Resistance

A term called resistance is used to unravel these complications. Resistance is a measure of the difficulty of the flow of electrons in a material. Hence

• insulators have a very high resistance
• conductors have a very low resistance
• semi-conductors are in between these two extremes

(Semi conductors are very important materials in the production of computer chips.)

A battery has a fixed voltage so how much current there is in a particular circuit depends on the resistance of that circuit.

e.g. Consider these two circuits, each of which has an identical battery:

 circuit 1 - low resistance (e.g. short circuit) low resistance High resistance current produced

N.B. It is worth noting that there can be some confusion about the statement that the current is the same everywhere in a series circuit. This is quite true for each of these circuits above. In each circuit, the current is the same everywhere within it and so, for example, no current is 'used up' as it passes through the high resistance in circuit 2. However, the current in circuit 1 is much larger than the current in circuit 2.

A battery is always marked as to its voltage, this provides the energy or the 'push' which causes the electrons to move in one direction around the circuit. As we have already said, the voltage (measured in Volts) is a measure of the energy per unit charge and this may be considered as driving the current.

A battery could not be marked with the size of the current that it puts out as this depends on the resistance of the circuit to which it is connected.

Resistance is measured in a unit called Ohms, after a German physics teacher called George Ohm. The unit is given the symbol W, the Greek letter Omega.

This now allows the current in a circuit to be controlled. For less current, the resistance needs to be high and for more current, the resistance low.

A variable resistor can be introduced into the circuit so that the current can be controlled:

As the resistance of the variable resistor is changed, the size of the current in the circuit also changes and the bulb's brightness alters.

Of course, a more drastic control is obtained by introducing a switch into the circuit which is then off or on.

In a series circuit, as more bulbs are added, the total resistance becomes higher and so the battery puts out less current. This current goes through each of the bulbs which are therefore dimmer.

It is a strange fact about a parallel circuit that the more bulbs that are added in parallel, the lower the total resistance of the circuit becomes! Hence, as more bulbs are added in parallel, the battery puts out a larger current so that, as long as all the bulbs are identical, they all still have enough current through them for each to be at normal brightness.

Children can make circuits from bulbs, buzzers, motors etc. and devise their own switches. It is interesting to note that a bulb glows because of the filament:

On an atomic scale, the moving electrons in the current collide with all the fixed atoms in the circuit which causes these atoms to vibrate more. The filament is just a very thin wire of high resistance and the vibrations due to the collisions with the moving electrons cause it to become hot and emit light.

Contents

Safety, Static Electricity
Charge
Current Electricity, Making a bulb light, Models
Atomic Theory
Conductors and insulators
Switches, Short circuits, Circuit diagrams
Current, Voltage
Electrical circuits
Measuring current
Power
Assessment/Discussion Material
Reference Material