Capacitors & Capacitance (Cambridge (CIE) A Level Physics)

Define the capacitance of a parallel plate capacitor.

Two capacitors, of capacitances, C_A and C_B, are connected in parallel to a power supply of electromotive force (e.m.f.) E, as shown in Fig. 1.1. 

Fig. 1.1

Determine an expression for the combined capacitance C_C of the two capacitors. 

Two capacitors of capacitances 18 μF and 51 μF, and resistor of resistance 4.5 kΩ, are connected into the circuit of Fig. 1.2.

Fig. 1.2

The battery has an e.m.f of 8 V.

(i) Calculate the combined capacitance of the two capacitors.

[1]

(ii) The two-way switch S is initially at position P, so that the capacitors are fully charged.  

Use the information in (c) (i) to calculate the total energy stored in the two capacitors.

 total energy = .................................. J [2]

Three capacitors, each of capacitance 27 μF, are connected as shown in Fig. 1.1.

Fig. 1.1

Calculate the total capacitance between points P and Q.

 capacitance = ..................................... μF 

The maximum safe potential difference that can be applied across any one capacitor is 4 V. 

Determine the maximum safe potential difference that can be applied between points P and Q.

 potential difference = ...................................... V 

A capacitor consists of an insulator separating two metal plates, as shown in Fig. 1.3.

Fig. 1.3

The potential difference between the plates is V. The variation with V of the magnitude of the charge Q on one plate is shown in Fig. 1.4.

Fig 1.4

Explain why the capacitor stores energy but not charge.

Use Fig. 1.4 to determine: 

(i) the capacitance of the capacitor

capacitance = ................................. μF [2]

(ii) the loss in energy stored in the capacitor when the potential difference V is reduced from 8 V to 6 V.

energy = ................................. mJ [2]

State two functions of capacitors connected in electrical circuits.

Three capacitors are connected in parallel to a power supply as shown in Fig. 1.1. 

Fig. 1.1

The capacitors have a capacitance C_A, C_B and C_C. The power supply provides a potential difference E. 

(i) Explain why the charge on the positive plate of each capacitor is different. 

[1]

(ii) Use your answer in (i) to show that the combined capacitance C_T of the three capacitors is given by the expression.

[2]

A student has available three capacitors, each of capacitance 24 μF. 

Draw circuit diagrams, one in each case, to show how the student connects the three capacitors to provide a combined capacitance of: 

(i) 36 μF

[1] 

(ii) 16 μF

[1]