Current, Potential Difference, Resistance & Power (Edexcel International A Level Physics)

Two resistors of resistance and are connected in parallel in a circuit.

Derive a formula for the total resistance  of the combination.

The diagram shows a combination of five resistors, M, N, O, P and Q. Each resistor has a resistance of 5.0 Ω.

i) Show that the resistance between points A and B is about 3 Ω.

(3)

ii) A student is told to modify the combination of resistors so that the combined resistance between A and B is greater than 5.0 Ω. She cannot change the arrangement of the resistors, but she can replace one of the 5.0 Ω resistors with a 20.0 Ω resistor.

Explain, without further calculations, which of the five resistors should be replaced.

(2)

The circuit diagram shows three identical resistors and two switches, X and Y.

The battery has negligible internal resistance.

Each resistor consists of a wire of diameter 0.181 mm and length 55.0 cm. At room temperature, the resistivity of the wire is 1.10 × 10⁻⁶ Ω m.

Show that the resistance of one resistor at room temperature is about 24 Ω.

A student suggests that the maximum power output from the resistors in this circuit will be greater than 12 W.

Assess whether the student’s suggestion is correct.

Switch X was open and switch Y was closed.

i) Calculate the number of conduction electrons passing through the resistor every second immediately after switch Y was closed.

Number of electrons = ............................(3)

ii) After switch Y had been closed for a few minutes, the power dissipated by the resistor decreased.

Explain why.

(3)

A student set up the circuit shown.

Calculate the number of electrons passing point X each second.

Number of electrons in one second = ..............................

Another resistor, Y, is added to the circuit as shown.

The student wrote the following statement.

Evaluate the student’s statement.