A device for heating a cup of water in a car connects to the car\'s battery, whi

Question

A device for heating a cup of water in a car connects to the car's battery, which has an emf EMF = 10.0 V and an internal resistance r_int = 0.05 ohms . The heating element that is immersed in the cup of water is a resistive coil with resistance R.

David wants to experiment with the device, so he connects an ammeter into the circuit and measures 12.0 A when the device is connected to the car's battery. From this, he calculates the time to boil a cup of water using the device. If the energy required is 100 kJ, how long does it take to boil a cup of water?

Neglecting heat losses in the system, what is the amount of time t will it take to boil the water in the cup?

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Explanation / Answer

resistance = 10/12 = 0.833 ohms = Rint + R = 0.05 + R hence R = 0.783 ohms 100000 = 12^2 *0.783 *t hence t = 886.9 seconds

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