# Resistance of a water heaters heating element?

1. Jul 23, 2017

1. The problem statement, all variables and given/known data
Household water heaters use a 240 V rather than a 120 V source. What is the resistance of a water heater's heating element if it heats 40 gallons (151 kg) of water from 15 C to 60 C in 15 min

2. Relevant equations
P = IV
P = I^2R
I = dq/dt
3. The attempt at a solution
Honestly no clue. If anyone can give me some hints I would really appreciate this.

So far I have electrical units, V, and then I have kg, Celcius, and seconds. I'm not sure how the given equations help me relate the units at all.

I need current and power, but those units do not help me at all.

I know I = dq/dt, and I thought 45 C / 900 s looks very similar to current but the units don't make sense to me...

2. Jul 23, 2017

### Staff: Mentor

Do you know how to determine how much heat it takes to raise the temperature of 1 kg of water from 15 C to 60 C?

3. Jul 23, 2017

I'm not sure actually.. this chapter that I'm studying is called Current and Resistance..

It has to do with specific heat, right? Also, does heat = thermal energy?

4. Jul 23, 2017

### Staff: Mentor

Yes. Did you not study heat in freshman physics?

5. Jul 23, 2017

I don't think I studied it enough..

So if the specific heat of water is 4.187 x 10^3 J/(kg * k )

That means it requires 4.187 x 10^3 J to raise 1 kg of water by 1 kelvin, correct?

6. Jul 23, 2017

### scottdave

Yes, heat = thermal energy. To raise a kg water 1°C, you need 4184 Joules of heat (thermal energy).

7. Jul 23, 2017

### Staff: Mentor

What is the mass in kg of 40 gallons of water?

Last edited: Jul 23, 2017
8. Jul 23, 2017