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ELectricity power

A 1 Kohm resistor is to be connected to a supply voltage of 25 V. Calculate the current that will flow and the required power rating for the resistor. What would the required power rating be if the supply voltage were 20 V.

I have done the calculation using P=VI but the exact values do not match the answer that I have been given. So I get, I= 0.025 amps, for 25 V p= 0.625 W ( given answer is 1W), for 20V P=0.4 W (given answer is 0.5W). How have they rounded this?
Original post by Outofthisworld
A 1 Kohm resistor is to be connected to a supply voltage of 25 V. Calculate the current that will flow and the required power rating for the resistor. What would the required power rating be if the supply voltage were 20 V.

I have done the calculation using P=VI but the exact values do not match the answer that I have been given. So I get, I= 0.025 amps, for 25 V p= 0.625 W ( given answer is 1W), for 20V P=0.4 W (given answer is 0.5W). How have they rounded this?

You havent included the question - in the real world resistors are bought out of a catalogue and come in a range of standard maximum power ratings...

if it's a more realistic type of question it might have stated what power ratings resistors are available in... and you'll have to pick a resistor with a max power rating greater than the calculated power (otherwise the resistor would get too hot)
Original post by Joinedup
You havent included the question - in the real world resistors are bought out of a catalogue and come in a range of standard maximum power ratings...

if it's a more realistic type of question it might have stated what power ratings resistors are available in... and you'll have to pick a resistor with a max power rating greater than the calculated power (otherwise the resistor would get too hot)



"A 1 kW resistor is to be connected to a supply voltage of 25 V. Calculate the current that will flow and the required power rating for the resistor. What would the required power rating be if the supply voltage were 20 V." This is just the question as it was given. Can you please explain what you mean by the resistor with max power rating and power ratings ?
Original post by Outofthisworld
"A 1 kW resistor is to be connected to a supply voltage of 25 V. Calculate the current that will flow and the required power rating for the resistor. What would the required power rating be if the supply voltage were 20 V." This is just the question as it was given. Can you please explain what you mean by the resistor with max power rating and power ratings ?

resistors dissapate energy as heat - there is a limit to the amount of power a particular design of resistor can handle before it gets too hot - that's it's maximum power rating.
it's imporant to not exceed the max power rating of resistors.
Original post by Joinedup
resistors dissapate energy as heat - there is a limit to the amount of power a particular design of resistor can handle before it gets too hot - that's it's maximum power rating.
it's imporant to not exceed the max power rating of resistors.


Okay, so for this question, we wouldn't be able to answer it unless we had the max power rating of a resistor mentioned in the question? But what should I do when I have been given a question like this , do i make a guess?
what is the source of the question?

if it doesn't give you a list of available power ratings in an earlier section you'd not be able to answer it - knowing standard resistor power ratings isn't on the A level syllabus and it'd be a pretty useless thing to memorise imo - if you had a real world application you'd pick from a catalogue in front of you... or more likely a component suppliers website these days.
Original post by Joinedup
what is the source of the question?

if it doesn't give you a list of available power ratings in an earlier section you'd not be able to answer it - knowing standard resistor power ratings isn't on the A level syllabus and it'd be a pretty useless thing to memorise imo - if you had a real world application you'd pick from a catalogue in front of you... or more likely a component suppliers website these days.

Oh, this is for my first year of Uni, a question on a lecture. After what you've mentioned about the catalogue and given power ratings of a resistor r, I went through the lecture again and it say
Common standard ratings are 0.25 W, 0.5 W, 1 W and 2 W for resistor. So sorry, my bad but I guess they will be given in the exam. Also this is under DC circuits, I'm wondering if these common standard ratings for resistors used are only common for DC circuits or AC circuits as well?
Yeah the same resistors are used for AC and DC

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