# Homework Help: Purchase a heater with a high resistance because P= I^2 R

1. Jan 2, 2012

### heartOFphysic

A student goes out to purchase an electric heater for his flat. The salesman says that, to
get more heat, he should purchase a heater with a high resistance because P= I^2 R
but the student thinks that a low resistance would be best, because P= V^2/R

explain who is correct?

Hmmm I'm rather stumped on this one, any pointers/ tips to head me off in the right direction?

Thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 2, 2012

### MartinCarr

Re: Power

Yes, but looking at those two equations what can you change at home? Voltage or current? In practice all you can change is the current flow

3. Jan 2, 2012

### Staff: Mentor

Re: Power

Consider the power supply in the flat. What are its characteristics?

4. Jan 2, 2012

### heartOFphysic

Re: Power

The current is alternating in a house supply, I guess?

5. Jan 2, 2012

### Staff: Mentor

Re: Power

While that is certainly true, what other specifications does it have?

6. Jan 2, 2012

### heartOFphysic

Re: Power

A current causes a heating effect!

7. Jan 2, 2012

### MartinCarr

Re: Power

In your house you can only change the current. Think of it a different way, something with a high resistance reduces current flow, but gets hot in the process

8. Jan 2, 2012

### Pengwuino

Re: Power

Sure, but what can you change at the flat? The power that you receive when you plug into an outlet is fixed at 120V/240V/whatever, depending on the country you live in. You can't change that.

$P = I^2 R$ is dependent on the current and the resistance. $P = V^2/R$ is dependent only on the voltage. Since the voltage is fixed, the 2nd equation tells you everything you need to know to answer your question. The problem with the first equation is that the current is not fixed and when you change the resistance, the current will change as well, so you can't immediately know "what happens when I change R".