Resistance of a water heaters heating element?

  • #1
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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

Homework Equations


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...
 

Answers and Replies

  • #2
20,578
4,425
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
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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?
 
  • #4
20,578
4,425
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?
 
  • #5
321
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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?
 
  • #8
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What is the mass in kg of 40 gallons of water?
I think the asker had the converted number already [emoji846]. They wrote 151 kg.
 
  • #9
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Anyhow, using the heat you can now calculate the power (J/s). See that the heat is the 'J' and the time you have the 's'.

Now think about how you can get current!
 

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