VISUAL PHYSICS ONLINE

8.1 FROM THE UNIVERSE TO THE ATOM

THE ATOM

P81 006

Crookes developed a gas discharge tube to investigate the electrical nature of matter. When a high voltage is applied to the tube, the glass behind the metal cross glows (fluoresces) and a shadow of the cross appears.

(a) Explain the appearance of the shadow. If the cross drops to the horizontal position, describe and explain the pattern observed.

(b) Explain the fluoresces of the glass.

(c) When a magnet is moved towards the gas discharge tube, the shadow moves and is distorted. Explain.

(d) How did the use of discharge tubes by scientists contribute to the understanding of atomic structure?

(e) You have performed investigations using discharge tubes. Explain how you were able to get them to function.

View solution below only after you have completed the answering the question.

Solution

(a) The shadow of the cross showed that the cathode rays travelled in straight lines. Without the cross there is no shadow.

(b) The glass fluoresces because the energy carried by the electrons are absorbed by the glass molecules. The molecules of the glass become and excited and then loss energy by emitting photons of visible light.

(c) The cathode rays are negatively charged electrons – negative charged particles moving in a magnetic field experience a force – electron beam deflected causing a distortion of the shadow of the cross.

(d) Electrons are constituent of atoms, investigate properties of electrons, measure the q_e/m_e ratio.

(e) Need a high voltage source such as an induction coil.