Significance of Thomson's Experiment (AQA A Level Physics): Revision Note

Comparing specific charge

• Thomson demonstrated the deflected particles in magnetic and electric fields must be negatively charged

• Previously, scientists had calculated the specific charge of a hydrogen ion (now known to be a proton)

• Thomson's specific charge for an electron was around 1800 times larger than that of the hydrogen ion, as shown in the table below

Thomson's experiment

• Recall that specific charge is defined as:

$\frac{\mathrm{particle}\mathrm{charge}}{\mathrm{particle}\mathrm{mass}}$

• The fact that this number for an electron was much larger than for a hydrogen ion meant either:

  • the electron had a much smaller mass

  • the electron had a much larger magnitude of charge

• Specific charge is a ratio so there was no way of knowing which was correct

  • Further experiments had to be performed to calculate the charge of an electron