Bending a beam of electrons

A cathode ray tube like the one J.J. Thomson used to discover the electron. Change the voltage on the plates and the current in the coils, and watch how electric and magnetic fields push the beam.

Screen, face on

0.0 mmhow far the spot has moved

Grid squares are 1 cm. The screen keeps glowing for a moment after the beam moves.

Controls

6.3 V supply to filament

Speeds up the electrons. Higher voltage means faster electrons.

Positive: the upper plate is positive. Negative: the upper plate is negative.

Positive and negative currents make magnetic fields in opposite directions.

The small buttons change the current by 1 or 10 thousandths of an amp (mA).

Readings

Meters

Accelerating voltage Va
Plate voltage Vp
Coil current I

What the markers mean

Blue arrows between the plateselectric field
⊗ and ⊙ symbolsmagnetic field
Blue arrow on the beamelectric force
Orange arrow on the beammagnetic force

Investigation

  1. Switch the heater on and watch the cathode. When it is hot enough, it gives off electrons. The anode is positive, so it pulls the electrons towards it, and they shoot through its hole as a narrow beam.
  2. Switch the heater off, then on again. What happens to the beam? What does this tell you about where the electrons come from?
  3. Set a plate voltage but keep the coil current at zero. Which plate does the beam bend towards? Reverse the voltage. What does this tell you about the charge on an electron?
  4. Increase the plate voltage slowly. How does the amount of bending change? What happens if you go too far?
  5. With the plate voltage fixed, change the accelerating voltage. Are fast or slow electrons easier to bend? Suggest why.
  6. Set the plate voltage back to zero and switch on a coil current. Describe the shape of the beam's path in the magnetic field. What changes when you reverse the current?
  7. Now use both fields together. Set a plate voltage, then adjust the coil current until the spot moves back to the centre of the screen. What must be true about the electric and magnetic forces when this happens?
  8. Thomson found that cathode rays bend in both electric and magnetic fields. Explain how this supports the idea that the rays are made of charged particles.

How the model is simplified

  • The fields are shown only between the plates. Real fields spread out a little beyond them.
  • You can see the beam here to make it easier to follow. In a real tube the inside is a vacuum and the beam is invisible. You only see a glow where it hits the screen or the glass.
  • The electrons are shown moving very slowly. Real ones cross the tube in a few billionths of a second.
  • Gravity is ignored, because its effect on an electron is far too small to notice.

In 1897 J.J. Thomson used a tube like this to show that cathode rays are streams of particles we now call electrons. It was the first discovery of a particle smaller than an atom.