Charged-Particle Motion in a Uniform Field
Charged-Particle Motion in a Uniform Field
- A uniform electric field gives a charged particle a constant force. That force produces a constant acceleration.
From field strength to acceleration
Worked example
An electron between parallel plates
The plates are 0.041 m apart and have a potential difference of 58 kV. Find the electron acceleration.
- Convert kilovolts, then find the field strength.
- Find the magnitude of the electric force.
- Use Newton’s second law.
Answer
This three-step chain is assessed in (9702/42/M/J/25 Q6(b)).
Motion across a uniform field
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- Before entering the plates, the particle has a velocity across the field.
- Between the plates, the electric force has constant magnitude and direction. The velocity across the plates remains constant while the velocity along the field changes.
- The path inside the plates is a parabola. After the particle leaves the field, no electric force acts, so the path becomes a straight line.
- A positive particle bends towards the negative plate. An electron bends towards the positive plate.
Recent sketch marks require the smooth curve inside the plates, correct deflection direction and straight path outside (9702/41/M/J/24 Q5(b)(iii)).
Your turnquick check
An electron enters horizontally between parallel plates. The top plate is positive. Which way does the electron curve, and what shape is its path inside the field?
Show worked answer
The electric field points downwards, but the electron is negative, so its force is upwards. It follows a parabola towards the positive top plate.