# Resistance of a water heaters heating element?

## Homework Statement

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

P = IV
P = I^2R
I = dq/dt

## 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...

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Chestermiller
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?

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?
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?

Chestermiller
Mentor
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?
Yes. Did you not study heat in freshman physics?

Yes. Did you not study heat in freshman physics?
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?

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

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

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