Rays inside a glass tube
Cathode rays are streams of electrons.
An almost empty tube
Imagine a glass tube with almost all the gas pumped out, and a metal electrode at each end. A high voltage is applied across the electrodes.
What do you think J. J. Thomson saw in 1897?
What this lesson covers
The idea
In a gas at very low pressure under high voltage, cathode rays travel from the cathode to the anode; they are streams of negatively charged particles, electrons, much lighter than atoms and emitted from them.
An almost empty tube
Imagine a glass tube with almost all the gas pumped out, and a metal electrode at each end. A high voltage is applied across the electrodes.
What do you think J. J. Thomson saw in 1897?
- Rays travelling from one electrode to the other
- Nothing at all, because there is hardly any gas
- The two electrodes swap their charges
Thomson's tube
Press Next step to see what Thomson found.
Rays made of particles
The rays turned out to be a stream of tiny particles, each with a negative charge.
In a gas at very low pressure under a high voltage, cathode rays travel from the cathode to the anode. They are streams of negatively charged particles, called electrons, with a much smaller mass than atoms, emitted from atoms.
Notes
Cathode rays travel from the cathode to the anode. They are streams of negatively charged electrons, much lighter than atoms and emitted from them.
Check yourself
In the cathode ray tube, in which direction do the rays travel?
Thomson found that cathode rays are streams of …
Compared with the mass of an atom, the mass of a particle in a cathode ray is …
What were these particles later called?
- From the anode to the cathode. The rays travel from the cathode, the negative electrode, to the anode, the positive electrode.
- From the middle of the tube to the two electrodes. The rays start at the cathode and end at the anode.
- From the cathode to the anode — correct. Yes!
- negatively charged particles — correct. Yes!
- positively charged particles. The rays behave as negatively charged particles in electric and magnetic fields.
- particles with no charge. In electric and magnetic fields the rays showed that the particles are charged, and the charge is negative.
- much bigger. The particles have a much smaller mass than atoms.
- much smaller — correct. Yes!
- the same. The particles have a much smaller mass than atoms, and they are emitted from atoms.
- Protons. Protons are the positively charged particles in the nucleus, discovered and named by Rutherford.
- Neutrons. Neutrons have no charge and were discovered by Chadwick in 1932.
- Electrons — correct. Yes!