Grade 12 Physics Knowledge area Matter and materials

Grade 12 Physics Knowledge area: Matter and materials

Photoelectric effect 1. 3 Work function and threshold frequency Work function (W 0) of a metal: It is the minimum amount of energy required for an electron to escape from the surface of a specific metal. The work function is metal-specific. Definition

Photoelectric effect The following table represents the work function of a number of elements: Element Aluminium Beryllium Cadmium Calcium Carbon Caesium Cobalt Work function (e. V) 4, 08 5 4, 07 2, 9 4, 81 2, 1 5 Element Copper Gold Iron Lead Magnesium Mercury Nickel Work function (e. V) 4, 7 5, 1 4, 5 4, 14 3, 68 4, 5 5, 01 Element Potassium Platinum Selenium Silver Sodium Uranium Zinc Work function (e. V) 2, 3 6, 35 5, 11 4, 73 2, 28 3, 6 4, 3

Photoelectric effect Threshold frequency (f 0) of a metal: It is the minimum frequency with which the metal must be irradiated to release electrons from the surface of the metal. Definition

Photoelectric effect The following table describes the different situations associated with various amounts of energy. less Energy of photons more No photoelectrons released Photoelectrons are only just emitted. Photoelectrons are emitted with a quantity of EK. If hf < W 0 the electron cannot be emitted. If hf = W 0 the photon has enough energy to emit the electron. If hf > W 0 the electrons have more energy than needed for emission.

Photoelectric effect less Energy of photons more No photoelectrons released Photoelectrons are only just emitted. Photoelectrons are emitted with a quantity of EK. The transferred energy is either irradiated again in the form of another photon or distributed to the metal’s atoms. It increases the internal energy (temperature) of the metal. The frequency of the photon is equal to the threshold frequency of the metal. The photoelectrons has no EK and is attracted to the metal again, because the metal now has a positive charge. The remaining energy is converted into kinetic energy (EK). The photoelectrons can now move away from the metal.

Photoelectric effect

Photoelectric effect

Photoelectric effect

Photoelectric effect Examples A group of learners conduct an investigation to compare the effect of two types of radiation on the release of photoelectrons from zinc. They place a zinc plate on top of the disc of a negatively charged electroscope. Ultraviolet and red light are shone in turn onto the zinc plate, as indicated below, while the electroscope is fully charged in each case.

Photoelectric effect

Photoelectric effect They note the following observations: Radiation Observation Ultraviolet light Gold leaves fall down. Red light No effect on the gold leaves 1. 1 Write down an INVESTIGATIVE QUESTION for this investigation. 1. 2 Explain the observation made for the ultraviolet light.

Photoelectric effect 1. 3 What conclusion can be drawn from this investigation? The following safety measures are printed on an ultraviolet light source: SEVERE EXPOSURE TO ULTRAVIOLET LIGHT IS A HEALTH RISK 1. 4 Name ONE health risk associated with severe exposure to ultraviolet light.

Photoelectric effect 2 The learners have access to the following information: Work function of zinc 6, 88 × 10 -19 J Frequency of ultraviolet light 4, 29 × 1014 Hz Frequency of red light 4, 29 × 1014 Hz 2. 1 Define the term work function of a metal. 2. 2 Name ONE type of electromagnetic radiation with a higher frequency than that of ultraviolet light.

Photoelectric effect 2. 3 Use a calculation to explain why red light does not succeed in releasing photoelectrons from the surface of the zinc plate. 1. 1 Which type of radiation will release photoelectrons from zinc? OR Which one, red light or ultraviolet light, will release photoelectrons from zinc?

Photoelectric effect Note This type of question is marked as follows: • The dependent and independent variables are named. • Ask a question about the relationship between the dependent and independent variables.

Photoelectric effect 1. 2 Ultraviolet light releases photoelectrons from the surface of the zinc plate. Electrons in the gold leaf move upwards in the disc of the electroscope due to the shortage of electrons. Fewer negative charges are left in the gold leaf, therefore there is less repulsion between the gold leaves. 1. 3 Ultraviolet light/radiation will release photoelectrons from the surface of zinc. Red light, however, will not release photoelectrons from the surface of zinc.

Photoelectric effect 1. 4 It could cause skin cancer or damage the skin. It could cause cataracts or damage the eyes. 2. 1 It is the minimum energy needed by an electron in a metal to be released from the surface of the metal. 2. 2 X-rays or gamma rays 2. 3 E = hf = (6, 63 × 10 -19)(4, 29 × 1014) = 2, 84 × 10 -19 J E < W 0 – no electrons emitted.

Photoelectric effect Examples The threshold frequency of a metal is 1, 8 × 1015 Hz. 1. Determine the work function of the metal. W 0 = hf 0 = (6, 63 × 10 -34)(1, 8 × 1015) = 1, 19 × 10 -18 J

Photoelectric effect 2. Calculate the speed at which electrons are emitted if a light with a wavelength of 160 nm shines on it. (The mass of an electron is read off the information table as 9, 1 × 10 -31 kg. ) E = W 0 + ½mv 2 hc λ = W 0 + ½mv 2 (6, 63 × 10 -34)(3 × 108) (160 × 10 -9) = (1, 19 × 10 -18) + ½(9, 1 × 10 -31)v 2 v = 3, 42 × 105 m⋅s-1