The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Power Equations

at what situations do i use these equations?
1)P=I^2*R
2)P=V^2/R
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.
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
Reply 3
Avatar for djmans
djmans
OP
2
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
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
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.
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

P=V2R P=\dfrac{V^2}{R}

as well as this one

P=I2RP=I^2R

should be on the formula sheet
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,

P=VIP=VI is the equation you are thinking of using right? along with V=IRV=IR...

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

If you did:
P=2000W;V=230VP=2000W; V=230V
I=PV=2000230≈8.7I=\dfrac{P}{V} = \dfrac{2000}{230} \approx 8.7

V=IRV=IR, Therefore
R=2308.7≈26.45R=\dfrac{230}{8.7} \approx 26.45

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

P=VIP=VI, but we know that V=IRV=IR...
So obviously substitute in and we get a nicer, one step equation to work with:
P=I2RP=I^2R.

The same can be done for current:
P=VIP=VI, I=VRI=\dfrac{V}{R}
∴P=V2R\therefore P=\dfrac{V^2}{R}.

Using this equation makes the operation doable in one step:
R=23022000=26.45R=\dfrac{230^2}{2000} = 26.45.

TL;DR combining is not a necessity and the end result can be achieved using the standard equations. But it makes it much nicer! :smile:
Reply 8
Avatar for djmans
djmans
OP
2
Original post by The-Spartan
Just to sum up,

P=VIP=VI is the equation you are thinking of using right? along with V=IRV=IR...

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

If you did:
P=2000W;V=230VP=2000W; V=230V
I=PV=2000230≈8.7I=\dfrac{P}{V} = \dfrac{2000}{230} \approx 8.7

V=IRV=IR, Therefore
R=2308.7≈26.45R=\dfrac{230}{8.7} \approx 26.45

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

P=VIP=VI, but we know that V=IRV=IR...
So obviously substitute in and we get a nicer, one step equation to work with:
P=I2RP=I^2R.

The same can be done for current:
P=VIP=VI, I=VRI=\dfrac{V}{R}
∴P=V2R\therefore P=\dfrac{V^2}{R}.

Using this equation makes the operation doable in one step:
R=23022000=26.45R=\dfrac{230^2}{2000} = 26.45.

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


thx

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you