# Electric Circuit Help (Emf and Internal Resistance)

1. Nov 22, 2007

1. Find the current in, and the potential difference across, each resistor in the circuit shown below:

The circuit is a simple parallel circuit with 2.0 ohms at the first resistor, 10.0 ohms at the second, and 20.0 ohms at the third

The problem also gives you Emf which is 12.0 V and the internal resistance "r" which is 0 ohms

2. Voltage = Current*Resistance
Possibly: Voltage = Emf - Current*Resistance
Req (total resistance) = [(1/Resistance 1) + (1/Resistance 2) + (1/Resistance 3)]-1

3. I would attempt the problem but I've tried so many times and I'm just lost. I was absent the day this question was answered in class.

I thought you were supposed to find the Req first then use the voltage given to find the current in the system, but thats not helping me at all. I don't understand Emf or internal resistance so I'm not sure of its purpose.

I hope this helps. I'm on a laptop so I can't scan any images at all.

2. Nov 22, 2007

### stewartcs

In parallel circuits the voltage through each branch is the same. Use Ohm's Law to find the current.

3. Nov 23, 2007

### Bill Foster

I don't believe that's correct.

$$\frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_2}$$

$$R_{eq}=\frac{R_1 R_2 R_3}{R_1 R_2 + R_1 R_3 + R_2 R_3}$$

This isn't really needed to find the current through each resistor.

Ohm's law, applied in this case:

$$V = I_1 R_1 = I_2 R_2 = I_3 R_3$$

Therefore...

$$I_1=\frac{V}{R_1}$$
$$I_2=\frac{V}{R_2}$$
$$I_3=\frac{V}{R_3}$$

You can use $$R_{eq}$$ to see if you got it right...

$$V=I R_{eq}$$
$$I=\frac{V}{R_{eq}}=V\frac{R_1 R_2 + R_1 R_3 + R_2 R_3}{R_1 R_2 R_3}$$

and...

$$I = I_1 + I_2 + I_3$$

Last edited: Nov 23, 2007