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Terminal Voltage of a Battery

by LHC
Tags: battery, terminal, voltage
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LHC
#1
Aug22-08, 02:00 PM
P: 25
There's a problem in my textbook where it gives the emf of a battery, its internal resistance, and the net resistance of the circuit that it is connected to. Then it asks for the terminal voltage.

Actually, this is just a problem set (not exactly a textbook), so it doesn't teach me from previous examples. I'm still fumbling my way around these problems... Could someone please give me a hint on what to do?

So far, from what I've read on the internet, you can calculate the terminal voltage as: V = emf - ir

I have the emf, and the internal resistance...but no current is given. Haha, I'm so confused.
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chroot
#2
Aug22-08, 02:04 PM
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You need to find the current through the total circuit, which is composed of both the external and internal resistances.

Next, you need to find the voltage drop across the internal resistance. You can use Ohm's law.

Finally, you know how much voltage is being "lost" on the internal resistance, so you know what voltage will appear on the battery's terminals.

- Warren
LHC
#3
Aug22-08, 02:15 PM
P: 25
Quote Quote by chroot View Post
You need to find the current through the total circuit, which is composed of both the external and internal resistances.

Next, you need to find the voltage drop across the internal resistance. You can use Ohm's law.

Finally, you know how much voltage is being "lost" on the internal resistance, so you know what voltage will appear on the battery's terminals.

- Warren
First of all, I'd like to thank you for your quick reply. However, I'm not quite sure if I understand you correctly.

Ok, so the battery's emf is 6V, internal resistance is 0.6 Ohms, and the circuit's net resistance is 7.20 Ohms.

When you said:

Quote Quote by chroot View Post
You need to find the current through the total circuit, which is composed of both the external and internal resistances.
I took that as...the total circuit has a resistance of 7.8 Ohms.
So, I have a current of 6.0/7.8 = 0.769 Amps ??

Quote Quote by chroot View Post
you need to find the voltage drop across the internal resistance. You can use Ohm's law.
So...I found that as Current X Internal Resistance = 0.462 V

6.0 V - 0.462 V = 5.54 V, which is the answer.

So...I don't need to worry about the resistance through the circuit?

Topher925
#4
Aug22-08, 03:12 PM
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Terminal Voltage of a Battery

Not when you are only concerned about the voltage across the battery terminals, then the rest of the circuit is irrelevant. Remember voltage is the potential between two points, where current is a conserved flow.
chroot
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Aug22-08, 03:16 PM
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Quote Quote by LHC View Post
which is the answer.
Good work!

So...I don't need to worry about the resistance through the circuit?
You DID worry about it -- you included it in the total resistance, so you could find the current.

- Warren


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