J.J. Thomson used cathode ray tubes in his research - Leaving Cert Physics - Question 11 - 2020

Electrons are deviated in a cathode ray tube by electric and magnetic fields. When a voltage is applied across the electrodes, electrons experience a force that alters their trajectory.

The maximum kinetic energy (KE) of an electron accelerated through a potential difference (V) of 4 kV is given by:

$KE = eV$

Where:

• $e$ is the charge of an electron (approximately $1.6 \times 10^{-19}$ C)
• $V$ is the potential difference (4000 V)

Substituting the values, we find:

$KE = (1.6 \times 10^{-19} C)(4000 V) = 6.4 \times 10^{-16} J$

Using the relation between kinetic energy and speed:

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

Solving for the speed ($v$) gives:

$v = \sqrt{\frac{2 \cdot KE}{m}}$

Where:

• $m$ is the mass of an electron (approximately $9.11 \times 10^{-31}$ kg).

Plugging in the values, we find:

$v = \sqrt{\frac{2 \cdot (6.4 \times 10^{-16} J)}{9.11 \times 10^{-31} kg}} = 3.75 \times 10^7 m/s$

To demonstrate the diffraction of light in the laboratory, the following apparatus can be used:

1. Diffraction grating - A tool that disperses light into its component wavelengths.
2. Light source - Such as a laser or monochromatic light to provide coherent light.
3. Spectrometer - An instrument to measure the angles and intensities of diffracted light.

The Geiger counter operates on the principle of gas ionization. When ionizing radiation passes through a gas-filled tube, it ionizes the gas molecules. This creates free electrons and positive ions. An applied voltage collects these charges, resulting in a measurable current pulse which indicates the presence of radiation.

The Geiger-Marsden experiment, also known as the gold foil experiment, involved directing alpha particles at a thin foil of gold. The setup included a source of alpha particles, a gold foil, and a detector.

Observations: Most alpha particles passed straight through the foil, while a small fraction were deflected at large angles, and some even bounced back.

Conclusions: These results suggested that the atom is mostly empty space, with a small, dense, positively charged nucleus at its center, leading to the Rutherford model of the atom.

In the Bohr model, electrons occupy distinct energy levels around the nucleus. When an electron transitions between these levels, it emits or absorbs energy in the form of light. The labelled diagram should include:

• Nucleus - Containing protons and neutrons.
• Electrons in different energy levels - Representing quantized states.
• Photon/light emitted - Indicating energy transitions.