Watt the hell?

[Tha-Flash]

If a new PC has a power supply rated at 240W with a standard mains supply what current [in Amps] would it draw? � Show your working out.

This is homework and I dont understand it even though teacher has explained it to me like 3 times. I really need help with this.

[kempie]

Power (Watts) = Voltage (Volts) x Current (Amps)

http://en.wikipedia.org/wiki/Watt#Definition

UK mains is nominally rated at 230V (used to be 240V in the good old days).

[Tha-Flash]

ah I actually put 240 down... I'll change it now. Thanks :D

[wildwood]

So, what answer did you get, Tha-Flash.?

1.04 Amps.?

[Tha-Flash]

One of the answers I needed to know was what the standard mains power supply was. But its the maths part which I'm confused on.

I would probably just write 1. :S

[wildwood]

kempie has the formula so we know that 240 = 230 x ???

So we divide 240 by 230 which is 1.04347826.........

[CJ2]

Kempie, don't confuse the issue.
Anyway it depends where you come from. I suspect it may still depend on where you are. Bristol used to be 210V.
Its obvious the standard mains supply in the question was your old one, viz. 240V. No school question really expects you to divide 24 by 23. Its not cricket and cannot be done on one's fingers.

So the answer is the simplest number between 240/210 and 240/250, = 1.0000000 amps , or 1amp.

Tha-Flash, think of a jet of water.
The rate of flow (current) times the pressure (voltage) is the rate of working (rate of power production/consumption, eg. watts) it can achieve. (eg. if using the jet of water, directed horizontally at a water-wheel to turn it.).

So Power = (a constant) x current x voltage.

If the current is specified in amps, the voltage in Volts and the Power in Watts, it just so happens that the (a constant) in the above equation = 1, which is mightily convenient, I hope you'll agree.

And hence, dividing both sides by Volts, we get
Amps = Watts/Volts

[TeeGee]

I think we may be missing the point a little. A PC PSU rated at 240w will provide various voltage and wattages depending upon demand. When you access the hard drive, then that hard drive requires more power ( to move the heads for example ) than when it it just idling.

The answer I think you should give therefore is :-

W=V * A, or A = W / A = 240/240 =1Ampere MAXIMUM>/> .

[kempie]

CJ2 - I do not believe that I have confused the issue as much as you.

I have reproduced the formula to calculate the answer.
I have given the nominal voltage of UK mains electricity.
I have stated a previous UK mains voltage (which happens to fit the equation more neatly).
UK schools do not expect any student to do mental (or digital) arithmetic since the introduction of calculators in the 1980s.

[ducko]

Draw a triangle and put the letter W (watts) in at the top then
bottom left put the letter I (amps) and finaly at the bottom
right put the letter V (volts) put your finger over the one you want to find out and do the sum thats left.
EG. cover amps this leaves watts divided by volts or in your case 240 watts divided by 230 volts answer is
1.043 amps

[Tha-Flash]

Thank you all :)