Connecting People

Nokia

The capacitor is the subject of this lesson. Its function in electronic circuitry in instruments such as mobile phones is described. The formula relating charge, voltage and capacitance is described. The influence of physical dimensions is also explained. Series and parallel capacitors are also covered.

Contains the full lesson along with a supporting toolkit, including teachers’ notes. Lesson excerpt

Capacitors are important components in electronic circuits
In this lesson we look at the capacitor, a device that stores electric charge. In particular we refer to the small capacitors that appear in familiar devices such as the mobile phone. However, whether the capacitor is small or large, the underlying mathematical and physical principles are the same.

What is a capacitor?
A capacitor is a device for storing electrical energy. It stores the energy in an electric field between two closely spaced sheets of conductor separated by an insulator. This insulating material is called the dielectric. Capacitors can absorb or supply energy very quickly.

Current starts to flow into a capacitor when a voltage is applied across the terminals, but the current reduces and stops as thecharge builds up on the plates. The capacitor is then said to be charged. If the voltage source is then replaced by a resistance, the capacitor will discharge through it.

The first capacitors, called Leyden jars, were metal sheets pressed to the inside and outside of a glass jar. When a voltage was applied, current started to flow into it but the current reduced and stopped as equal and opposite charges built up on the two sheets of conductor. The quantity of the charge increased with the parallel area of the conductors, and also as the conductors were brought closer together (here, thinner glass walls). The charge also changed when a different dielectric was used (air, paper, etc.) The capacitor could be discharged by connecting the metal sheets together using a resistor or short circuit.

True or False?

1. The relationship involving capacitance, charge and voltage is CQ=V. false
2. The unit of capacitance is the coulomb. true
3. Capacitance is directly related to the area of the conducting plates. true
4. Capacitance is directly related to the distance between the conducting plates. false
5. An AC source will charge a capacitor. false
6. A DC source will continue to charge a capacitor as long as it is connected. false
7. A vacuum has no dielectric constant. false
8. The charge on a capacitor increases if the applied voltage increases. true
9. Materials with low dielectric constants are better insulators. false
10. Rubber has a higher dielectric constant than air. true