Chapter 13 - Quantum Physics

Question 1

How can the energy of a Wave be found?

Answer 1

E = hf = hc/λ

where h: Planck Constant, f: frequency, c: speed of light, λ: wavelength

Question 2

Define an Electron Volt?

Answer 2

1 Electron Volt is defined as the gain in kinetic energy by an electron when it is accelerated through a potential difference of 1 volt.
1 eV = 1/2 mv²

Question 3

How do you convert from Electron Volts to Joules?

Answer 3

1 eV = 1.6x10⁻¹⁹ J

Question 4

How can Planck’s Constant be experimentally calculated?

Answer 4

• Find the Threshold Voltage of different coloured LEDs.
• Plot a graph of Threshold Voltage against the inverse of the wavelength of light, with a linear line of best fit.
• The gradient is equal to hc/e
• Multiply the gradient by 1.6x10⁻¹⁹, then divide by 3x10⁸.
• This gives us Planck’s Constant.

Question 5

What is the Photoelectric effect?

Answer 5

When light is shone on a surface, if the energy is high enough, photoelectrons will be release.
If the Threshold Frequency is met, then the Work Function is met.
Intensity ∝ No. Electrons Emitted

Question 6

Why couldn’t the Photoelectric Effect be explain by Wave Theory?

Answer 6

• The energy needed to liberate electron should be able to met progressive if light was a wave, which is can’t.
• intensity should increase the kinetic energy of liberated electron, not the rate of electrons are liberated.

Question 7

What is the Work Function?

Answer 7

The minimum energy require to liberate electrons.
Work Function = Fundamental Frequency x Planck’s Constant
ɸ = fh

Question 8

What happens if light with a higher frequency than the Threshold Frequency is used?

Answer 8

The extra energy is converted to Kinetic Energy.

hf = ɸ + Ek

Question 9

What was De Broglie’s Wave Particle Duality Theory?

Answer 9

He stated “if wave-like light showed particle properties, particles like electrons should be expected to show wave-like properties”

Question 10

What is the De Broglie Equation?

Answer 10

λ = h / p
λ = h / mv