A+ Answers. Question 1

This question is worth 15 marks, and is designed to test your understanding of electric

potential and potential energy (Unit 9).

(a)Two point charges with q1=+3.2 × 10 separated by a distance of r =2.4 × 10 4 7 C and q

2= -4.8 × 10 7 C are initially m.

(i) How much energy is required to double their separation?

(ii) What is the electric potential at the point midway between the two charges when their

separation is 2r? (8 marks)

(b) A uniform electric field of magnitude 1.0 × 10 1 exists between two conducting plates, one of which is positively charged and the other of which is negatively charged. The plates are 10 mm apart.

(i) Calculate the magnitude of the potential difference between the plates.

(ii) If a proton is moved from the positive plate to the negative plate, what is the magnitude of the change in its electrostatic potential energy?

(iii) Sketch the electrostatic equipotentials between the two plates.

(iv) If this device operates as a capacitor, how is it able to store electrostatic potential energy?

Question 2

This question is worth 20 marks, and is designed to test your understanding of electrical circuits (Unit 10).

(a) State in words Kirchhoﬀ’s laws and Ohm’s law for electrical circuits. (4 marks)

(b)The circuit shown in Figure 1 can be used to measure the resistance of a platinum resistance thermometer (PRT). AB is a uniform resistance wire of length 1.0m and Cis a sliding contact on this wire. A standard resistor R is included in the circuit. The position of C is adjusted until the voltmeter V reads zero.

(i) By applying Kirchhoﬀ’s laws to loops ADCA and BCDB, deduce an expression for the resistance of the PRT in terms of l,land the value of the standard resistor. (7 marks)

(ii) The PRT consists of 9.0 m of wire of diameter 8.0 × 10 when l 1 1 2 mm. The voltmeter reads 0 V =0.44 m. If the standard resistor, R, has a resistance of 224 O, what is the resistivity of platinum? (7 marks)

(c) Indicate brieﬂy what factors might aﬀect the precision of the measurement when using such a simple circuit to measure the resistance of the PRT. (2 marks)