Double Award - Science for GCSE

Chapter 27

Currents and circuits

1.	Circuits and flow of charge An electric current is a flow of tiny negatively-charged electrons around an electric circuit. The negative terminal of the cell (or battery) gives up electrons which flow round the circuit to the positive terminal. So, electrons flow from the negative to the positive terminal around a circuit, but the conventional current is shown by an arrow pointing from the positive to the negative terminal on the connecting wires. A battery is two or more cells connected together. The different pieces of equipment in a circuit are called components. Each component is shown by a symbol in circuit diagrams.
2.	Series and parallel circuits In a series circuit, there is only one path for the current. The current is measured in amperes (A) by an ammeter connected in the circuit. Current is not used up as it passes round a circuit. So, in a series circuit, the current is the same at every point. In a parallel circuit, the current has a choice of routes. More current will take the easier path at a junction. The total current going into a junction equals the total current coming out of the junction. Make sure that you understand the information in the diagram below. shows the current at that point, shows the current at its point, etc. All the bulbs in the circuit are identical.)
3.	Cells and bulbs Cells in a circuit provide electrons at the negative terminal with electrical energy. As the electrons move round the circuit, they lose their electrical energy to components as heat, light (in bulbs) and sound (in buzzers and bells). This electrical energy or electrical driving force in a circuit results in a potential difference or voltage between the terminals of a cell. This is measured in volts (V). Bulbs and other components in a circuit hinder the flow of electrons. This causes a resistance which is measured in As the number of bulbs and other components in a circuit increases, the resistance increases. This results in a smaller current. Make sure you understand the information in the diagram below. (All the cells are identical and all the bulbs are identical.)

4.	Ohm’s Law Ohm’s Law states, the voltage across a resistor is proportional to the current through it, provided its temperature is constant. Resistors which obey Ohm's Law have a straight line of voltage against current as in the graph on the right. In filament lamps, the temperature of the wire (resistor) rises sharply as the current increases. This causes the resistance to increase and the gradient of the voltage-current graph rises (diagram A below). Thermistors and light-dependent resistors are made from semiconductors in which the resistance falls as the temperature increases. This means that the gradient of the voltage-current graph falls as the current increases and the temperature increases (diagram B below).