# Homework Help: D.C. Circuits

1. Apr 25, 2010

### Punch

1. The problem statement, all variables and given/known data

A relay switches on when the current through the coil reaches 0.0048A.

In a circuit, a battery of 12V is connected in series with this relay it switches on when another resistor of 800 Ohm is connected to it.

Calculate the resistance of the relay.

2. Relevant equations

3. The attempt at a solution

V= IR
12V = 0.0048A * (800 ohm + coil resistance)

12V / 0.0048A = 800 ohm + coil resistance

coil resistance = 2500 ohm - 800 ohm
=1700 ohm

However, I am not sure if I am on the right track....

Last edited: Apr 25, 2010
2. Apr 25, 2010

### ideasrule

Is the circuit with the battery and the resistor supposed to deliver 0.0048A? If so, your solution is right.

3. Apr 25, 2010

### Punch

from the first line, "A relay switches on when the current through the coil reaches 0.0048A."

so can i conclude that the current passing through the coil has reached 0.0048A? which means that current in the circuit is 0.0048A

however, this question is worth 4 marks and it seems to easy for a usual 4 mark question...