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# Internal Resistance and EMF watch

1. For part (b)

Increasing the resistance of the rheostat would increase the PD across it and decrease the PD across the internal resistance.

Hence, the Voltmeter across PQ will tend to a value that is closer to the actual EMF of the battery.

Would this be an acceptable answer ?
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2. The current in the starter motor, when first switched on, is 160 Amps.

Explain why the headlights appear dimmer when the S is closed.

Sidelight rating : 6W, 12V

Why would the headlights become dimmer ?
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3. BUMP
4. (Original post by Ari Ben Canaan)
For part (b)

Increasing the resistance of the rheostat would increase the PD across it and decrease the PD across the internal resistance.

Hence, the Voltmeter across PQ will tend to a value that is closer to the actual EMF of the battery.

Would this be an acceptable answer ?
I'd say yes, it's correct Only you could mention that increasing external resistance will make current decrease, hence the pd across internal resistance decreases.
5. (Original post by Ari Ben Canaan)
The current in the starter motor, when first switched on, is 160 Amps.

Explain why the headlights appear dimmer when the S is closed.

Sidelight rating : 6W, 12V

Why would the headlights become dimmer ?
When you have closed S, part of the current that previously went through headlights now goes through the motor M. Power dissipated in the headlights is proportional to the square of current going through them, therefore they become dimmer when the current decreases.
6. (Original post by jaroc)
When you have closed S, part of the current that previously went through headlights now goes through the motor M. Power dissipated in the headlights is proportional to the square of current going through them, therefore they become dimmer when the current decreases.
But I thought the point of connecting components in parallel was to ensure each one received the voltage and current it required/was rated for.
7. (Original post by Ari Ben Canaan)
But I thought the point of connecting components in parallel was to ensure each one received the voltage and current it required/was rated for.
You would have to do numerical calculations to check whether the lights are working at nominal ratings.

I did not refer to this at all. I just said that if you add resistance in parallel, smaller current will go through elements that already are in the circuit.

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