Confused by resistance question

BigCon

In the question there's a 24vdc power supply connected to a bank of 10 6W, 24vdc solenoid valves in another cabinet by a 100m copper cable (resistivity of copper given).
The first question is to calculate the resistance of the cable (no bother).
The next question is to calculate the resistance of each solenoid? How the hell am I supposed to do this, I don't think there's enough info provided?
Any help appreciated...

robfitz

This should be solvable using Watt's Law or similar.

Michael Collins

Yeh it's solvable using Joule's Law. The trick is that the solenoids consume 6 W when an ideal voltage source of 24 V is placed across them.

You can work out the resistance from this.

mawk

sorry.. maybe im way off here but

p=VI

so

I=6/24

.25

and
p=r X I^2

=>

6/.25^2=r

r= 16ohms

sum R's = 160ohms?

am i missing something important? probably.. im pretty tired

Michael Collins

All good except for your calculations! But that's the method alright.

You don't need to find the current really, just use P=V²/R.

No need to add up the resistances. The question wants in individual resistance of each coil, which is the same for each and

close to 100 ohms!

mawk

Michael Collins wrote:

All good except for your calculations! But that's the method alright.

[/SPOILER]

well i did them in my head at two in the morning..

BigCon

Cheers Guys, looks obvious now...

mawk

yeah no problem, any time