EK waves questions

[JFK90787]

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OK these two questions were right next to each other:

650: If the amplitude of the wave were doubled, the frequency of the wave would:
A Increase by a factor of 4
B Decrease by a factor of 2
C Remain the same
D Increase by a factor of 2

651: Which of the following would NOT increase intensity of a harmonic wave?
A Increase in velocity
B Increase in frequency
C Increase in amplitude
D Increase in period

Answers were C and D. I got 650 right since I thought amplitude/intensity were only a measure of energy and had no relation to v or frequency/period. EK says for the answer to 651 that Intensity is proportional to Frequency^2, but that contradicts their answer in 650. So why does decreasing the period increase amplitude in 651?

 

[Akarat]

I'm not too good when it comes to waves, but for 651, the intensity of wave is equal to: I = P/A, where P is the power and A is the surface area. Since power is equal to joules per second, if you increase the period you're basically increasing the time that it takes for a source emit another wave. This would decrease the power because the time, t, would increase which would then decrease the intensity.

That's how I approached 651.

For 650, I just memorized that increasing amplitude has no effect on either the wavelength or the frequency. Although memorizing stuff really does no good when it comes to the MCAT. 🙄

 

[JFK90787]

Wait, amplitude and intensity are not synonymous for waves? Well that would clear things up

 

[Akarat]

Wait, amplitude and intensity are not synonymous for waves? Well that would clear things up

I'm pretty bad when it comes to the specifics of waves. =/

However, I would assume that they are closely related. If I remember correctly, the amplitude is the energy of a wave, while the intensity is the power per surface area. The two are related in that energy per unit time is equal to power.

For 651, an increase in amplitude would increase the intensity, which would not be the answer to the question since it's asking for what would decrease the intensity of a wave.

 

[JFK90787]

Right. Either way that helps a lot, I've just been assuming they were two different words for the same thing for a long time

 

[Eye Cue]

I = (0.5)(density)(angular f^2)(A^2)(v)

So, intensity increases with the square of the amplitude.

 

[Econ2MD]

period (T) is 1/f. If frequency increases, period decreases. If period increases, frequency decreases.

Using units helps to keep it straight, f=(1/s)=Hz; T=(s)

Increasing frequency has the opposite effect of increasing period.

I don't see any contradiction in the questions. increasing amplitude increases intensity and is independent of frequency.

 

[Econ2MD]

Don't get bogged down with equations.

Think of big waves in the ocean crashing into shore. Amplitude is the height of the waves. Period is the time between waves. Frequency is how often the waves hit.

Taller waves=more energy.
More frequent waves = more energy.
More frequent waves = less time between waves.
More time between waves = less energy
Taller waves =/= more frequent waves.
Etc.