AQA Physics

Investigating charge and capacitance

Specification references

  • 3.7.4.2
  • 3.7.4.3
  • 3.7.4.4
  • MS 3.8
  • 7.3.1, 7.3.2, 7.3.3, 7.3.4
  • 8.1

Learning outcomes

After completing the worksheet you should be able to:

  • determine the capacitance of a capacitor from a suitable graph
  • calculate the energy stored by a capacitor from a graph
  • consider the errors involved in an investigation involving capacitance.

Information

Capacitors are used to store electrical charge, which means that they can also store electrical energy. Electrons will flow onto and off plates when a capacitor is connected to a dc source, such as a cell, battery, or power pack.

Task

The apparatus is arranged as shown in Figure 1. Once the switch is closed, charge will move onto and off the plates. Adjusting the variable resistor setting will keep the value of the current constant until the capacitor becomes fully charged. Having closed the switch, a stopwatch is used to record the time and the voltmeter and microammeter the values for the current and potential difference at regular intervals until the current falls to zero. At this point, the capacitor has become fully charged. The charge, Q, stored on the plates is then obtained from the equation QI×t.

Figure 1Apparatus to measure the potential difference across a capacitor

Potential difference, V/V / Average current,
I/mA / Time current flowed for, t/s / Charge,
Q/C
2.0 / 7.7 / 12.2
4.0 / 12.7 / 14.8
6.0 / 17.2 / 16.4
8.0 / 16.8 / 22.4
10.0 / 25.3 / 18.6
12.0 / 33.8 / 16.7
14.0 / 55.3 / 11.9

Table 1Data showing the potential difference, average current, and time for the charging capacitor

Questions

1Complete the table by calculating the charge present on the fully charged capacitor and recording the values in each of the cells of the final column.

(2 marks)

2Circle any data that is likely to be erroneous and suggest why this might be the case.

(2 marks)

3Plot a graph of charge, Q, against potential difference, V.

(3 marks)

4Use the graph to determine a value for the capacitance of the capacitor.

(2 marks)

5The actual capacitance of the capacitor is stated as 50 mF. Find the percentage error in the value obtained from the graph.

(1 mark)

6Calculate the energy stored by the capacitor when it is fully charged.

(1 mark)

7In Figure 2 the charger charges from uncharged until the potential difference across the capacitor is 18 V and the average current flowing is 1.2 mAfor 90 seconds.

Figure 2

Calculate:

athe charge

bthe energy stored on the capacitor.

(2 marks)

8Describe the improvements that could be made in this investigation to obtain a more accurate and precise value for the capacitance of the capacitor.

(4 marks)

9Describe how the arrangement in Figure 3 could be used to determine an accurate value for the capacitance of the capacitor, C.

Figure 3

(4 marks)

© Oxford University Press 2016

This resource sheet may have been changed from the original1