# Homework Help: Relation between Electric field and Magnetic field?

1. Sep 29, 2014

### Gaurav

An observer in an inertial frame finds that at a point P the Electric field vanishes but the Magnetic field does not. This implies that in any other inertial frame the electric field E and the magnetic field B satisfy:
[these values in vectors]
1. |E|2 = |B|2
2. E . B = 0
3. E x B = 0
4. E = 0

I know the answer is E.B = 0 but why is that?

2. Sep 29, 2014

### Orodruin

Staff Emeritus
What quantities (involving EM fields) do you know that are invariand under Lorentz transformations?

3. Sep 29, 2014

### Khashishi

What is the relativistic definition of a scalar?

4. Sep 30, 2014

### Gaurav

I don't know these values. This is a question of my exam. There is nothing given more.

5. Sep 30, 2014

### Orodruin

Staff Emeritus
If you do not know the expressions for the invariants of the electromagnetic field, then you have no chance (or rather 25% chance) to answer this question correctly. This suggests that your examiner expects you to know. Do you have some course litterature where you can look them up?

6. Sep 30, 2014

### Gaurav

I have only its paper, I attached it, It is question no. 2 in it.

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7. Sep 30, 2014

### Gaurav

Its all I have

8. Sep 30, 2014

### Orodruin

Staff Emeritus
So I am assuming this is a general physics test of some sort. Your problem will not be solvable until you know some special relativity and how it describes the electromagnetic field. In order to fully understand the solution, you will also need to know what is meant by a Lorentz scalar and use some of the properties of the electromagnetic field tensor (see, e.g., http://en.wikipedia.org/wiki/Electromagnetic_tensor#Properties).

If you find the correct argumentation to use, this problem is solved in one line. In accordance with forum guidelines I will refrain from posting it here.

9. Sep 30, 2014

### Gaurav

Thanks a lot. I will check it.

10. Sep 30, 2014

### Orodruin

Staff Emeritus
Just another small pointer: When they talk about a Lorentz invariant in the Wikipedia link above, this means that they are talking about a quantity which has the same value in all inertial frames. Given this information along with the Wikipedia information, you should be able to deduct the correct answer. Please tell me when you have figured it out and ask if you have further questions.

11. Jul 23, 2017

### Amit pokhriyal

I'm still unable to get this relation. E.B =0
I guess for inertial frame, it should be easier