Ask A Scientist© Physics Archive

name         Ben
status       student
age          17

Question -   I am confused about the concept of 'back emf'.
Have you got a clear, concise and logical explanation of just what it is?
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Hi, Ben  !!

Well, as you probably already heard before, the back-emf is
the same as counter emf (cemf). It is a voltage produced in
a conductor that tends to neutralize the present voltage. It is
a phenomenon that always tends towards the contrary of what happens!!
Let´s suppose: if lines of a magnetic field are cut by a conductor,
than a voltage is generated in this conductor, which causes a
current of electrons to flow in one direction. At the same time,
like trying to avoid this, ANOTHER voltage is created that tends
to neutralize this effect, and it forces the electrons to flow in the
contrary direction. As you know, the back-emf is directly pro-
portional to the velocity of the magnetic field. It is proportional
to the relative motion between them.

I am sure that you know all of this. But, to give you an example,
think of a wire with a switch. Suppose the electrons are running
from a direction into another and suddenly, you open the switch.
What happens?? Well, when electrons run, they give rise to a magnetic
field. When you open the switch, the electrons should stop their
flow. But - as a kind of inertial action - they try to continue the flow,
and ionize the air, in an electric arc to find its way. Or let it be,
the originally initial magnetic field, in a circumference around
the wire, changes its sense, and try to avoid stopping the original
flow.  It is possible to see this in an oscilloscope, as a negative
peak in certain experiences where you make use of a coil and
- abruptely - stops the electron flow.

I know you know all of this. But, that is it!!! It is the way things are.
Just try to answer this question: what is electrical charge?
Why is that we have positive and negative charges?? Answer :
nobody knows it!!!  We must accept this condition!!! It belongs
to the laws of the universe, since the big bang created it like as it is.

Just to speak a little bit more about this subject, you know that
when an electrical induction "squirrel" motor runs, you have a
magnetic field that circulates around the squirrel. There is a
speed difference between them, what in turn generates a voltage
and a force appears, which drives the "squirrel". A back-emf
also appears, which lowers the net electric electron flow. The
current through a rotating electric motor is greatly reduced because
of this back-emf and if you avoid the moviment of the rotor, the
current should be so large that could damage the equipment.

And last but least, back-emf corresponds in a way to inertial
forces in the movement of a mass through space.

Inertia is a mass and has no intrinsic force.
The electron has a mass ( ca. 1/1840 of the mass of a proton ).
Just to compare, the Newton´s first law states that a mass
will maintain its linear direction of movement, unless the resultant
of the forces applied on it be different from zero. When you
interrupt a current, the flow of electron tend to continue, like
having inertia (no voltage difference, no electrical pressure and
the flow should stop). Nevertheless, magnetic fields
appear alongside the current which act like forcing
electrons keep on their movement.

Alcir Grohmann
Beschaffung SAM
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Dear Ben,

The short answer to your question is "No", but some words about it may
help.  The universe is put together in such a way that whenever the
magnetic field through a loop changes, a voltage or emf is induced in
that loop.  Now, in principle that emf could be in such a direction as
to oppose the change that produced it or, in the opposite direction
around that loop, to aid and increase the change that produced it.

Lenz's Law says the induced emf always opposes the change producing
it, leading to such terminology as "back emf".  The argument, by the
way, is very simple. If the current aided the change that produced it,
the induced current would change the field more, producing more emf,
thereby producing more field, and so on to infinity, thereby blowing
up the universe.  This does not happen!

To take a practical example, an electric motor running under no load
uses very little power.  If the motor were frictionless and
superconducting, it would use no power.  This is because the back emf
opposes the imposed voltage.  As the load on the motor increases, the
back emf decreases as must be the case; the motor must be given energy
to produce energy.  To see why the back emf decreases under load is
often complicated, having to do with the angle between coils in the
motor and the rotating magnetic field which is driving the coils.

Best, Dick Plano
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