# EMF and voltage

## Homework Statement

Can somebody explain what ems is for a person (me) who has only heard of voltage?

http://imgur.com/Y1JQy7w

Lets say the batteries volt is 10V. Is it always gonna be 10? Is the batteries ems 10V?

## The Attempt at a Solution

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gneill
Mentor
I think you mean "emf", or electromotive force.

An ideal battery will always present the same emf. Real life batteries depend upon chemical reactions and non-ideal materials that, over time and use, change the properties of the cell. Even if the emf provided by the underlying chemistry remain the same, the emf presented at the batteries terminals will degrade due to factors like internal resistances.

okay but am I ever gonna use emf? For example in this formula: P=VI Is V the emf or is it the ems? basically is ems always ε or can the ems value be put in an equation that containc V?

gneill
Mentor
okay but am I ever gonna use emf? For example in this formula: P=VI Is V the emf or is it the ems? basically is ems always ε or can the ems value be put in an equation that containc V?
I am not familiar with the term "ems". Can you provide a definition or a source for where you found it?

A good approximate model for a real battery is an ideal cell with a particular emf (otherwise known as its voltage) in series with a small resistance that represents the battery's internal resistance -- the non-ideal manifestation of the real chemistry and materials that make up the battery. The analysis and design of practical circuits might involve this resistance.

This internal resistance may come into play when you need to determine how the voltage that appears at a battery's terminals changes with the electrical load on the battery (current drawn), or how much power is lost inside the battery (causing it to heat up!) for a given load. It also plays a role in charging a (rechargeable) battery, since it can limit the charging current for a given charger voltage.

gneill
Mentor
It occurs to me that "ems" may be the same thing as "emf", but coming from a different language. Can you tell me the source language of the document where you found ems used?

It is in swedish :P I am wondering if the ems is used in P=UI and when do I need the ems value?

gneill
Mentor
It is in swedish :P I am wondering if the ems is used in P=UI and when do I need the ems value?
Ah! Swedish. That would explain it then. So I then interpret ems to be the same as emf in English.

In your circuit, if the battery is said to have an ems of 10 Volts, and since no internal resistance is given for that battery then that ems of 10 V will be impressed across the load resistor R (5 Ohms). This results in a current $I = ems/R = 10/5 = 2 A$ to flow, as shown in your figure.

okay so lets say the internal resistance is 1 ohm then the volt is U= 10 - (1*2)= 10 - 2 = 8
so the voltage is 8 then?

gneill
Mentor
okay so lets say the internal resistance is 1 ohm then the volt is U= 10 - (1*2)= 10 - 2 = 8
so the voltage is 8 then?
You would have to calculate the new current given the new total resistance; Adding resistance to the circuit will decrease the current.

oh right I forgot!

the total resistance is 6 ohms
the voltage is 10 V
The current is 10/6

U = 10 - (1*10/6) = 10 - 10/6 = (60/6) - (10/6) = 50/6 = 8.3333333....

is this correct?

gneill
Mentor
oh right I forgot!

the total resistance is 6 ohms
the voltage is 10 V
The current is 10/6

U = 10 - (1*10/6) = 10 - 10/6 = (60/6) - (10/6) = 50/6 = 8.3333333....

is this correct?
Yes. You could also have found the potential drop across R using the current flowing through it: V = (10/6 A)(5 Ω)

ehild
Homework Helper
Yes and we also write U=RI not V=RI