# Induced emf is proportional to the magnetic flux

According to the faraday's law,the induced emf is proportional to the magnetic flux change linking the conductor.However,I wonder why there is induced emf when conductor is moving in uniform magnetic field,because i think that the magnetic flux linking to it is always the same as it move in the uniform magnetic field.
ps:I suppose that magnetic flux is continuously flowing.
Can anyone point out where I wrong?Thank you.

Drakkith
Staff Emeritus
A moving conductor in a static magnetic field is the same as a static conductor in a moving magnetic field. When one is moving in relation to the other you will have induced EMF. It doesn't matter which one is moving.

I think that magnetic flux is the amount of magnetic field passing through the surface of conductor.Therefore,even though there is relative motion ,but the magnetic flux linking to the conductor don't change(no flux change)in uniform magnetic field,isn't it?Or,in other words,the flux linking to the conductor in uniform magnetic field is same no matter the conductor moves or not,isn't it?

Drakkith
Staff Emeritus
I think that magnetic flux is the amount of magnetic field passing through the surface of conductor.Therefore,even though there is relative motion ,but the magnetic flux linking to the conductor don't change(no flux change)in uniform magnetic field,isn't it?Or,in other words,the flux linking to the conductor in uniform magnetic field is same no matter the conductor moves or not,isn't it?

As the conductor moves every point in it experiences a change in the magnetic field, resulting in a change in magnetic flux. I don't believe there is a way to move something and keep the flux the same.

Born2bwire
Gold Member
If you are moving the conductor then you are imparting a velocity to the charges in the conductor. A moving charge experiences a force from a magnetic field via the Lorentz force. Hence, the magnetic field will induce an EMF in a moving conductor. If you looked at the problem in the reference frame of the moving conductor, you would have to apply a Lorentz transformation to the magnetic field. This transformed field will have an electric field and this electric field can exert a force on the now stationary charges in the conductor via the Lorentz force and induce the same EMF.

A tape recorder illustrates induction very well. The individual bar magnets on the tape are unchanging, i.e. a fixed static mag field. When the tape is still & contacts the head, no induction takes place since the flux is static. But pressing the "play" switch moves the tape past the heads resulting in induction. Although the bar magnets on the tape surface remain static, the field encountered by the head is time changing. As each magnetic particle passes the head, the mag field at the location of the head gap is continuously changing, thus induction takes place.

Adapters for car tape players can accept a CD & generate a time changing mag field from the CD signal. When the module is inserted into the tape player, the mag field emitter comes in close proximity to the tape mag head. The head gap now encounters a stationary device with a mag field that varies with time. Induction takes place & the CD signal is picked up by the tape head & amplified.

Induction takes place when the incident mag field in the pickup circuit encounters time change. The said time change can be due to a varying field, or motion, or the combination. Faraday's Law has been repeatedly affirmed since the mid 19th century. It is known to be valid.

Claude

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dlgoff
Gold Member
Here's a calculator to find the http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir2.html#c1"

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Thank you for your detail explanation.I just can't understand how the flux change with the relative motion in uniform magnetic field(the tape in tape recorder still experience flux change as it pasts the magnet,so there is induced emf).I try to describe my question more clearly.

Lets say we are holding a "basin" (conductor),walking in rain.If the "rainfall" (magnetic field) is constant,then the "rainwater collected in the basin per second"(magnetic flux linking to the conductor per second) is independent to our "walking speed"(relative motion)(unless we go upward),isn't it?
I know this is a bit weird ,but it is close to what i am thinking.
Thank you.

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Redbelly98
Staff Emeritus