Electricity tutorial

1. 5.2(ii) - Ammeters, Voltmeters
& Potential Dividers
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2. Voltmeters
They measure the difference in potential between two points so
they are connected in parallel.
Ideal Voltmeters
They should have an
infinite resistance so that
no current is taken from
the circuit.
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3. Ammeters
Ideal Ammeter
This has zero resistance so that it doesn’t lower the current in
the circuit.
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4. Sensors
Thermistor Light Dependent Strain Gauge
Resistor
As it gets hotter, more More light, releases As the strain gauge is
charge carriers are more electrons, which stretched the wire inside it
released by the material lowers the Resistance. gets thinner, increasing
lowering the Resistance the Resistance.
Temperature↑ R↓ Light↑ R↓ Strain↑ R↑
All of these sensors can be used with Variable Resistors and
Potential Dividers to switch Output devices ON or OFF
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5. Potential Dividers
20Ω 40Ω
24V 24V
V=12V V=16V M
20Ω 80Ω
Because the resistors This time the With the thermistor
are equal the potential is divided instead of a fixed
potential difference into different resistor, the attached
across each one is proportions. motor will go faster
equal. when the thermistor
gets colder. 5

6. Potential Divider Equation
You can either calculate Vout using
ratios and your knowledge of
Potential Difference.
OR
You can use an equation (which
does the same thing):
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7. Automatic Light Switch
How does it work?
When light shines on the LDR
the resistance is very low.
This makes Vout low, so the
switch is off.
When it gets dark the LDR’s
resistance increases.
This makes Vout much larger and
the switch comes on.
What simple change would make the switch come on in the light?
Swap R1 and the LDR 7

8. Fire Alarm
When the temperature rises
the resistance of the
thermistor falls.
The potential difference across
the thermistor becomes low.
Most of the potential
difference is now across R2
which turns on the Electronic
Switch.
The electronic switch can be used to ring
a bell or turn on a sprinkler system
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9. Strain Gauge
As the Strain gauge is
stretched by heavy loads
it’s resistance rises.
The pd across the strain
gauge becomes larger.
The voltmeter can be
monitored.
Low values mean large
strains in this circuit.
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10. Using a Potential Divider to investigate Ohms Law
This arrow shows that the potential
divider can be varied – this is also
called a Rheostat.
The Potential Divider allow the Potential Difference across the bulb to be
varied.
The Ammeter measures the current through the bulb.
The Voltmeter measures the PD across the bulb
By collecting pairs of V and I, an Ohms Law graph can be plotted.
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