Batteries / Charging + Resistance.

Discussion in 'Homework Help' started by stewi3, Feb 24, 2013.

1. stewi3 Thread Starter New Member

Feb 24, 2013
2
0
Hey,

Got this question on my tutorial sheet and 2 different teachers have given different answers. No one else in the class really knows for certain which way is right, and it's doing my head in to be honest.

I've attached a picture of the question, and a solution that one teacher gave us.

Could someone please tell me if this is the correct way of working this stuff out?

2. THE_RB AAC Fanatic!

Feb 11, 2008
5,430
1,311
The method looks ok. Check all your calcs and values.

3. #12 Expert

Nov 30, 2010
18,076
9,678
Where Rb = 1.2 ohms and I x Rb = 2 x10,

Maybe it is 1.2 ohms times 10 amps?

4. WBahn Moderator

Mar 31, 2012
23,848
7,379
As The_RB says, the method looks reasonable, but check your calculations.

I wouldn't actually recommend this approach to actually charge the batteries, but for a paper exercise in circuits that's not the point.

What was the other method that was suggested?

5. stewi3 Thread Starter New Member

Feb 24, 2013
2
0
Find Rt for the circuit by doing Total Circuit voltage, Vt =[110 - (V=E-Ir)]

Divided by Total Amps (10). = Total Circuit resistance, Rt

Rt = Rs + Ir
Therefore Rs = x

However, you get a different answer.

6. WBahn Moderator

Mar 31, 2012
23,848
7,379
You KNOW this is wrong because the units don't work out.

Rt and Rs have units of resistance. Ir has units of voltage. The two can't be added together.

Always, always, always check the units!

The basic approach is valid, it just goes off the tracks midway. The total resistance is the resistance of the batteries plus the resistance you are going to put into it, call that Rb and Rx respectively.

The total voltage pushing current through that total resistance is 110V-Vb, where Vb is the voltage of the battery stack.

Your current is then

I = (110V-Vb)/(Rb + Rx)

Solve for Rx.

You should get the same result as before.