Answer:
The correct option is D
Explanation:
From the question we are told that
The speed of the first charge is 
This is because it is at rest
The speed of the second charge is 
Generally the force exerted by a magnetic field on a charge is mathematically represented as
Now looking at this above equation we can see that 
can only be maximum at 
and this only obtained when the direction of the charge (i.e its velocity ) is perpendicular to the direction of the magnetic field
so the correct option for this question is D
Answer:
The first harmonic is: 250Hz, second harmonic 500Hz, third harmonic 750Hz.
Explanation:
Use the frequency f, speed v, and wavelentgh L relationship:
We are given the speed v=400 m/s. The base wavelength on a string of length 80cm is twice the length of the string (a "half wave" along the full length of the string), so:
The fundamental frequency (first harmonic) is 250 Hz
The second harmonic is produced by one full wave across the string (adding one node in the middle), so L=80cm in this case, therefore the second harmonic frequency is: f2 = 2*250=500Hz
the third harmonic add another node (and a half wave) to the pattern and the wavelength will be 2/3 of 80cm, so f3=3*250Hz = 750Hz
....................................
Yea what he said good luck
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