Ohm's Law and Resistance
Ohm's law and the definition of resistance comes from looking at what happens to the current through a component when you change the voltage across it. You use the circuit below. The variable resistor or rheostat can be used to change the voltage.
Let's begin with a resistor.
Let's begin with a resistor.
If we plot a graph of our results, we get a straight line. Comparing this to the equation for a straight line, we find that voltage is directly proportional to current or
Voltage (V) = Current (A) x Resistance (Ohms) The constant was callled resistance and it can be thought of as how much energy each electron needs to get through the resistor or how much it slows the electrons down. This is known as Ohm's Law 
You need to know the VoltageCurrent graphs for two other components: a diode and a filament light bulb.
No current flows in a diode if the voltage is negative. Once the voltage is positive, currently only flows after a particular point called the switch on voltage, and then increases with voltage.

You can see that for a very small region near the origin there is a straight line and in this 'Ohmic region' Ohm's law is obeyed. When the voltage gets too high, the rate of current increase decreases: the graph begins to curve. This is because the filament in the bulb gets hot. This causes the metal atoms to vibrate faster and further from their positions, which means the electrons collide with them more and so are hindered more: this increases the resistance.
Current can flow in both directions through a filament bulb. 