# Homework Help: Water equivalent of a kettle

1. Mar 4, 2015

### ande

1. The element of a heater kettle has a constant resistance of 105 ohms and applied voltage of 240V.

1. Calculate the time taken to raise the temperature of water from 15 degrees Celsius to 90 assuming that. 80% of the power input to the kettle is usefully employed. I worked this out and got 11 min 56sec which is the same answer that appears on the textbook

2. If the water equivalent of the kettle is 150g, find how long it will take to raise a second litre of. Water through the same temp range immediately after the first time. I get 10min 08sec but the textbook says 10min 58sec. Please shed some light

2. Q= mcdt P= V^2/R and efficiency = Po/Pi

3. I attempted the question by subtracting the equivalent mass from the mass of water thus giving a net mass of 0.85 but the answer I got doesn't correspond with the textbook. Assume 1L = 1kg

2. Mar 4, 2015

### Bystander

This is a clue.
Think about the assumption you made to get this number, then try it again.

3. Mar 4, 2015

### ande

Correct me of I'm wrong but if the second litre is heated immediately after the first, 150g of the system is theoretically at 90 degrees correct? You'll have to pardon my reasoning, I'm a first year electrical engineering student.

4. Mar 4, 2015

### Bystander

Figure the capacity of the kettle is at least two liters. If you can heat one liter plus the kettle in time x, at whatever efficiency, if the efficiency is constant, you can heat two liters plus the kettle in what time? What is the difference between the two times?