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Current-Potential Graph for a filament
bulb
Use your apparatus from last lesson.
This time you are going to use your rheostat as a
variable resistor to vary the current through a
filament bulb.
But you will measure the voltage across the bulb
this time as the current varies.
Now replace an LED for the bulb and
repeat the investigation. Record
and plot your results. Turn the
diode, now what do you notice.
Next replace the LED with a
Thermistor, but repeat the
investigation at two temperatures
of water. Record and plot your
results.
Next replace the Thermistor with an
LDR, but repeat the investigation
at two different light intensities
(Cover with different thicknesses
of tracing paper. Record and plot
your results.
Variable Resistors
The electronic thermometer
To CALIBRATE the thermistor, you must expose it to a range of temperatures that you
can measure using a mercury thermometer.
The thermistor is surrounded by HOT water in a small beaker.
The resistance will show a much LOWER value than at room temperature.
A thermometer is placed in the water.
As the water cools, the resistance rises and results should be recorded at regular
intervals of temperature.
To test your calibrated electronic thermometer, move your thermistor and meter to
another beaker of water which your teacher will have.
Allow it to read the resistance.
Use your graph to give the temperature.
Tell the teacher your answer.
Light Dependant Resistor
LDRs (light-dependent resistors) are used to detect light levels, for
example, in automatic security lights. Their resistance decreases as
the light intensity increases:
LDR symbol

In the dark and at low light levels, the resistance of an LDR is
high, and little current can flow through it.

In bright light, the resistance of an LDR is low, and more
current can flow through it.

LDRs are also useful for controlling how long the shutter should
remain open on a digital camera. Changes in the resistance are
measured and, if the light level is low, the shutter stays open for
longer than if the light level is high.
Record how resistance varies with light intensity.
Add sheets of tracing paper over an LDR, record in a table how
resistance varies with the number of sheets of tracing paper. Plot a
graph of your results.
Number of Resistance 1
sheets of
(Ohms)
tracing paper
Cover Work –
Resistance 2
Resistance 3
(Ohms)
(Ohms)
Book Work Nelson Thornes
Resistance
Average
(Ohms)
p.154-155
Variable Resistors
Copy the graph of current – potential for a resistor (figure 3 p. 155) and label it .
Define what an ohmic conductor is, how does resistance vary with the direction of
current?
Answer Qs 1 and 2 p. 155 in full in your book.
More Current-Potential Graphs p. 156-157
Draw how the current and potential difference varies through a filament bulb (Fig
1). Use the information to describe why the graph is this shape.
Use R = V/I and the graph to answer question a.
Draw how the current and potential difference varies through a diode (Fig 2). Use
the information to describe why the graph is this shape.
Answer question b
Draw how the current and potential difference varies through a Thermistor and LDR
(Figs 4 & 5). Use the information to describe why the graph is this shape.
Answer Questions c & d
Complete the summary questions on page 157 in full.