module 4 - quantum physcis Flashcards
what is a photon?
a quantum of electromagnetic energy
What is the energy of 1 electronvolt?
the energy of 1eV is defined as the energy transferred to or from an electron when it moves through a potential diffference of 1 V.
explain how to set up a practical to determine plancks constant?
set up a simple circuit containing an LED, a battery, a voltmeter and a resistor (to protect the LED)
LEDs convert electrical energy into light energy. they emit visible photons when the p.d. across them is above a critical value (threshold p.d.).
When the p.d. reaches the threshold p.d. the LED lights up and starts emitting photons of a specific wavelength. At this p.d. the work done is given by W=VQ. use voltmeter to measure threshold p.d. Place a black tube over the LED to show exactly when the LED lights up
How do you calculate the plancks constant using the experiment data?
we need to know the wavelength of the photons emiited by the LED to determine plancks constant.
At the threshold p.d the energy transferred by an electron in the LED is approximately equal to the energy of the single photon it emits.
Ve = hf
eV = hc/λ.
to be more accurate obtain more data for different wavelength LEDs. plot a graph of V against 1/λ. determine plancks constant from the gradient of the graph, hc/e .
What is the defintion of the photoelectric effect?
the emission of photoelectrons from a metal surface when electromagnetic radiation above a threshold frequency is incident on the metal.
what is a gold- leaf electroscope?
- gold leaf stands up because it is negatively charged
- charge is lost due to photoelectric effect of UV on zinc
- gold leaf droops back down (showing that charge is lost)
- the incident ray causes the free electrons to be emitted from the zinc plate.
What were the 3 key observations from the photoelectric effect?
- Electrons were emitted only if the incident radiation was above the threshold frequency.
- If the incident radiation was above the threshold frequency, emission of electrons was instantaneous.
- If the incident radiation was above the threshold frequency, increasing the intensity of the radiation did not increase the maximum kinetic energy of the electrons. instead more electrons were emitted.(rate of emission increases) the only way to increase the maximum kinetic energy was to increase the frequency of the incident radiation
how can you use photons to illuminate photoelectric effect?
einstein suggested that each electron in the surface of the metal must require a certain amount of energy in order to escape from the metal, and that each photon could transfer its exact energy to one surface electron in a one-to-one interaction.
as a single photon delivers its energy to a single surface electron in a one to one interaction. if a photon does not carry enough energy on its own to free an electron, the number of photons makes no difference. the only thing that does is the energy which is dependant on the frequency.
what is the defintion of work function?
the minimum energy needed to remove a single electron from the surface of a metal.
what is threshold frequency?
The minimum frequency of the electromagnetic radiation that will cause the emission of an electron from the surface of a metal.
what does the energy of each individual photon do? (photelectric effect)
- it frees a single electron from the surface of the metal in a one-to-one interaction.
- any remainder is trasnferred into the kinetic energy of the photoelectron.
what is the wave-particle duality?
A theory that states that matter has both particle and wave properties and also electromagnetic radiation has wave and particulate nature.
what shows the wave nautre of electrons?
diffraction. they can be made to spread out like waves as they pass through a tiny gap.
what shows the particulate nature of electrons?
they have mass and charge. they can be accelerated and deflected by electric and magnetic fields.
