A Level Physics Practice Exam

What is the consequence of the kinetic energy of emitted electrons when a clean metal surface is irradiated with ultraviolet radiation?

1. Electrons are emitted with consistent energy levels

2. The kinetic energy of emitted electrons is at a maximum value

3. All electrons are emitted with zero kinetic energy

4. No electrons are emitted at low energies

The correct answer is: The kinetic energy of emitted electrons is at a maximum value

When ultraviolet radiation is directed at a clean metal surface, the energy of the incident photons can overcome the work function of the metal. The work function is the minimum energy required to remove an electron from the surface of the material. Once the photon energy exceeds this threshold, electrons can be emitted from the surface. The kinetic energy of the emitted electrons is determined by the equation: \[ KE = hf - \phi \] where \( KE \) is the kinetic energy of the emitted electrons, \( hf \) is the energy of the photons (with \( h \) being Planck's constant and \( f \) the frequency of the incident radiation), and \( \phi \) is the work function of the metal. If the energy of the photons is significantly greater than the work function, the excess energy contributes to the kinetic energy of the emitted electrons. Therefore, the emitted electrons can have a wide range of kinetic energies, with some being emitted with very high kinetic energies. In this scenario, the statement about the kinetic energy of emitted electrons being at a maximum value correctly reflects that the highest energy electrons will correspond to the highest energy photons, minus the energy needed to remove them from the metal surface. Thus, emitted electrons can indeed possess high kinetic