Electrical Resistance When the same p.d. is applied across different conductors, different currents flow. Some conductors offer more opposition or resistance to the passage of current. Definition The resistance, R, of a conductor is defined as the ratio of the potential difference V across it to the current I flowing through it. This can be expressed mathematically as: or V=IR

The unit of measurement of resistance is the ohm, . When a p.d. of 1 volt drives a current of 1 ampere, the resistance is 1 ohm.

I-V Graphs for different materials Metals and Alloys At a constant temperature they have a constant resistance. A I-V graph will show a straight line through the origin. I is proportional to V. They are called linear or ohmic conductors i.e. they obey Ohm’s law:

Ohmic conductors will always have a straight line characteristic. The resistance of a metallic conductor does not change with p.d. provided the temperature is constant. Note that the gradient of the line is I / V This is 1 / R (from V=IR and so R = V / I) Ohmic conductors will always have a straight line characteristic.

Filament Lamps What does the I-V characteristic look like? OR or

As the resistance increases, the gradient decreases.

I / mA V I / m A Semi-Conductor Diodes Current passes when the p.d. is applied in one direction, but is almost zero in the opposite direction. V I / m A 0.7V A diode has a small resistance in one direction but has a large resistance when the p.d. is reversed. Diodes are non-ohmic conductors. Note that in Forward Bias, it turns on at about 0.7V.

Remember that in Forward Bias, it turns on at about 0.7V. - + Forward Bias - conv. current Remember that in Forward Bias, it turns on at about 0.7V.

0.7V What is the p.d. across each bulb in these circuits? 3V - 0.7V Forward Bias Reverse Bias I = 0 Almost 3V