# Physics repost please check

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Posted by Mary on Monday, November 5, 2007 at 8:19pm.

A charged particle, passing through a certain region of space, has a velocity whose magnitude and direction remain constant.

(a) If it is known that the external magnetic field is zero everywhere in this region, can you conclude that the external electric field is also zero?

Explain.

(b) If it is known that the external electric field is zero everywhere, can you conclude that the external magnetic field is also zero?

Explain.

Physics - bobpursley, Monday, November 5, 2007 at 9:07pm
I will be happy to critique your thinking. It is not nice to post a lot of questions with each a different name. Usually, slackers or answer moochers do that.

Physics - Mary, Monday, November 5, 2007 at 9:29pm
I am sorry but you are mistaken. This is the first time I have been on this website in about 3 weeks. I have posted the question just above this one as well and another one just a minute ago for you to check my working. I will make an educated guess and repost the question.

My educated guess:

a. Yes. The particle has zero acceleration because magnitude and direction of the velocity are constant. If a charged particle passes through an electric field some force will be exerted on it. However, the magnetic field is zero therefore the magnetic force is zero. Net force = electric force. So, if the net foce is zero, then the electric field must therefore be zero.

b. No. If the paricle moves parallel to the magnetic field then no force will act on it.

• Physics repost please check -

Both of your answers are correct. Good thinking on (b), especially. I had forgotten about the parallel B field situation, where there is no force.

• Physics repost please check -

Correct.

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