The resistance of an object is a measure of the how much current is to flow through that object for a given potential difference across it. It is given the symbol R and has the unit Ω ,which is a Greek letter omega and provided across it. The bigger the potential difference across its ends the bigger the current flow. The equation below is used to work out the resistance of a component from measurements of the current flowing through it and the potential difference across its ends.

Can you sketch a circuit diagram of a suitable circuit you could use to find out such information ?

You should be able to!

Where V = potential difference in volts (V)

I = current in amps (A) and

R = resistance in ohms (Ω)

Current is the rate of flow of charge. As soon as a potential difference is applied the electrons generally drift in one direction. The bigger the potential difference the faster their average speed and the more charge will move past a point in a second - bigger current ! Increasing the number of electrons experiencing the slope will mean more flow past a point in a second and so more charge passes a point in a second and more current flows.

There are four factors that affect the resistance of a wire:

♥ Resistance is proportional to length. If you take wires of different lengths and give each a particular potential difference across its ends. The longer the wire the less volts each cm. of it will get. This means that the 'electric slope' that makes the electrons move gets less steep as the wire gets longer, and the average drift velocity of electrons decreases. The correct term for this 'electric slope' is the potential gradient. A smaller potential gradient means current decreases with increased length and resistance increases.

♥ Resistance is inversely proportional to cross-sectional-area. The bigger the cross sectional area of the wire the greater the number of electrons that experience the 'electric slope' from the potential difference. This does not give rise to a straight line graph as cross sectional area is inversely proportional to resistance not directly proportional to it.

♥ Resistance depends on the material the wire is made of. The more tightly an atom holds on to its outermost electrons the harder it will be to make a current flow. If a shell is almost full, the atom is reluctant to let its electrons wander and the material is an insulator. If the outermost shell is less than half full then the atom is willing to let those electrons wander and the material is a conductor.

♥ Resistance increases with the temperature of the metallic wire. When a material gets hotter the atoms present in the material vibrates more. This makes it difficult for the electrons to move without interaction with an atom and increases resistance. The relationship between resistance and temperature is not a simple one.

R = R_{o}(1 + α.T)

α is the thermal resistance coefficient.

In Brief: For same material resistance decreases as the thickness of the wire increases, resistance increases as the length of the wire increases and resistance changes with temperature also