A student conducts an experiment to determine the Planck constant hhh using light-emitting diodes (LEDs) of different wavelengths λ\lambdaλ. An LED starts to conduct and emit monochromatic light when the potential difference across it exceeds a threshold value V0V_0V0.
The equation relating V0V_0V0 and λ\lambdaλ is:
V0=(hce)×1λ+K V_0 = \left(\frac{hc}{e}\right) \times \frac{1}{\lambda} + K V0=(ehc)×λ1+Kwhere:
The student plots a graph of V0V_0V0 against 1λ\frac{1}{\lambda}λ1. The gradient of the student's straight line of best fit is found to be 1.35×10−6 V m1.35 \times 10^{-6}\text{ V m}1.35×10−6 V m.
Determine the value of the Planck constant hhh to 2 significant figures using the gradient of the line of best fit.
Calculate the percentage difference between this experimental value of hhh and the accepted value of 6.63×10−34 J s6.63 \times 10^{-34}\text{ J s}6.63×10−34 J s. Give your answer to 2 significant figures.
Identify one source of random error in determining V0V_0V0 and suggest how this error can be minimized.
26 exam-style questions on OCR A A Level Physics Quantum physics, covering Photons, The photoelectric effect, and Wave–particle duality. Each one has a worked solution and a mark scheme showing where the marks go.