Transcript P2 Topic 2 LDRs_and_thermistors_and__diodes

Current (A)
Explaining the shape of the currentvoltage graph for the bulb
Voltage (V)
Cold metal = low resistance
A low resistance means that the
electrons can move easily, therefore
the current will be high.
Hot metal = high resistance
If the metal is heated, the metal ions
will vibrate more. This means that there
is a greater chance of an electron
bumping into a metal ion and so the flow
of electrons is reduced. i.e. the
resistance is increased.
The Diode
The diode is a non-ohmic conductor.
to flow in one direction only.
Doesn’t allow the current to flow
Allows the current to flow
in the forward direction
(has infinite resistance in the reverse direction)
Current (A)
Plotting a current-voltage
graph for a diode
produces this shape line
It allows the current
diode
Voltage (V)
which bulb will light up (notice the position of the diodes).
A
B
C
Circuit 1
answer
A
B
C
Circuit 1
A & B only
which bulb will light up (notice the position of the diodes).
A
B
C
Circuit 2
answer
A
B
C
Circuit 2
None of them
which bulb will light up (notice the position of the diodes).
A
B
C
Circuit 3
answer
A
B
C
Circuit 3
A & C only
LDRs and Thermistors
LDR investigation
You will carry out an investigation to measure the resistance of an LDR at
different light intensities.
An LDR is an electric component whose resistance varies with light intensity.
Method:
•
Connect the LDR to a multimeter and set it to 20kΩ,
•
Place a desk lamp so that the bulb is 10cm away from the LDR and record the
resistance
•
Vary the desk lamp distance and measure the resistance accordingly.
•
Light intensity can be expressed as an ordered variable: bright, medium and dim.
•
Plot a graph of resistance against light intensity.
While carrying out the experiment:
Explain how you are keeping your test fair
Explain how you are making your readings accurate
Draw a diagram of the experiment
Thermistor investigation
You will carry out an investigation to measure the resistance of a thermistor at
different temperatures.
A thermistor is an electric component whose resistance varies with temperature.
Method:
1. Heat 250ml of water using a bunsen burner to 80ºC then turn the bunsen burner off,
2. Immerse the thermistor in the hot water,
3. Connect the thermistor to a multimeter and set it to 20kΩ,
4. Record the temperature of the water as it cools down
5. Record the resistance of the thermistor for each corresponding temperature
6. Plot a graph of resistance against temperature.
While carrying out the experiment:
Explain how you are keeping your test fair
Explain how you are making your readings accurate
Draw a diagram of the experiment
Current (A)
Resistance
Thermistor
temperature
high temperature
low temperature
Voltage (V)
LDR
Light intensity
Current (A)
Resistance
Thermistor – resistance DECREASES when temperature INCREASES
light
dark
Voltage (V)
Light dependant resistor – resistance DECREASES when light intensity INCREASES
Thermistors and LDRs can work as Sensors: A sensor is a component that
allows a physical quantity to change its resistance and hence control the
amount of current flowing in a circuit.
A
A
Light Dependent Resistor/ LDR: Its
resistance decreases as the light
intensity increases. (The higher the
illumination, the higher the current
Temperature Dependent Resistor
/Thermistor: Its resistance decreases
as the temperature increases. (The
higher the temperature, the higher the
current).
In this circuit, the LDR can work as
a lightmeter. The higher the
ammeter reading, the higher the
illumination.
In this circuit, the thermistor can work
as a thermometer. The higher the
ammeter reading, the higher the
temperature.