Waves Flashcards
Frequency
The total number of waves which pass a specific point per second.
(Symbol, f, and measured in hertz, Hz = 1/s)
Amplitude
The maximum displacement from the rest position
In metres
Period
Time taken for one complete wave to pass a point
Symbol, T, and expressed in seconds
Speed of a wave
The distance covered by the wave per unit time (s)
Symbol, V, and measured in m.s-1
Equation for energy of a photon
E = hf
h is Planck’s constant
When does an electron gain kinetic energy?
When it is accelerated in a potential difference
One electron volt = ….J ?
1 eV = 1,6x10^19 J
When did Hertz discover the photoelectric effect?
He was investigating radio waves
What did Planck show?
That energy is quantised (E=hf)
The rate of photo emission for a single frequency radiation beam is proportional to…
The intensity of the radiation
The emitted photo electrons have kinetic energy ranging from…
Zero to a maximum
Maximum kinetic energy depends on…
Frequency
Emission begins as soon as…
The surface is irradiated with effective radiation
Photoelectric current depends on…
Intensity
Wave theory
An electromagnetic wave produces an electric field, which exerts a force on the electrons on the surface of a metals the force will push the electrons from the surface.
Why does a higher intensity of electromagnetic radiation cause?
A high electric field which then produces a bigger electric force on the electrons. The force will push off the electrons with a higher speed
What is required for emission in terms of frequency?
Emission will occur at any frequency, as the electrons absorb energy from the incoming radiation until they have enough energy to escape
Why threshold frequency? Quantum theory is the answer
Quantum theory - particle theory of light
- EM radiation consists of small particles or packets of energy called photons, which all carry energy proportional to their frequency.
- There are free electrons in metals.
- When a light is directed onto a metal surfaceD a photon will collide with a free electron.
- The interaction between a photon and an electron is equal to one correspondence.
- The photon can then be reflected without a change in its kinetic energy or it transfers all its kinetic energy to the electron.
- The electron gains all the kinetic energy from the photon.
- If the energy gained is sufficient, the electron will escape from the metal surface. That is the process of photoelectric emission which results in the photoelectric effect.
- Part of the energy gained by the electron is used to release it from the surface (to overcome the attraction between the electrons and the metal ions) and the rest of the energy is the kinetic energy of the electron as it leaves the metal.
- The minimum energy required to overcome the forces is the work function.
- The magnitude of this energy is a few electron volts.
- The frequency that corresponds to this energy is the threshold frequency.
- Electrons are only emitted if the frequency radiation is greater than the threshold frequency (hf>W).
Equation to work out max speed of an electron
hf = W + 1/2mv^2
Why is the photoelectric effect important?
- Helped explain the particle nature of light.
- It’s the basis of the quantum theory.
- It’s used in photocells (eg. Solar calculators and alarms)
Wavelength
The wavelength of a wave is the distance between two crests or two troughs on consecutive waves
(Defined by lambda and measured in metres)
