0

1. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Other Resistors
Learning Outcomes
• I know how current varies with voltage for DIODE (forwards and
backwards)
• I know how the Resistance of an LDR changes with brightness
• I know how the Resistance of an Thermistor changes with temperature
Summary of Resistance so Far
Filament Lamp
We can draw the voltage and current being negative which refers to the voltage
and current running in the opposite direction (reverse bias).
I / A
V /v
The Filament lamp:
 Non-Ohmic
 Has a higher resistance at higher currents
 Has the same behaviour for forwards and reverse current.
1

2. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Fixed Value Resistor
I / A
V /v
The Fixed Value Resistor:
 Ohmic
 The same resistance at all different currents
 Has the same behaviour for forwards and reverse current.
2

3. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
The Semi-Conducting DIODE
Research Question:
Quickly investigate how the diode conduct both forwards and backwards. Is the
diode OHMIC?
Apparatus
We can again use a ‘test circuit’.
Method
1. Set up the circuit as shown
2. Select a potential difference on the powerpack to get the voltage close to 0.2V,
Measure and record the current.
3. Continue for voltages of 0.2V to 0.8V and then in the opposite direction;
-0.2V to -0.8V
4. (Do this quickly - it will be fiddly and will not work well with this equipment)
3
A
V

4. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Results
 Only spend two minutes on this – it is very difficult with this equipment.
 As a start just set the PD to 0.4V and see what happens forwards and
backwards.
Voltage Current Current Average Current
/ V Repeat 1 / A Repeat 2 / A / A
0.00 0.00
0.20
0.40
0.60
0.80
-0.20
-0.40
-0.60
-0.80
Semi-Conductor DIODE
4
I / A
V / V
Forwards Bias
Reverse bias

5. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
The Diode:
 Non-Ohmic
 The diode has low resistance and will …conduct…. when in …forwards…
bias and very high resistance and does not …conduct…….. in …
reversed….. bias (backwards).
 The diode is electrically, a “…one…-…way… street”.
Uses of a semi-conducting diode:
• Rectification of mains electricity to charge batteries (I-pods P-pods, laptops,
mobile phones etc….)
• *Some diodes can emit light and are
LEDs and are often used to show us
when a circuit is conducting (battery
chargers for mobile phones etc…)
5

6. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Light-Dependent Resistor (LDR)
Resistance
/ Ω
Light Intensity /
W/m2
(brightness)
The LDR:
 Is a better conductor in …bright... conditions
 It has lower resistance in …bright... light and higher resistance in …dark….
conditions.
 It is also a semiconductor (eg. Silicon)
Uses:
• Light meter in a camera
• Burglar alarm (sets off the alarm when the safe is opened and light enters)
6

7. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Thermistor
Resistance
/ Ω
Temperature / K
The Thermistor:
 Is a better conductor when it is …hot temperatures …
 It has lower resistance at …hot… temperatures and higher resistance at …
cold… temperatures.
 It is also a semi-conductor (eg. Silicon)
Uses:
• Electronic Thermometer (has a huge range of
temperatures)
• Cooking temp probe.
• Thermostat (controls household heating and aircon
systems)
7

8. FUNDAMENTAL Physics – Circuits: 3.8 Other resistors
Extension
How can a semi-conductor change its resistance?
Metals have lots of free electrons and are good conductors.
Non-Metals (eg. Plastics) have no free electrons and are insulators.
Think of the electrons in the semi-conductor:
• Semi-conductors have a limited number of free …electrons…..
which is why they are semi-conductors.
• Some semi-conductors can release electrons to create more free
electrons.
• Extra free electrons are released by light energy (photons) in
LDRs.
• Extra free electrons are released by thermal energy in Thermistors.
• The increased number of free electrons means that the device is a
better conductor and has lower resistance.
Homework – Revise for the test.
8