A group of scientists perform an investigation by shining five different light sources A, B, C, D and E of different wavelengths onto a platinum metal surface - NSC Physical Sciences - Question 10 - 2017 - Paper 1

The work function can be calculated using the formula:

$W_0 = hf_0$

where $h$ is Planck's constant, $6.63 \times 10^{-34} \, J.s$, and $f_0$ is the frequency at which the kinetic energy of the ejected electrons is zero (intercept on the frequency axis). From the graph, $f_0$ corresponds to approximately $1.4 \times 10^{15} \, Hz$.

Calculating the work function:

$W_0 = (6.63 \times 10^{-34}) (1.4 \times 10^{15}) \approx 9.28 \times 10^{-19} \, J$

10.3.1 The intensity of light source A is increased.

The number of ejected electrons will increase.

10.3.2 When light source B is used instead of light source A.

The number of ejected electrons remains the same.

To find the speed of the ejected electron when light source C is used, we first need to calculate the maximum kinetic energy using the information from the graph.

From the graph, the maximum kinetic energy $E_k$ for source C is approximately $2 \times 10^{-18} \, J$. The kinetic energy of an ejected electron is given by the equation:

$E_k = \frac{1}{2}mv^2$

Solving for $v$, we find:

$v = \sqrt{\frac{2E_k}{m}}$

Where $m$ is the mass of an electron, approximately $9.11 \times 10^{-31} \, kg$. Plugging in the values:

$v = \sqrt{\frac{2(2 \times 10^{-18})}{9.11 \times 10^{-31}}} \approx 1.24 \times 10^6 \, m.s^{-1}$