emf

hi i really dont get what emf is could someone explain it to me and how it differes from normal voltage? also what calculations will i have to do with it for as physics?

thanks alot

Reply 1

AT82

I don't know much about physics but I do have a GCSE and Electronics and my degree invovles a bit of electronics to a certain extent. EMF is electro magnetic field. It is a present electrical currencies. I don't really know much more than that. I think the product it gives off is a kind of radiation. Anything with a electromagnet gives of this kind of radiation. Motors, cathode ray tubes give off a lot of it and it can cause cancer. Its very bad to be near a washing machine for a long time for this reason.

Reply 2

Perplexed

The EMF of a circuit is the number of volts that is supplied to each coulomb.

Provided the internal resistance of power supply is ignored:

In a series circuit ε=V1+V2+V3
In a parallel circuit ε=V1=V2=V3

Or, if it isn't ignored:

ε=IR+Ir

This can be simplified, since we know V=IR, to:

ε=V+Ir

(Where R=external resistance and r=internal resistance)

Reply 3

Abraxis

emf stands for electomotive force, even though it is a measure for energy and not force.
The emf of a power supply is a measure of the total energy that it supplies to each coulomb of charge.
That's all I got for now...

Reply 4

elpaw

the emf is specifically the (theoretical maximum) voltage supplied by a voltage source (i.e., because all real sources will have some internal resistance, it is the voltage supplied when the current is 0).

theres no physical difference between the terms "voltage" and "emf", it is just from a linguistic perspective that the voltage accross a voltage source is called "emf".

Reply 5

Douglas

lalalaura

hi i really dont get what emf is could someone explain it to me and how it differes from normal voltage? also what calculations will i have to do with it for as physics?

thanks alot

In the engineering community, the words "emf" are never used. Since emf is measured in volts, I see it as useless in engineering.

I can't speak for it's use in physics, other than it may be the time rate of change of the magnetic flux or the work required to carry a unit charge around a loop. In either case, I doubt it would be measured in volts.

Reply 6

Jonatan

lalalaura

hi i really dont get what emf is could someone explain it to me and how it differes from normal voltage? also what calculations will i have to do with it for as physics?

thanks alot

The emf is pretty much the theoretical voltage when you considder the limits of the power source to be due to an "imaginary" internal resistance (I wrote imaginary in quotes because technically all physical concepts are things we assume in order to make calculations. Also known as approximations... )

Take a battery as an example. Say you were to short-circuit the poles using a wire with zero resistance. According to Ohm's law this would give you infinite current. However, a battery cannot supply infinite current, thus when the resistance decreases towards zero, the voltage across the poles drop.

This is often represented in circuit diagrams as an "internal" resistance thought of as a resitance within the battery itself. The emf is then the voltage across the entire circuit, including this internal resistor, whereas the voltage across the battery is the voltage across the circuit excluding this resistor. This sounds horrible, but lets considder an example:

Say a battery with emf 1.5V and internal resistance 1Ohm is connected to a circuit with resistance 9Ohm. What will the voltage across the battery poles be?

Total current
I = V/R = 1.5v / 10ohm = 0.15A

The voltage across the poles will be just the voltage across the external circuit so:

V = IR = 0.15A * 9ohm = 1.35V

Hence the battery can only deliver 1.35V when a current of 0.15A is drawn through it.

As you can see the concept of emf and internal resistance is very useful as the emf will not depend on the current drawn , as opposed to the external voltage. All variations in voltage due to differences in current are incorporated into the internal resistance, thus rather than having to deal with varying voltages one can deal with a constant internal resistance.