A stationary particle of charge q = 3.6 10-8 C is placed in a laser beam (an ele

Question

A stationary particle of charge q = 3.6 10-8 C is placed in a laser beam (an electromagnetic wave) whose intensity is 1.8 103 W/m2. Determine the magnitudes of the (a) electric and (b) magnetic forces exerted on the charge. If the charge is moving at a speed of 3.7 104 m/s perpendicular to the magnetic field of the electromagnetic wave, find the magnitudes of the (c) electric and (d) magnetic forces exerted on the particle.



(a) F

= ---Select--- N T V J W



(b) F

= ---Select--- T J N V W



(c) F

= ---Select--- J N V W T



(d) F

= ---Select--- T N V J W

Explanation / Answer

This is a poorly worded question; you might want to bring this to the attention of your professor. Because the light represents an EM WAVE, the electric and magnetic fields both oscillate. This means the magnitude of the average force in all cases is zero. I think the intent of the problem is for you to find the magnitude of the MAXIMUM force experienced by each particle... but the problem is very vague. It happens. Physics problems are written by people, and people (even physics professors...) make mistakes.

My point is that I am going to answer the problem the way I think the person who wrote it wants it answered. If he wanted average force, the problem would be silly. But if he wanted MAX force, he should have stated this.

Anyway...

First find the amplitude, i.e. the MAX value of the E and B fields:

MAX E field = sqrt (240 pi * intensity) = sqrt (240 pi * 1800) = 1165

and MAX B field = E / c = 1165 / 3 x 10^8 = 0.000003883

Now...

a) MAX force on particle = qE = 3.6 x 10^-8 * 1165 = 0.00004194 Newtons

b) force is zero because particle is stationary

c) MAX force is still 0.00004194 N because E force is not affected by speed

d) MAX force = qvB = 3.6 x 10^-8 * 37000 * 0.000003883 = 5.17 x 10^ -9 Newtons

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