Lesson test: Electromagnetic waves

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Electromagnetic waves

1.[3p]
The wave equation for $E$ is obtained by

differentiating one coupled equation with respect to $x$ and substituting the other for $\partial B / \partial x$

Correct

integrating Gauss's law over a closed surface

The answer is: differentiating one coupled equation with respect to $x$ and substituting the other for $\partial B/\partial x$

assuming the wave travels at the speed of light and checking consistency

The answer is: differentiating one coupled equation with respect to $x$ and substituting the other for $\partial B/\partial x$

setting the charge density to zero in Coulomb's law

The answer is: differentiating one coupled equation with respect to $x$ and substituting the other for $\partial B/\partial x$

2.[3p]
Taking $ε_{0} = 8.854 \times 10^{- 12}$ and $μ_{0} = 1.2566 \times 10^{- 6}$ , what is $1 / \sqrt{ε_{0} μ_{0}}$ , in units of $10^{8}$ metres per second?

CorrectNot quite: 3.00

3.[3p]
Why was the agreement between $1 / \sqrt{ε_{0} μ_{0}}$ and the measured speed of light such strong evidence?

Both constants were measured electrically, with no adjustable parameter available to force the match

Correct

The two numbers agreed to twelve significant figures

The answer is: Both constants were measured electrically, with no adjustable parameter available to force the match

Maxwell had predicted the speed of light before it was measured

The answer is: Both constants were measured electrically, with no adjustable parameter available to force the match

Fizeau's measurement was made with the same apparatus

The answer is: Both constants were measured electrically, with no adjustable parameter available to force the match

4.[2p]
A radio station transmits at 100 MHz. What is the wavelength, in metres?

CorrectNot quite: 3.0

5.[2p]
In an electromagnetic wave in vacuum, the electric and magnetic fields carry equal amounts of energy.

True

Correct

False

The answer is: True

6.[3p]
A beam delivers 1000 W/m². What is the rms electric field in it, in volts per metre?

CorrectNot quite: 614

7.[3p]
A perfectly reflecting surface receives 2000 W/m². What radiation pressure does it feel, in units of $10^{- 5}$ pascals?

CorrectNot quite: 1.33

8.[3p]
Which of these are places where the classical theory of this course fails?

Select all that apply

The photoelectric effect, where the electron energy depends on frequency rather than intensity

Correct

The stability of the atom, since an orbiting electron should radiate and spiral in

Correct

The Michelson-Morley result, which found no medium for the waves to travel in

Correct

The propagation of radio waves over long distances

The answer is: The photoelectric effect, where the electron energy depends on frequency rather than intensity, The stability of the atom, since an orbiting electron should radiate and spiral in, The Michelson-Morley result, which found no medium for the waves to travel in

9.[3p]
Match each experimenter to what they contributed to the story of light as a wave.

Weber and Kohlrausch
Fizeau
Maxwell
Hertz

measured the ratio of electrical units
identified the two speeds as the same thing
measured the speed of light with a toothed wheel
generated and detected the waves in a laboratory

Show the answer

Weber and Kohlrausch: measured the ratio of electrical units Fizeau: measured the speed of light with a toothed wheel Maxwell: identified the two speeds as the same thing Hertz: generated and detected the waves in a laboratory

1.[3p] The wave equation for E is obtained by

2.[3p] Taking ε0=8.854×10-12 and μ0=1.2566×10-6, what is 1/ε0μ0, in units of 108 metres per second?

3.[3p] Why was the agreement between 1/ε0μ0 and the measured speed of light such strong evidence?

4.[2p] A radio station transmits at 100 MHz. What is the wavelength, in metres?

5.[2p] In an electromagnetic wave in vacuum, the electric and magnetic fields carry equal amounts of energy.

6.[3p] A beam delivers 1000 W/m². What is the rms electric field in it, in volts per metre?

7.[3p] A perfectly reflecting surface receives 2000 W/m². What radiation pressure does it feel, in units of 10-5 pascals?

8.[3p] Which of these are places where the classical theory of this course fails?

9.[3p] Match each experimenter to what they contributed to the story of light as a wave.

1.[3p]
The wave equation for $E$ is obtained by

2.[3p]
Taking $ε_{0} = 8.854 \times 10^{- 12}$ and $μ_{0} = 1.2566 \times 10^{- 6}$ , what is $1 / \sqrt{ε_{0} μ_{0}}$ , in units of $10^{8}$ metres per second?

3.[3p]
Why was the agreement between $1 / \sqrt{ε_{0} μ_{0}}$ and the measured speed of light such strong evidence?

4.[2p]
A radio station transmits at 100 MHz. What is the wavelength, in metres?

5.[2p]
In an electromagnetic wave in vacuum, the electric and magnetic fields carry equal amounts of energy.

6.[3p]
A beam delivers 1000 W/m². What is the rms electric field in it, in volts per metre?

7.[3p]
A perfectly reflecting surface receives 2000 W/m². What radiation pressure does it feel, in units of $10^{- 5}$ pascals?

8.[3p]
Which of these are places where the classical theory of this course fails?

9.[3p]
Match each experimenter to what they contributed to the story of light as a wave.