The photoelectric effectAQA A-Level Physics: Subtopic test

1. 1
   A physics class directs light of different colours and intensities at a clean metal surface inside an evacuated tube. A sensitive ammeter detects any electrons emitted from the surface. The work function of the metal is 3.4 × 10⁻¹⁹ J.

   (a)

   Which statement explains why electrons are emitted only when the frequency of the light is above a threshold value?

   [1 mark]

   • AOne photon gives all its energy to one electron, which escapes only if the photon energy is at least equal to the work function
   • BElectrons absorb energy from many photons until they have enough to escape
   • CHigher frequency light contains more photons per second
   • DThe intensity of low frequency light is too small to release electrons

   (b)

   What is the threshold frequency of the metal?

   [1 mark]

   • A1.9 × 10⁻¹⁵ Hz
   • B2.3 × 10⁻⁵² Hz
   • C5.1 × 10¹⁴ Hz
   • D5.1 × 10⁻¹⁴ Hz

   (c)

   The intensity of light below the threshold frequency is increased greatly, but no electrons are emitted. Explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a vacuum photocell a reverse potential difference is applied across the tube and increased until the photocurrent just falls to zero. This potential difference is the stopping potential. For one frequency of light the fastest photoelectrons leave the metal surface with a kinetic energy of 2.4 × 10⁻¹⁹ J.

   (a)

   Which statement defines the work function of a metal?

   [1 mark]

   • AThe maximum kinetic energy of the emitted electrons
   • BThe energy of each incident photon
   • CThe energy gained by an electron moving through a potential difference of 1 V
   • DThe minimum energy needed to remove an electron from the surface of the metal

   (b)

   What is the stopping potential?

   [1 mark]

   • A0.67 V
   • B1.5 V
   • C3.8 × 10⁻³⁸ V
   • D2.4 V

   (c)

   Explain why the photoelectrons are emitted with a range of kinetic energies, up to a maximum value.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A clean metal surface with a work function of 3.6 × 10⁻¹⁹ J is illuminated with ultraviolet light of wavelength 320 nm.

   (a)

   Calculate the maximum kinetic energy of the photoelectrons.

   [3 marks]

   (b)

   The ultraviolet source is replaced by a very intense red laser of wavelength 650 nm. Deduce whether photoelectrons are emitted.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A manufacturer is choosing the metal for the light-sensitive surface of a photocell in a visible-light sensor. Metal X has a work function of 3.6 × 10⁻¹⁹ J and metal Y has a work function of 1.0 × 10⁻¹⁸ J. Visible light has wavelengths from about 400 nm to 700 nm.

   (a)

   A technician finds that a photocell with a surface of metal X gives a current immediately when dim blue light of wavelength 450 nm is shone on it, but gives no current at all with bright red light of wavelength 700 nm. Explain these observations using the photon model, and explain why the wave model of light cannot account for them.

   [6 marks]

   (b)

   Evaluate which metal is more suitable for the photocell, and discuss how the current from the photocell depends on the intensity of the light.

   [6 marks]

   Total for question 4: 12 marks

End of questions