internal resistance

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Manna

Cards (7)

The internal resistance of a cell causes a loss of voltage before the circuit begins.
Most cells have some internal resistance, they reduce the voltage output of the cell before it reaches the circuit.
Internal resistance arises because the cell is made up of chemicals which will resist the flow of charge.
Explain how we measure the emf of a cell.
Connect the cell to two wires, one to the positive and one to the negative terminal.
Use a voltmeter to measure the voltage between these two wires, this is known as the open circuit voltage and is the same as the EMF.
Potential dividers can be used to supply constant or variable potential differences from a power supply.
Often in electronics, the power supply has a potential difference that is too high for our intended use.
Another problem is that we might want a variable potential difference when we only have a constant potential difference power supply.
A potential divider circuit uses the resistance of a resistor to change the potential difference across a component.
When the second resistor has a high resistance, there is a reduction of the potential difference across the component.
If a thermistor is now in place of R1, the potential difference across the component is altered based on the temperature of the thermistor.
When the temperature is high, the thermistor resistance is close to zero and so the component gets nearly the full power supply of potential difference.
When the temperature is low, the component gets a smaller share of the full power supply of potential difference.
If a light dependent resistor (LDR) is now in place of R1, the potential difference across the component is altered based on the light intensity incident on the LDR.
When the intensity is high, the LDR resistance is close to zero and so the component gets nearly the full power supply of potential difference.
When the intensity is low, the component gets a smaller share of the full power supply of potential difference.