Electrical Resistance

1
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 VIR
2
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.
3
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 Ohms law
4
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 VIR and so R V / I)
Ohmic conductors will always have a straight line
characteristic.
5
Filament Lamps
What does the I-V characteristic look like?
OR
or
6
As the resistance increases, the gradient
decreases.
7
Semi-Conductor Diodes
I /
mA
Current passes when the p.d. is applied in one
direction, but is almost zero in the opposite
direction.
V
m
I /
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.
8
-

Forward Bias - conv. current
Remember that in Forward Bias, it turns on at
about 0.7V.
9
What is the p.d. across each bulb in these
circuits?
0.7V
3V - 0.7V
2.3V
Forward Bias
Reverse Bias
3V
I 0 Almost