# Homework Help: Motional EMF

1. Apr 21, 2015

### ZARATHUSTRA

• Member warned about lack of template
consider the graph below, the bar of length ζ moving to the right with speed v in a uniform magnetic field B going out the page.

i don't understand why does the textbook say that there is a change in the magnetic fluxΦ? i think the area doesn't change, although it's moving. the original saying is that: During a time interval dt, the area of the loop increases by a differential rectangular strip of area dA=ldt this change in area causes a differential change in flux given by dΦ=B⋅dA .............

2. Apr 21, 2015

### TSny

Only the blue bar is moving. The resistor is at rest. A is the area between the resistor and the bar.

Last edited: Apr 21, 2015
3. Apr 22, 2015

### ZARATHUSTRA

right, so..... what does that mean?

4. Apr 22, 2015

### ZARATHUSTRA

is it true that the moving object in motional emf has to be a conductor?

5. Apr 22, 2015

### ZARATHUSTRA

and the emf created is from the seperation of charges, therefore creating a potiential difference?

6. Apr 22, 2015

### ZARATHUSTRA

i feel sorry to ask those stupid questions. i don't have a teacher currently teaching me AP Physics.

7. Apr 22, 2015

### TSny

As shown in the figure below, as the rod travels to the right the shaded area A increases. So, the amount of magnetic flux through the circuit also increases.

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8. Apr 22, 2015

### ZARATHUSTRA

interesting!!! let me think hard about it, and thank you for your help

9. Apr 22, 2015

### rude man

You can also look at his from a different perspective. Starting with the Lorentz force F = qv x B you can determine that the emf across the moving bar is Blv. This view is "flux cutting" of a B field (uniform or not) by a bar of length l moving with veloc ity v.

In some cases, using Faraday's law, as you did with emf = -dφ/dt, in moving media does not work. So the "Blv" law is preferred whenever media (like your bar) is moving.
Yes, but not a perfect conductor. In fact, anything other than a perfect insulator.

Last edited: Apr 22, 2015
10. Apr 22, 2015

### ZARATHUSTRA

right, i found out that there are many ways to analyse motional emf, for example, relativity, parallel capacitor, potential difference or magnetic flux, etc

11. Apr 22, 2015

### ZARATHUSTRA

but the results is the same,this Blv expression occured in a lot of the results, am i right?

12. Apr 22, 2015

### rude man

Right.
Truth to tell, I'm not sure what to make of your comment. Bit of a jumble, seems like.
I restricted my comment to the generation and determination of emf by magnetic means.

13. Apr 27, 2015

### ZARATHUSTRA

thanks for the reply, i found it hard to understand the magnetic field and therefore don't like to use magnetic means, too.