calculate the battery current and the p.d. across the pair.[3]
3. (c)Calculate both branch currents and verify your answers with a junction check.[3]

A temperature sensor runs from a 10 V supply: a fixed 5.0 kΩ resistor and a thermistor form a series chain, with the output wired across the thermistor. At 20 °C the thermistor is 15 kΩ; at 80 °C it is 5.0 kΩ.

1. (a)Calculate the output at 20 °C and at 80 °C.[4]
2. (b)Explain the direction of the output change as the sensor warms.[2]
3. (c)A cooling fan should switch on when the sensor gets hot. State whether it should trigger on a rising or falling output, and why.[2]

A student builds the same divider twice: a 5.0 kΩ fixed resistor and an LDR across a 10 V supply. In circuit A the output is across the LDR; in circuit B the output is across the fixed resistor. The LDR is 1.0 kΩ in bright light and 15 kΩ in darkness.

1. (a)Calculate circuit A’s output in bright light and in darkness.[4]
2. (b)State, without recalculation, how circuit B’s two outputs compare with A’s.[2]
3. (c)Which circuit triggers a dusk street lamp on a RISING output? Explain briefly.[2]

An unknown resistor R is connected in series with a 6.0 Ω resistor across a 12 V supply. A voltmeter across the 6.0 Ω resistor reads 4.0 V.

1. (a)Calculate the current in the circuit.[2]
2. (b)Calculate the p.d. across R and its resistance.[4]
3. (c)State how the readings change if R is short-circuited by a switch placed across it.[2]