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induced emf ,back emf difference..

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i read some where that induced emf and back emf are the same
but apperantly it not

i have solves the 3rd question

but the 5th says its the opposite on the induced emf

http://i26.tinypic.com/2o61f.jpg

so what is the difference between them.

induced emf comes as a result of a change in flux
so does the back emf

why the one is the opposite of the other
??
 

Boron

New Member
In the third question, the switch has been opened. Therefore when the magnetic field in the inductor collapses, the emf induced is the only source of emf, since it has been disconnected from the original power supply. Being the sole emf present, they refer to it as simply "induced emf"

In question 5, the inductor is continuously connected to the power supply. As a result, any emf generated by the inductor itself will have to contend with the emf of the power supply as well. Since these two emf's will be of opposite polarity, the emf of the inductor is in essence fighting back against the supply emf. This is why in this case its referred to as "back emf".

Technically I suppose its the same phenomenon, its just in one case it has to contend with the supply emf, which causes a drop and gets it named specifically back emf.
 
In the third question, the switch has been opened. Therefore when the magnetic field in the inductor collapses, the emf induced is the only source of emf, since it has been disconnected from the original power supply. Being the sole emf present, they refer to it as simply "induced emf".
i dissagree
when we open the switch and the current drops
the dropping of the current also creates a magnetic fiels->induced emf .

In question 5, the inductor is continuously connected to the power supply. As a result, any emf generated by the inductor itself will have to contend with the emf of the power supply as well. Since these two emf's will be of opposite polarity, the emf of the inductor is in essence fighting back against the supply emf. This is why in this case its referred to as "back emf".

Technically I suppose its the same phenomenon, its just in one case it has to contend with the supply emf, which causes a drop and gets it named specifically back emf.
if we have a current source with changing current
then it creates emf (induced emf)

so there is only one emf here
 

Boron

New Member
I fail to see how you are disagreeing. Unless maybe I worded it incorrectly saying magnetic field collapse. Another way of saying it would be that when the current drops to zero, that causes a change in magnetic flux, which in turn induces an emf in the inductor. Seeing that it is no longer connected to a source of current (and therefore source of emf) this emf generated upon opening the switch is the only emf present, hence we simply refer to it as "induced emf"

A current source with changing current also has a changing voltage or emf. This source emf is attached to the inductor. Now since the voltage and therefore current of the source is fluctuating, this again causes a change in magnetic flux which induces emf in the inductor. But contrary to the first case, not only is there the emf induced in the inductor but the source has an emf feeding the inductor as well. The emf generated in the inductor is of opposite polarity to the source and opposes it, which is why it is called "back emf"
 
regarding the second part:

"A current source with changing current also has a changing voltage or emf."

"Now since the voltage and therefore current of the source is fluctuating, this again causes a change in magnetic flux which induces emf in the inductor."

i see it as the same change i current
i see only one enduced emf here
the second change in the inductor
was already described by you as the change of voltage.
its the same one emf
 

Boron

New Member
It is somewhat difficult to explain... maybe I can use an example.

I dunno if you ever noticed, but let's say you hook up a 24 volt battery to an electric motor. When the motor is disconnected and you measure the voltage, you will indeed find it reads 24. However when you hook up only the motor to the battery and measure the voltage, you will find it has dropped to, lets say, 18 volts. This drop in voltage is due (in an ideal motor with no resistance) to back emf. As the motor spins, it generates its own proper emf in addition to the 24 volts of the source. In this case when the motor is spinning it acts somewhat like an inductor and the induced emf is 6 volts. Since it is of opposite polarity, this 6 volts "fights" the 24 of the source. This is why you measure only 18 volts.

It is the same idea in an inductor. Whenever we are talking about "back emf" it is a secondary emf generated by an inductive load which opposes the source.

You can read a little more here:
Counter-electromotive force - Wikipedia, the free encyclopedia
 

MrAl

Well-Known Member
Most Helpful Member
Hi there,


There is a simple way to understand what the back emf is.

Recall that a theoretical inductor may not have any resistance, or made
from very very low resistance wire, yet when we connect it to a sine source
of 18.85v for example we only get a current of 5 amps. How can that be?
It's just a wire inside the inductor, that's all (air core). Why dont we get
an infinite current flow with any applied voltage? One way of looking at this
is by imagining a counter emf that exactly cancels the applied emf and that
is where the term 'back emf' really comes from.

The back emf isnt always the same as the applied emf however. If we connect
a resistor of 1 ohm in series with the perfect inductor of 10mH and apply a sine
source of 5 amps we still get the same back emf as with the inductor alone
(the back emf is measured across the inductor) but the total voltage across
the whole network is about 19.5v now. Thus, the applied emf is 19.5v and the
back emf is 18.85v with the 1 ohm resistor included.
As a matter of fact, this is the way back emf is sometimes measured; by sensing
the current and knowing the resistance and calculating the drop across the
resistance and subtracting that from the total votlage and that gives the drop
across the inductance, which is the back emf.
 
Last edited:

Sydney

New Member
Maybe you woud discover the difference by a simple test.........use a battery and ammeter to test continuity of a primary of a transformer..................then holding the transformer leads take away the battery ....................discover the ring of back EMF

Think outside the square and square it
 
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