Q: A 0.5 m long organ pipe is open at both ends. What is the fundamental frequency if the speed of sound in air is 340 m/s?

$$f_1$$ = \dfrac{v}{2L} = \dfrac{340}{2 \times 0.5} = 340\ \mathrm{Hz}$$

Q: A sound wave has an intensity of $$5.5 \times 10$$^{-6}$$\ \mathrm{W/m^2$}. $$What is its sound level in decibels (dB)?

$$L = 10 \log_{10} \left( \dfrac{I}{I_0$} \right ) = 10 \log_{10} \left( \dfrac{5.5 \times 10$$^{-6}$$}{1.0 \times 10$$^{-12}$$} \right ) \approx 67\ \mathrm{dB}$$

Q: Two point charges, $$q_1 = +2$$\ \mu\mathrm{C}$$ and q_2$ = -3\ \mu\mathrm{C}$$, are separated by 0.5 m. What is the magnitude of the force between them? ($$k = 8.99 \times 10^9$\ \mathrm{Nm^2$/C^2$$}$)

$$F = k \dfrac{|q_1$ q_2$|}{r^2$} = 8.99 \times 10^9$ \dfrac{2 \times 10$$^{-6}$$ \times 3 \times 10$$^{-6}$$}{0.25} = 0.216\ \mathrm{N}$$

Q: A proton is placed in a uniform electric field of $$1.0 \times 10^4$\ \mathrm{N/C}. $$What is the magnitude of the force on the proton?

$$F = qE = 1.6 \times 10$$^{-19}$$ \times 1.0 \times 10^4$ = 1.6 \times 10$$^{-15}$$\ \mathrm{N}$$

Question 1

A string of length 2.0 m is fixed at both ends and vibrates in its fundamental mode. If the speed of the wave on the string is 200 m/s, what is the frequency of the fundamental mode?

Accepted Answer

$$f = \dfrac{v}{2L} = \dfrac{200}{2} = 100\ \mathrm{Hz}$$

Question 2

Which of the following statements correctly describes the difference between transverse and longitudinal waves?

Accepted Answer

In transverse waves, particle motion is perpendicular to wave propagation; in longitudinal waves, it is parallel.

Question 3

A sound wave of frequency 440 Hz travels through air at 20°C. What is its wavelength?

Accepted Answer

$$\lambda = \dfrac{v}{f} = \dfrac{343}{440} \approx 0.78\ \mathrm{m}$$

Question 4

A tuning fork produces a sound wave in air. Which part of the wave corresponds to a region of compression?

Accepted Answer

The crest of the sinusoidal wave.

Question 5

A 0.5 m long organ pipe is open at both ends. What is the fundamental frequency if the speed of sound in air is 340 m/s?

Accepted Answer

$$f_1$$ = \dfrac{v}{2L} = \dfrac{340}{2 	imes 0.5} = 340\ \mathrm{Hz}$$

Question 6

Which equation gives the speed of a wave on a string under tension $$F$$ and with linear mass density $$\mu$$?

Accepted Answer

$$v = \sqrt{\dfrac{F}{\mu}}$$

Question 7

A sound wave has an intensity of $$5.5 	imes 10$$^{-6}$$\ \mathrm{W/m^2$}. $$What is its sound level in decibels (dB)?

Accepted Answer

$$L = 10 \log_{10} \left( \dfrac{I}{I_0$} ight ) = 10 \log_{10} \left( \dfrac{5.5 	imes 10$$^{-6}$$}{1.0 	imes 10$$^{-12}$$} ight ) \approx 67\ \mathrm{dB}$$

Question 8

A car is moving toward a stationary observer at 30 m/s. If the car's horn emits a frequency of 500 Hz and the speed of sound is 340 m/s, what frequency does the observer hear?

Accepted Answer

$$f' = f \dfrac{v + v_o}{v - v_s} = 500 \dfrac{340}{310} \approx 548\ \mathrm{Hz}$$

Question 9

Which of the following best describes the conservation of electric charge?

Accepted Answer

Electric charge is always conserved in an isolated system.

Question 10

Two point charges, $$q_1 = +2$$\ \mu\mathrm{C}$$ and q_2$ = -3\ \mu\mathrm{C}$$, are separated by 0.5 m. What is the magnitude of the force between them? ($$k = 8.99 	imes 10^9$\ \mathrm{Nm^2$/C^2$$}$)

Accepted Answer

$$F = k \dfrac{|q_1$ q_2$|}{r^2$} = 8.99 	imes 10^9$ \dfrac{2 	imes 10$$^{-6}$$ 	imes 3 	imes 10$$^{-6}$$}{0.25} = 0.216\ \mathrm{N}$$

Question 11

What is the electric field at a distance $$r$$ from a point charge $$q$$?

Accepted Answer

$$E = k \dfrac{q}{r^2$}$$

Question 12

A proton is placed in a uniform electric field of $$1.0 	imes 10^4$\ \mathrm{N/C}. $$What is the magnitude of the force on the proton?

Accepted Answer

$$F = qE = 1.6 	imes 10$$^{-19}$$ 	imes 1.0 	imes 10^4$ = 1.6 	imes 10$$^{-15}$$\ \mathrm{N}$$

Question 13

An electron is accelerated from rest through a potential difference of 100 V. What is its final kinetic energy?

Accepted Answer

$$K = qV = 1.6 	imes 10$$^{-19}$$ 	imes 100 = 1.6 	imes 10$$^{-17}$$\ \mathrm{J}$$

Question 14

Which of the following best describes the direction of electric field lines?

Accepted Answer

Electric field lines start on positive charges and end on negative charges.

Question 15

A sound-insulating door reduces the sound level by 40 dB. What fraction of the original sound intensity passes through the door?

Accepted Answer

$$\dfrac{I}{I_0$} = 10$$^{-4}$$

Question 16

A neutron (mass $$m = 1.67 	imes 10$$^{-27}$$\ \mathrm{kg}) is $$suspended at rest in a uniform electric field $$E$$ that balances gravity. What is the magnitude of $$E if $$the neutron has charge $$q = -1.6 	imes 10$$^{-19}$$\ \mathrm{C}$$?

Accepted Answer

$$E = \dfrac{mg}{|q|} = \dfrac{1.67 	imes 10$$^{-27}$$ 	imes 9.8}{1.6 	imes 10$$^{-19}$$} \approx 1.02 	imes 10$$^{-7}$$\ \mathrm{N/C}$$

Question 17

A parallel plate capacitor is charged to a potential difference of 50 V. If the separation between the plates is 0.01 m, what is the magnitude of the electric field between the plates?

Accepted Answer

$$E = \dfrac{V}{d} = \dfrac{50}{0.01} = 5000\ \mathrm{N/C}$$

Waves, Sound, and Electricity: Physics 103 Review Notes

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