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Further electricity for OCR A-level Physics

This page covers the following topics:

1. Conservation of charge and energy
2. Electromotive force
3. Capacitance

The Law of conservation of charge states that, for an isolated system, the net charge will remain constant. This is because, whenever a charge is created, an equal and opposite charge is created simultaneously. The Law of conservation of energy states that energy cannot be created or destroyed, but can only be converted from one form to another.

The electromotive force of a cell, emf, ε, is the energy per unit charge converted into electrical energy by the cell and is measured in Volts. The terminal potential difference is the potential difference between the two ends of a cell in a closed circuit. The internal resistance of the cell opposes the charge that flows through it, therefore some of the energy converted by the cell will be used up to overcome this resistance. The following relationship can be constructed for the cell of a closed circuit: ε = V + Ir.

Capacitors are used to store charge and energy. They are made up of two parallel conducting plates separated by an insulator. When connected to a cell, charge flows from the negative terminal to one of the plates and from the other plate to the positive terminal until the voltage across the capacitor is equal to that across the cell. The plates of the capacitor will always have equal and opposite charge and no charge will flow between the plates, thus storing charge. Capacitance is the ability to store electrical energy and is calculated in Farads (F) using the following formula: Capacitance = Charge//Voltage. Since energy is used to charge capacitators, they store energy. The amount of energy stored by capacitators can be calculated using the following formula: E = (1/2)QV = (1/2)CV².

1

State the Law of conservation of charge.

The Law of conservation of charge states that, for an isolated system, the net charge will remain constant.

2

Define electromotive force.

The electromotive force of a cell is the energy per unit charge converted into electrical energy by the cell.

3

Calculate the potential difference across a 0.0004 F capacitor with a charge of 0.0005 C on it.

Using C = Q/V, 0.0004 F = 0.0005 C/V, therefore V = 1.25 V.

4

Use the conservationn of charge to describe the charge flowing out of a battery and the charge flowing back into it at the end of a loop through a circuit.

The Law of conservation of charge states that, for an isolated system, the net charge will remain constant. Therefore, the charge flowing into and out of the battery are equal.

5

A cell of emf 3.5 V and internal resistance 0.8 Ω is connected to a 15 Ω resistor. Calculate the current through the resistor.

Using ε = V + Ir and Ohm's Law, 3.5 V = I(15 Ω) +I(0.8 Ω), so 3.5 V = 15.8I, therefore I = 0.222 A (to 3 significant figures).

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