BR Physics 8 Electrostatics & electromagnetism

[Natnow]

Hi all,
I have a couple of question I need some help with answering and understanding from BR Physics Electrostatics

Ex. 8.8b To increase the oscillation frequency of an electron (isolated in a vacuum) that oscillates because it is stimulated by an electromagnetic wave, one must:
I. decrease the wavelength of the wave
II. increase the electric field amplitude of the wave
III. increase the magnetic field amplitude of the wave
IV. increase the speed of the wave

A. I only
B. II or III only
C. I and IV only
D. None of the above

I was thinking that item "I" is correct due to the velocity=(wavelength)(frequency) equation.
I was thinking to figure out if item III was correct I could use F=qvB equation, solving for v or B shows that they are inversely proportional, increasing B would decrease v, which would decrease the frequency (using v=(wavelength)(frequency). I'm not to sure about this rationale though.


Item IV: increasing the wave speed, increases the frequency (?), thus IV would be true.
I thought I remember reading however that the frequency cannot be changed a change in velocity.

Thank you so much!

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[BrothaX]

What was the solution??

I have an idea but i dont want to lead you down the wrong path if my answer is incorrect.

 

[PKA<0]

Discard 2 and 3 amplitude won't affect frequency. Think of a pendulum because this oscillation is simple harmonic motion. In pendulum releasing the bob with the same size string from different angles creates different amplitudes, but frequency remains constant.

Your reasoning for 1 and 4 are right and that alone should give you your answer.

Edit: I thought about how frequency doesn't change as you move from medium to medium and I justified that it would change in this case because you are in the same medium.

I'm not exactly sure, but if you think of it in terms of energy it makes sense for the frequency to change.

E=hf and adding velocity adds kinetic energy

 

[sps27]

Example 10.1b (From older BR)

To increase the oscillation frequency of an electron (isolated in a vacuum) that oscillates because it is stimulated by an electromagnetic wave, one must:

I. decrease the wavelength of the wave.
II. increase the electric field amplitude of the wave.
III. increase the speed of the wave.

A. I only
B. II only
C. I and III only
D. None of the changes will increase the oscillation frequency of the electron.

Solution

The electron is being stimulated by electromagnetic radiation in a vacuum, so we should start by noting that the speed of EM radiation in a vacuum is constant. This means that if the frequency of the radiation were to increase, the wavelength would have to decrease (v = fk). This makes Statement I valid, which eliminates choices B and D. We need to only consider Statement III at this point. Because the speed an EM wave in a vacuum is constant, Statement III can have no impact. The best answer is choice A.

 

[Natnow]

Thank you!

I have a couple more questions (from the Electric Circuit chapter)

9.9b: Doubling the maximum voltage in an AC circuit will increase the rms voltage by a factor of:

A. 1
B. sqrt 2
C. 2
D. 4

I think I understand that for the Vrms equation for AC circuits you are using the average voltage. But if I don't know what the voltage is how can I get the answer?
I was thinking that Vrms uses half of Vmax (or the average V), so would Vmax be V and thus doubling the max voltage be 2V for the max voltage, which half of that would be Vavg=2, answer C?

If you could explain your answer that would be appreciated too!

 

[sps27]

Vrms is not average of Vmax. I think it is more like Vrms = k.Vmax (constant k = 1/sqrt 2) . Same with Irms = k.(Imax) where k = 1/sqrt 2. So if you double Vmax, the Vrms also doubles to keep the relationship btwn the two still valid.

Here is the link for official MCAT Q & A forum. Post ur questions in the Q & A forum. In all likely hood, the questions will get answered much quicker........

http://forums.studentdoctor.net/forums/mcat-study-question-q-a.134/