A Level Physics Practice Exam

How is kinetic energy of an electron related to the energy of the photon incident on it?

1. The kinetic energy is equal to the photon's energy

2. The kinetic energy is dependent only on the work function

3. The kinetic energy is the difference between the photon energy and work function

4. The photon energy has no effect on electron kinetic energy

The correct answer is: The kinetic energy is the difference between the photon energy and work function

The correct relationship between the kinetic energy of an electron and the energy of an incident photon can be understood through the photoelectric effect. When a photon strikes an electron, it may transfer energy to the electron. The energy of the incident photon is given by \( E = hf \), where \( h \) is Planck's constant and \( f \) is the frequency of the photon. For an electron to be emitted from the surface of a material, it must overcome the work function, which is the minimum energy required to release an electron from the material. If the energy provided by the photon is greater than the work function, the excess energy is converted into the kinetic energy of the emitted electron. Therefore, the kinetic energy (\( KE \)) of the electron is the energy of the incoming photon minus the work function (\( \phi \)) of the material: \[ KE = hf - \phi \] This accurately describes why the kinetic energy of the electron is the difference between the energy of the photon and the work function of the material. Thus, the answer correctly identifies the relationship as the kinetic energy being dependent on both the energy of the photon and the work function.