Electromagnetic Radiation(quiz #1)

Question 1

What did Oersted do to advance our understanding of electromagnetism?

Answer 1

Oersted found that an electrical current in a conducting wire produces a magnetic field perpendicular to the current(Showed that electricity can lead to magnetism)

1st hand rule, compass over wire experiment

Question 2

What did Faraday do to advance our understanding of electromagnetism?

Answer 2

A changing magnetic flux will induce current in a conductor(magnetism can induce electricity)

Bar over open circuit, modified third hand rule

Question 3

What did James Clerk Maxwell do?

Answer 3

Confirmed that a changing magnetic flux will induce current in any object, including briefly in insulators.

He also believed that electric and magnetic fields could exist in space(don’t need an object to induce magnetic or electrical field), therefore modifying Oersted and Faraday’s principle.

Question 4

What was Maxwell’s principle?

Answer 4

A changing electric field in space will generate a changing magnetic field(and vice versa). The interaction between these fields propagates an electromagnetic wave through space

Question 5

An electromagnetic wave is ______________(1) meaning that it keeps producing itself and propagates on its own in space.

Answer 5

(1) self propagating

Question 6

What were Maxwell’s four predictions about EM waves?

Answer 6

1. Accelerating charges produce EM waves
2. The frequency of oscillations of the charges(how many oscillations in 1 second) is equal to the frequency of the EM wave(the EM wave is oscillating) that is produced.
3. The oscillating field are perpendicular to each other and to the direction of wave propagation
4. All EM waves:
   - Travel at 3.00 x 10^8 m/s in a vacuum and obey universal wave equation
   - Can reflect, retract, diffract, interfere, and become polarized

Question 7

How are AC’s an example of accelerating charges?

Answer 7

The electrons are oscillating back and forth, meaning that the direction is constantly changing resulting in acceleration

Question 8

Describe & draw the apparatus of Heinrich Hertz’s experiment(the EMR experiment)

Answer 8

Two wires placed beside each other, each with gap between them. Wire #1 has a high voltage source attached to it, which produces a current in wire #1. Wire #2 has no power source attached.

Wire #1 has an oscillating electron inside. If the electrostatic force is enough and the voltage source is high enough, it will allow an electron to jump across the gap in wire #1 and produce a spark. The accelerating electron in wire #1 will produce an EMR wave that will induce a spark across the gap in wire #2. The electric field in the EMR wave affects the stationary charge in wire #2.

When the electric field is downward(i.e. the trough of the wave), the electron hops down. When the trough turns into a crest, the electron jumps up.

Remember that magnetic fields only affect moving charges while electric fields affect both stationary and moving charges. This is why the electric field produce by wire #1 must be in line with the gap in wire #2

Question 9

What were the observations from Hertz’s experiment

Answer 9

1. Rotating the second gap by 90 degrees does not produce a spark
2. The speed of EM wave determined (using a zinc reflector plate) to be in the range of 3.00 x 10^8 m/s, even if the frequency of oscillation is changed

Question 10

What were the conclusions from Hertz’s experiment?

Answer 10

1. A spark is only produced across the second gap when the electric field is in line with the gap(stationary election only affected by the electric field and not the magnetic field) and when the magnetic field is perpendicular to the gap. This also demonstrates the perpendicular nature of EM waves.
2. EM waves exist
3. Visible light is an EM wave

Question 11

What was John Dalton’s version of the atom?

Answer 11

All elements are composed of tiny indivisible atoms. The atom is the smallest possible particle.

Question 12

What was J.J. Thomson version of the atom?

Answer 12

Raisin bun/plum pudding model: The atom is a sphere of positive charge and contains an equal number of electrons. Overall, the atom is neutral. Although incorrect, Thomson’s model explains the basic principles of electrostatics(i.e. the transfer of electrons from a positively charged object to a negatively charged object).

Question 13

Describe the apparatus of Rutherford’s scattering experiment.

Answer 13

A lead box with a hole inside is placed. Lead is used because its good at absorbing alpha particle making it a safe way to contain the alpha particles that you don’t want leaving the box. At the end of the hole, there is uranium. Uranium is radioactive and it undergoes alpha decay which is a type of radioactive decay (that makes uranium unstable and causes it to emit alpha particles).

The alpha particles are fired at gold foil from the radioactive uranium source. Gold foil is used because it could be pounded down to a layer which is only a few atoms thick.

The deflected alpha particles hit a zinc sulphide screen, which is a phosphorescent light source that will glow when the high energy particles hit it.

The microscope then is used to observe where on the screen the alpha particles strike.

Question 14

What were the predictions and the results of Rutherford’s scattering experiment?

Answer 14

According to the Thomson model, most alpha particles should travel through the gold foil undeflected because of the uniform charge distribution. Only the alpha particles that pass near an electron will be slightly deflected due to electrostatic forces. Alpha particles have a lot of momentum and the electrostatic force is not enough to overcome the inertia of the alpha particles.

Results: As predicted, majority of the alpha particles traveled through undeflected and some alpha particles were slightly deflected. However, a few alpha particles were deflected at large angles and the Thomson model could not explain this.

Question 15

Describe Rutherford’s Planetary Model. How did it explain the observations Rutherford made while conducting his scattering experiment?

