# Flux and emf in coil with DC

by skylines
Tags: coil, flux
 P: 3 Hi, When coil is excited with ac because of varying nature of current ,flux of varying nature will be developed in coil and because of varying flux emf will induce in coil. what will happen if coil is excited with dc? intially cureent will increase from zero to maximum and is constant.Because of this varying flux will develop for very short period of time .Does this flux is sufficient to induce emf in coil ? Thanks
P: 758
 Quote by skylines Hi, When coil is excited with ac because of varying nature of current ,flux of varying nature will be developed in coil and because of varying flux emf will induce in coil.
This is but the question related to electromagnetism.

Ok , anyways I think you are talking about a single coil firstly connected to mains. Yes there will be varying magnetic flux created in the coil. Ewing postulated his statement that every molecule and every electron behaves as a dipole or a free magnet. Current will be AC if either its intensity keep on changing or its direction or both in the coil. So flux of varying nature will be developed in coil but total magnetic intensity will be very less.

 what will happen if coil is excited with dc? intially cureent will increase from zero to maximum and is constant.Because of this varying flux will develop for very short period of time . Thanks
Please imagine this situation yourself. Have you conducted any experiments on electronics ?
If you connect a circuit to the battery connected to an ammeter then instantaneously within no time ammeter will give you its reading of current in amperes in the circuit. Current is not increasing from 0 to x amperes ! As soon as you connect the circuit to battery ammeter will give you the current reading. So after all unless you don't do any change in current intensity or resistance or change battery (etc), current will be constant in a circuit. In other words current intensity will remain constant. You must also realize there is also net flow of current in one particular direction. This means its direction is not changing with time.

Hence the current will be DC. There will be no varying flux. There will be same magnetic flux induced in coil in which direct current is flowing.

 Does this flux is sufficient to induce emf in coil ?
In DC the potential difference between two electrodes in open circuit is emf. If current intensity is high , the turns in coil are more , soft pure iron core is placed between coil , or laminating iron core will help to create stronger magnetic flux.

Note : If DC current is flowing in a circuit then flux is not inducing emf in a circuit.
 P: 1,506 AC flowing in the coil will produce an AC magnetic flux which will produce an AC emf. This is called a BACK EMF. When the coil is connected to DC you are correct to realise that an emf will only be induced when the current is switched ON or OFF. At switch ON the induced emf opposes the applied emf and this results in a slowing of the rise in the current. This is especially notable in a coil with many turns and an iron core (to make the magnetic effect large) At switch off the current stops instantaneously and this rapid collapse of the magnetic field can produce a large emf. This emf can be large enough to cause a spark at the switch contacts. In practice this is a problem in relay coils and it is put to use to generate high voltages in Induction coils.
P: 758
Flux and emf in coil with DC

 Quote by technician AC flowing in the coil will produce an AC magnetic flux which will produce an AC emf. This is called a BACK EMF.
Correct , current (AC) induced in coil by the process of electromagnetic induction will either have directions changing or intensity changing or both changing in the respective coil. So it will produce an AC magnet flux which will produce an AC emf.

 When the coil is connected to DC you are correct to realise that an emf will only be induced when the current is switched ON or OFF.
Yes I am.
That's what I typed in my previous post.

 At switch ON the induced emf opposes the applied emf and this results in a slowing of the rise in the current. This is especially notable in a coil with many turns and an iron core (to make the magnetic effect large)
Incorrect ; technician , unless you have the constant magnetic flux due to the flow of direct current , no electromotive force is induced by that DC magnetic flux.
There will be the emf which is applied at the electrodes in battery in an open circuit when current does no external work.

Look at the leftmost face of coil. Instead of clock rule , use Lenz's law. Imagine you are moving bar magnet in of solenoid. According to Lenz's law , the polarity of the left most face of coil opposes the motion of bar magnet. So polarity of leftmost face of coil is North pole. Magnetic flux will be from N.P. to S.P. of bar magnet with high intensity.
Similarly if you move bar magnet out of solenoid the polarity of the left most face of coil opposes the motion of bar magnet. So polarity of leftmost face of coil is North pole. Magnetic flux will be from N.P. to S.P. of bar magnet with little low intensity.

But in DC magnetic intensity is same so only emf is at end terminals of battery.
 At switch off the current stops instantaneously and this rapid collapse of the magnetic field can produce a large emf. This emf can be large enough to cause a spark at the switch contacts. In practice this is a problem in relay coils and it is put to use to generate high voltages in Induction coils.
Correct but vague partially. I think you are talking about self induction. Yes if current is switched on sometimes due to self induction the direction of induced current is same as primary current.

If you open the circuit there will be spark and current will decrease. So the induced current will oppose the primary current due to change in magnetic flux which reverses the direction of induced current in a circuit. Hence eddy currents are developed.

In AC :

IN DC :

 P: 1,506 I think that my wording has been misunderstood! At switch On the current begins to increase in the coil, This increasing current produces an increasing magnetic flux in the coil which, (because it is changing) will produce an induced emf. This induced emf will oppose the change producing it (Lenz's law)... that is it will oppose the Increasing current. If the coil has lots of turns and has an iron core to increase magnetic effects this slowing in the rise of the current is easy to demonstrate.....I have done it many times. Your last diagram Sankalpmittal shows this perfectly your e is opposing the increasing current When the current is switched off the magnetic field collapses rapidly giving an induced emf that tries to maintain the current. A large emf (can be 1000V) is developed across the terminals of the coil.This is easy to demonstrate and particularly if the demonstrator is touching the terminals of the coil..... you get an electric shock. I hope this clears up any confusion in my previous wording
 P: 3 thanks now i am more clear abt emf.
P: 758
 Quote by technician I think that my wording has been misunderstood! At switch On the current begins to increase in the coil, This increasing current produces an increasing magnetic flux in the coil which, (because it is changing) will produce an induced emf. This induced emf will oppose the change producing it (Lenz's law)... that is it will oppose the Increasing current. If the coil has lots of turns and has an iron core to increase magnetic effects this slowing in the rise of the current is easy to demonstrate.....I have done it many times. Your last diagram Sankalpmittal shows this perfectly your e is opposing the increasing current When the current is switched off the magnetic field collapses rapidly giving an induced emf that tries to maintain the current. A large emf (can be 1000V) is developed across the terminals of the coil.This is easy to demonstrate and particularly if the demonstrator is touching the terminals of the coil..... you get an electric shock. I hope this clears up any confusion in my previous wording
Woops sorry for the typo !

 Quote by sankalpmittal I think you are talking about self induction. Yes if current is switched on sometimes due to self induction the direction of induced current is same as primary current. If you open the circuit there will be spark and current will decrease. So the induced current will oppose the primary current due to change in magnetic flux which reverses the direction of induced current in a circuit. Hence eddy currents are developed.
Quoting myself. The bold text of my post is wrong !

Here is the correct one :

I think you are talking about self induction. Yes if current is switched on sometimes due to self induction the direction of induced current is opposite to primary current.Hence eddy currents are developed.

If you open the circuit there will be spark and current will decrease. Thus magnetic flux will collapse. This will develop an emf and hence the induced current will flow in the same direction as the primary current due to change in magnetic flux which reverses the direction of induced current in a circuit.

I think this clarifies my confusion. Thanks !

 Quote by skylines thanks now i am more clear abt emf.
I am very glad to hear that !

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