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5.2(ii) - Ammeters, Voltmeters & Potential Dividers

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Voltmeters

They measure the difference in potential between two points so they are connected in parallel.

Ideal VoltmetersThey should have an infinite resistance so that no current is taken from the circuit.

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Ammeters

Ideal AmmeterThis has zero resistance so that it doesn’t lower the current in the circuit.

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Sensors

Thermistor Light Dependent Resistor

Strain Gauge

More light, releases more electrons, which lowers the Resistance.

As it gets hotter, more charge carriers are

released by the material lowering the Resistance

As the strain gauge is stretched the wire inside it

gets thinner, increasing the Resistance.

Light↑ R↓Temperature↑ 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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Potential Dividers

24V

20Ω

20Ω

24V

40Ω

80ΩV=16V MV=12V

Because the resistors are equal the potential difference across each one is equal.

This time the potential is divided into different proportions.

With the thermistor instead of a fixed resistor, the attached motor will go faster when the thermistor gets colder.

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Potential Divider Equation

𝑉 𝑜𝑢𝑡=𝑉 𝑖𝑛(𝑅2

𝑅1+𝑅2

)

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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Automatic Light SwitchHow 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

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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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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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Using a Potential Divider to investigate Ohms Law

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.

This arrow shows that the potential divider can be varied – this is also called a Rheostat.