# Energy changes during motional emf

Abhishekdas
I had already posted this question in the homework category but since i did not get any replies i thought maybe i should post it here again.......

A U shaped conducting tube is placed in a constant magnetic field in the downward direction. On the left of the tube is a resistance R.All other rods are assumed resistance less.On this tube a conducting rod is moving with constant velocity towards right with the help of an external force.....Now i was trying to figure out the energy chnges associated during the process.......

Now we know that a motional emf is induced in the rod and a current is generated and due to this current a force is excerted on the magnetic field towards left. Now i wanted to figure out the energy changes in the process but i was not confident of what exactly was happening......Correct me wherever i am wrong....

Initially lets say the external force does work with power Fv and the magnetic field does equal amount if negative work....So net work on rod is 0.
Now coming to the emf part.....The magnetic field creates the emf.....Now normally when we talk about emf in a battery we say that chemical energy is converted to the energy required in doing the work required to push the charges arong the circuit....But in this emf where does the energy come from......?Is it the magnetic field which excerts forces on the electrons responsible for this energy?.....And what about the energy which is dissipated by the resistance in the form of heat? Where does all these energies come from and what conversions take place? Im confused about that.....So i would really appreciate some help on this......I want to develop my concepts on energy conversions in electricity and magnetism which i feel is much tougher that energy conversions in mechanics which seem much simpler......

PS: I have attached a digram to describe the basic setup....

#### Attachments

• motional emf 1.bmp
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## Answers and Replies

Staff Emeritus
Is this homework? If so, I'd just bump your other thread up with a reply see if you get a response.

Curl
Conservation of energy is just as simple in electrodynamics as in mechanics.

The work done by the force (pulling the bar) is exactly matched by the work lost to the resistance (joule heating if it's a standard resistor).

I don't see where the confusion is coming from.

Abhishekdas
hi...curl....thanks for the reply....

Ya you are right that the work done in total by mechanical force (on the bar) is equal to the work lost in joule heating but it is not a direct transformation is it? I want to know each step......In between it must have been converted to Electrical energy(work done by emf) and then something else......Here i want to know all the steps or conversions of energy.....

Hope you can tell me more about that......

Curl
Bar moving gives rise to a current. The current is slowed by the resistor.

I kick a ball, the grass slows it down --> The grass absorbed the energy from my leg.

Same thing, nothing too magical.

Abhishekdas
In between is there no involvement of electrical energy?

Ok lets go step by step....External force does work in moving the bar....Now this work is converted to electrical energy ie the emf now pushes the charges to set up a current(if i am not wrong)....and on the other hand the magnetic forces dont do any work so thats out(because velocity of electron is perpendicular)......So the entire work done by emf is converted to heat (joule heating)......

Now am i right in all these? Just tell me that.....Am i missing out on any steps?