OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) motion of charged particles in a uniform electric field.
Full specification wording
Uniform electric field: (c) motion of charged particles in a uniform electric field.
When a charged particle enters a uniform electric field, it experiences a constant electric force F = qE.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a charged particle enters a uniform electric field, it experiences a constant electric force F = qE. This force causes acceleration a = F/m = qE/m. If the particle enters perpendicular to the field, its motion is analogous to projectile motion: constant velocity parallel to the plates and constant acceleration perpendicular to them. For example, an electron entering midway between two horizontal plates with speed v horizontally will follow a parabolic path, deflecting towards the positive plate. If it enters parallel to the field, it accelerates in a straight line. You must be able to describe the path, calculate acceleration, time of flight, deflection and final velocity components, and explain how the motion depends on charge, mass, field strength and initial velocity.
What earns the marks
- State that the electric force on a charged particle is F = qE and is constant in a uniform field.
- Use Newton's second law to find acceleration: a = qE/m.
- Describe the path as parabolic when the initial velocity is perpendicular to the field.
- Calculate time of flight from the horizontal velocity and plate length.
- Calculate vertical deflection using suvat equations with acceleration a = qE/m.
- Explain that the motion is independent of the sign of the charge except for the direction of deflection.
Common mistakes and how to put them right
Treating the electric force as varying.
Put it right: In a uniform field the force is constant, so acceleration is constant.
Forgetting to use the correct mass and charge of the particle.
Put it right: Use the specific values for an electron, proton, etc., as given.
Mixing up the directions of velocity and acceleration.
Put it right: Resolve motion into components parallel and perpendicular to the field.
Assuming the particle moves in a circle.
Put it right: In a uniform electric field the path is parabolic (if entering perpendicular) or straight (if parallel).
Exam tips
- Draw a clear diagram showing the field direction, initial velocity and expected path.
- Use suvat equations separately for the direction parallel to the field and perpendicular to it.
- Remember that the electric force does not depend on the particle's speed.
Try the lesson
The start of the Uniform electric field: (c) motion of charged particles in a uniform electric field. lesson, made from checked teaching notes for this specification point.
Listen
When a charged particle enters a uniform electric field, it experiences a constant electric force F = qE. This force causes acceleration a = F/m = qE/m. If the particle enters perpendicular to the field, its motion is analogous to projectile motion: constant velocity parallel to the plates and constant acceleration perpendicular to them. For example, an electron entering midway between two horizontal plates with speed v horizontally will follow a parabolic path, deflecting towards the positive plate. If it enters parallel to the field, it accelerates in a straight line. You must be able to describe the path, calculate acceleration, time of flight, deflection and final velocity components, and explain how the motion depends on charge, mass, field strength and initial velocity. A strong answer earns marks for these points. State that the electric force on a charged particle is F = qE and is constant in a uniform field. Use Newton's second law to find acceleration: a = qE/m. Describe the path as parabolic when the initial velocity is perpendicular to the field. Calculate time of flight from the horizontal velocity and plate length. A common slip: Treating the electric force as varying. Instead, in a uniform field the force is constant, so acceleration is constant. Exam tip: Draw a clear diagram showing the field direction, initial velocity and expected path.
Key words
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Uniform electric field
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Projectile motion analogy
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Electric force and acceleration
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See every definition
- Uniform electric field
- A field in which the electric force on a charged particle is constant, so the acceleration is constant.
- Projectile motion analogy
- When a particle enters perpendicular to the field, its motion is analogous to projectile motion: constant velocity parallel to the plates and constant acceleration perpendicular to them.
- Electric force and acceleration
- The constant electric force F = qE causes acceleration a = F/m = qE/m.
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