Special resistors
Book page 84 – 87
Syllabus2.12 – 2.13
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Multiple-choice quiz
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What am I?
LDR, LED or thermistor?
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Starter
LDR’s and thermistors are both types of resistor which can
change their resistances.
What change in conditions do you think might cause each to
change its resistance?
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Aim
• Describe how resistance varies with conditions for
LDR’s and thermistors, and how this can be
investigated experimentally
Key words
• LDR (Light Dependent Resistor) 조명 의존형
저항기
- diode
• Thermistor 서미스터, 저항기
• Conditions 상태 , 조건
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How do street lights automatically switch on and off?
The big picture
Have you ever wondered?
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What is a diode?
A diode is a component made
from a semiconducting material
such as silicon.
Diodes have the unique
property of only allowing
current to pass through them
in one direction.
The arrowhead on the symbol
shows the direction of current flow.
Some diodes emit light when they
conduct electricity. These are called
Light Emitting Diodes (LEDs).
LDRs LEDs and Thermistors
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diode and its symbol
LDRs: light and resistance
The resistance of a light
dependent resistor (LDR)
is not fixed. It is dependent
on the intensity of incident light.
An LDR has a high
resistance in the dark but a
low resistance in the light.
LDR symbol
resistance (k)
The graph shows how the
resistance of an LDR decreases
as the light intensity increases.
light intensity
This means that LDRs can be
used in light sensing circuits,
because their output is dependent
on the light conditions.
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Thermistors: temperature and resistance
The resistance of a
thermistor varies
depending on temperature.
It has a high resistance
when cold but a low
resistance when hot.
thermistor
symbol
resistance (k)
This is unusual, as resistance
normally increases with
increasing temperature.
temperature (°C)
Thermistors are useful in the
sensor circuits of a
thermostat, as their output
varies with temperature
fluctuations.
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LDR Task
You have:
- 1 LDR
- An ammeter
- Power packs and wires
- 3 pieces of paper of known thickness
I have 3 pieces of paper of unknown thickness.
You need to tell me:
Easy: How the resistance of an LDR changes with light level
Harder: The thicknesses of my pieces of paper
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Thermistor Task
You have:
- 1 Thermistor
- An ammeter
- Power packs and wires
- 3 containers of water at known temperatures
I have a glass of water of unknown temperature.
You need to tell me:
Easy: How the resistance of a thermistor changes with
temperature
Harder: The temperature of my water.
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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
Light Dependent Resistor/ LDR: Its
resistance decreases as the light
intensity increases. (The higher the
illumination, the higher the current
A
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
In this circuit, the thermistor can work
a lightmeter. The higher the
as a thermometer. The higher the
ammeter reading, the higher the ©cgrahamphysics.com
ammeter
reading, the higher the
2016
illumination.
temperature.
Summary
• Thermistors have a resistance that decreases
with increasing temperature
• Resistance of an LDR decreases as greater
levels of light fall on it.
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Plenary: Resistance of LDR and thermistor
1. Match the follow electrical symbols for components with their
names:
cell
power supply
filament lamp
switch
fixed resistor
voltmeter
V
2. Draw the symbols for the following components:
a) LDR
b) variable resistor
c) thermistor
d) ammeter
A
3. As the intensity (brightness) of the light increases, the resistance of
increases / decreases
the LDR _________________
4. As the temperature increases, the resistance of the thermistor
increases
/ decreases
_________________.
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Resistor quiz
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which bulb will light up (notice the position of the diodes).
Circuit 1
A
B
C
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answer
Circuit 1
A & B only
A
B
C
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which bulb will light up (notice the position of the diodes).
Circuit 2
A
B
C
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answer
Circuit 2
None of them
A
B
C
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which bulb will light up (notice the position of the diodes).
Circuit 3
A
B
C
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answer
Circuit 3
A & C only
A
B
C
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Key words
• LDR (Light Dependent Resistor) – A type of variable
resistor that is sensitive to light intensity. As light
intensity increases, its resistance decreases
• Diode – A resistor that only allows current to flow
through it in one direction
• Thermistor – A type of variable resistor that is sensitive
to temperature. As temperature increases, its
resistance decreases
• Conditions – an event will happen if some other thing
is taken place. If temperature increases (condition)
then resistance will decrease (thing taken place)
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