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 (which as we now know, is 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

Specific charges of the electron and the hydrogen ion

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