Answer 15

Most of the mass and all the positive charge is concentrated at the centre of the atom called the nucleus. The remainder of the atom, which makes up most of the volume, is spy space and a few orbiting electrons.

As an alpha particle get closer to the nucleus, it will experience a very strong repulsive electrostatic force that causes very few atoms to experience a strong deflective force. Since the nucleus makes up such a small volume of the atoms, only a few alpha particles experience such. strong repulsive force.

Question 16

What was the failure of Rutherford’s Planetary Model?

Answer 16

Maxwell’s principle of EM explained a failure in Rutherford’s Planetary model. The electrostatic force of attraction on the electrons from the nucleus acts a centripetal force, thus causing the orbiting electrons to constantly accelerate. Since accelerating charges emit EMR, the electrons lose energy in the form of kinetic energy. This loss of speed should cause electrons to spiral into the nucleus and explode. Since this does not occur, Rutherford’s Planetary model was proven incorrect.

Question 17

What is a blackbody?

Answer 17

A blackbody is an object that completely and perfectly absorbs any light energy that falls on it and reradiated this energy as light energy or EMR. Think of a black object.

Question 18

What is blackbody radiation? Draw the radiation diagram.

Answer 18

Radiation that is emitted when light energy falls on a blackbody(energy absorbed causes electrons to oscillate and accelerate to produce EMR). The type of light reradiated from the blackbody is dependant only on the temperature of the object. When a blackbody is heated, the electrons vibrate over a variety of frequencies, therefore a range of different frequencies. As temperature increase, the frequency of light emitted also increases.

Question 19

What part of blackbody radiation could classical physics not explain? Draw the graph that explains this discrepancy.

Answer 19

Classical physics predicted that increasing the temperature of a blackbody should progressively produce higher frequency EMR(including high frequency UV, x-rays, etc.)

The experimental data however showed that high frequency UV, x-rays, or gamma rays cannot be produce regardless of the temperature of the blackbody. There is a sharp drop off in the UV part of the spectrum.

Question 20

What are photons also referred to?

Answer 20

Quanta(multiple bundles) or quantum( a single bundle)

Question 21

What was Max Planck’s Quantum Hypothesis(1900)?

Answer 21

• Provided an explanation for why blackbody radiation curve appeared the way it did
• Assumed that the energy distributed among the molecular oscillators is not continuous but instead consists of a finite number of small discrete amounts.
• Energy acts as both a particle and wave: Bullets of energy that behave as wavelengths(diffraction, interference, polarization)

Question 22

What is the quantization of energy? How did it differ from what classical physics predicted about energy?

Answer 22

The energy of any molecular vibration can only be some whole number multiple of hf(the quantum of energy).

E = nhf

where n Is the number of discrete bundles of energy.

Classical physics predicted that energy could come in any size or amount.

Question 23

What does intensity refer to?

Answer 23

The # of photons

Question 24

What is the electron volt measure?

Answer 24

The amount of energy that one electron gains when it falls through a potential difference of 1V. Another, more convenient way to express joules.

Question 25

What is wave-particle duality? What key physics principle relates to this concept?

Answer 25

Depending on the situation, light will sometimes behave more like a wave and other times more like a particle. For low frequency EMR, wave nature dominates. For high frequency EMR, particle nature dominates.

Key physics principle #9(wave-particle duality)

Question 26

What is the photoelectric effect? Who observed the photoelectric effect? Draw the apparatus of his experiment.

Answer 26

The photoelectric effect was observed by Heinrich Hertz in 1887. He observed that when a high frequency light is shone on a metal cathode ray in a CRT, cathode rays will be emitted to the anode even without a voltage source. Photoelectric means to take light and turn it into electricity(i.e. solar panels)

Question 27

What were the 5 observations Hertz made about the photoelectric effect?

Answer 27

1. The photoelectric effect only occurs if the incident light has a frequency above a certain threshold frequency f(0)
2. Different metal cathodes have a different threshold frequency
3. The effect is instantaneous
4. Increasing the frequency of incident photons increase the kinetic energy of the photoelectrons(due to conservation of energy) but dos not increase the photocurrent.
5. Increasing the intensity of incident light increases the photocurrent but does not increase the speed of the photoelectrons.

Question 28

What does frequency refer to?

Answer 28

The energy of the incident photons(how much energy is in one photon)

Question 29

What does photocurrent refer to?

Answer 29

The number of electrons

Question 30

What model of light did Einstein use to explain Hertz’s observations about the photoelectric effect?

Answer 30

Max Planck’s photon model of light

Question 31

Derive the equation used for the photoelectric effect.

Answer 31

E(input) = E(output) —-> Conservation of energy
E(photon) = W + E(k, photon)
hf = W + (1/2) m(electron) v^2

Question 32

What were the three explanations Einstein provided?

Answer 32

1. Electrons on the surface of a cathode are held there with a certain binding energy. For electrons to become free of the metal, this binding energy must be overcome. Einstein called the amount of energy a photon of light must have to release an electron from the metal the work function W, which is calculated using W =hf(0)
2. Since each individual photon(with a frequency above the threshold) will free exactly one electron, increasing the number of incident photons(intensity) striking the cathode will increase the number of electrons bumped off the metal surface(photocurrent)
3. If the incident photon has more than the minimum energy required to cause the PW effect, any excess energy will be given to the electron as kinetic energy.