You are Here: Home >< Physics

# Power Equations Watch

1. at what situations do i use these equations?
1)P=I^2*R
2)P=V^2/R
2. (Original post by djmans)
at what situations do i use these equations?
1)P=I^2*R
2)P=V^2/R
In direct current, working out the power dissipated in a resistor.
3. (Original post by morgan8002)
In direct current, working out the power dissipated in a resistor.
also works for AC if it's a resistive load - you will be given RMS values in an exam.

P=I^2R means that if you know any 2 of P,I or R you can work out the third
P=V^2/R means that if you know any 2 of P,V or R you can work out the third

so you can tackle questions like
what's the resistance of a 2000W kettle element that operates at 230V?

or

what's the current in a 800W heater with resistance of 5.5 ohms
4. (Original post by Joinedup)
also works for AC if it's a resistive load - you will be given RMS values in an exam.

P=I^2R means that if you know any 2 of P,I or R you can work out the third
P=V^2/R means that if you know any 2 of P,V or R you can work out the third

so you can tackle questions like
what's the resistance of a 2000W kettle element that operates at 230V?

or

what's the current in a 800W heater with resistance of 5.5 ohms

what's the resistance of a 2000W kettle element that operates at 230V?
for this question cant i just use P=VI
5. (Original post by djmans)
what's the resistance of a 2000W kettle element that operates at 230V?
for this question cant i just use P=VI
those equations are derived from combining Ohms law and P=VI... they don't tell you anything you couldn't get from using P=VI and Ohm's law
6. (Original post by Joinedup)
also works for AC if it's a resistive load - you will be given RMS values in an exam.
Yeah. In its above form it can only be used to find instaantaneous power though.
7. (Original post by djmans)
what's the resistance of a 2000W kettle element that operates at 230V?
for this question cant i just use P=VI
this formula

as well as this one

should be on the formula sheet
8. (Original post by djmans)
what's the resistance of a 2000W kettle element that operates at 230V?
for this question cant i just use P=VI
Just to sum up,

is the equation you are thinking of using right? along with ...

Now the equations you have given are a combination of these two.

If you did:

, Therefore

See how long winded this is? we can combine the equations into one to make this easier.

, but we know that ...
So obviously substitute in and we get a nicer, one step equation to work with:
.

The same can be done for current:
,
.

Using this equation makes the operation doable in one step:
.

TL;DR combining is not a necessity and the end result can be achieved using the standard equations. But it makes it much nicer!
9. (Original post by The-Spartan)
Just to sum up,

is the equation you are thinking of using right? along with ...

Now the equations you have given are a combination of these two.

If you did:

, Therefore

See how long winded this is? we can combine the equations into one to make this easier.

, but we know that ...
So obviously substitute in and we get a nicer, one step equation to work with:
.

The same can be done for current:
,
.

Using this equation makes the operation doable in one step:
.

TL;DR combining is not a necessity and the end result can be achieved using the standard equations. But it makes it much nicer!
thx

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 29, 2016
Today on TSR

### Is this person a genius?

...with these A Level results?

### I think I'm transgender AMA

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.