If we measure the potential difference against a component in a circuit and the current through it, we can calculate the resistance by using the relationship Potential difference= current * resistance (Volts, V) (Amperes, A) (Ohms, ? ) Resistance is measured in ohms. A component has a resistance of 1 ohm if a current or 1 amp flows through it when there is a potential difference of 1 volt across it. The symbol ? is sometimes used instead of the word ohms. Resistors are used to reduce the size of electric currents in electronic circuits.
The resistors on a circuit board are the components with the coloured stripes. To help us understand what is happening in the current we use a model of the circuit. Scientists use models when they want to simplify things they are studying- this helps them to make predictions, which they can then test using experiments. Electricity flowing through wires behaves like water flowing through pipes. Water can be pumped through pipes using a pump. This is like electricity being pushed through wires by a battery. A piece of narrow pipes tends to decrease the rate at which the water flows- so this is like a resistance.
Increasing the number of cells in a battery would be like making the pump work faster- this would pump more water flow through more of the pieces of narrow pipes- making the water flow more slowly round the circuit. We can use special electrical components called resistors when we want to make a circuit that contains resistances that are known accurately. To find the resistance of resistors in series, add their individual resistances, and to find the total current through a circuit containing resistors in parallel, add the current through each resistor.
Cells connected in series behave in a similar way to resistors connected in series. Three cells with potential differences of 1. 5 volts, 2. 0 volts and 1. 8 volts will have a combined potential difference of5. 3 volts when connected together in series. The energy losses that occur when electricity is transmitted over long distances are due to the resistance of the wires carrying the current. The rate, at which energy is lost due to resistance, increases more rapidly as more current flows through the wire. Rate at which energy is= (current through wire) ? resistance of wire transferred to heat.
This means that if we increase the current through the wire by a factor of 2 we increase the rate at which energy is lost by a factor of 2? =4. Resistance occurs when the electrons travelling along the wire collide with the atoms of the wire. These collisions slow down the flow of electrons causing resistance. Resistance is a measure of how hard it is to move the electrons through the wire. Electricity is conducted through a conductor, in this case wire, by means of free electrons. The number of free electrons depends on the material and more free electrons means a better conductor, i. e. it has less resistance.