Terminal Voltage of a Battery


by LHC
Tags: battery, terminal, voltage
LHC
LHC is offline
#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.
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
chroot
chroot is offline
#2
Aug22-08, 02:04 PM
Emeritus
Sci Advisor
PF Gold
chroot's Avatar
P: 10,424
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
LHC is offline
#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
Topher925 is offline
#4
Aug22-08, 03:12 PM
Topher925's Avatar
P: 1,672

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
chroot is offline
#5
Aug22-08, 03:16 PM
Emeritus
Sci Advisor
PF Gold
chroot's Avatar
P: 10,424
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


Register to reply

Related Discussions
Voltage/Battery Terminal confusion Electrical Engineering 10
Car battery terminal corrosion Chemistry 2
Can a conducting metal plate connected to a +ve terminal of a battery gets charged? General Physics 2
EMF and Terminal Voltage Introductory Physics Homework 13
Terminal Voltage of unknown battery Introductory Physics Homework 9