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# Power Equations

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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

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