OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) charged particles moving in a region occupied by both electric and…
Full specification wording
Motion of charged particles: (c) charged particles moving in a region occupied by both electric and magnetic fields; velocity selector.
In a velocity selector, a charged particle moves through crossed electric and magnetic fields.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a velocity selector, a charged particle moves through crossed electric and magnetic fields. The electric force QE and magnetic force BQv act in opposite directions. When they balance, QE = BQv, so v = E/B. Only particles with this speed travel undeflected; slower or faster particles are deflected one way or the other. The selected speed is independent of charge and mass, which is why the device can pick a chosen velocity from a mixed beam. Example: if E = 4.0 × 10⁴ V m⁻¹ and B = 0.20 T, then v = E/B = 2.0 × 10⁵ m s⁻¹. Particles with this speed pass straight through; others curve towards one plate.
What earns the marks
- States that the electric and magnetic forces on the particle act in opposite directions in a velocity selector.
- Uses the balance condition QE = BQv to derive v = E/B.
- Explains that only particles with speed v = E/B pass through undeflected, independent of charge and mass.
- Applies v = E/B to calculate the selected speed from given field values.
Common mistakes and how to put them right
Thinking the selected speed depends on the particle's charge or mass.
Put it right: Q cancels in QE = BQv, so v = E/B is the same for all charged particles.
Adding the electric and magnetic forces instead of recognising they oppose.
Put it right: The forces must be equal and opposite for the particle to travel in a straight line.
Using the magnetic force as BQv when the particle is not moving at right angles to B.
Put it right: The velocity selector geometry requires v perpendicular to B.
Exam tips
- Write the force balance QE = BQv first, then cancel Q to obtain v = E/B.
- Check the directions of E and B so that the electric and magnetic forces oppose each other.
- Keep E in V m⁻¹ and B in T so that v comes out in m s⁻¹.
Try the lesson
The start of the Motion of charged particles: (c) charged particles moving in a region occupied by both electric and magnetic fields; velocity selector. lesson, made from checked teaching notes for this specification point.
Listen
In a velocity selector, a charged particle moves through crossed electric and magnetic fields. The electric force QE and magnetic force BQv act in opposite directions. When they balance, QE = BQv, so v = E/B. Only particles with this speed travel undeflected; slower or faster particles are deflected one way or the other. The selected speed is independent of charge and mass, which is why the device can pick a chosen velocity from a mixed beam. Example: if E = 4.0 × 10⁴ V m⁻¹ and B = 0.20 T, then v = E/B = 2.0 × 10⁵ m s⁻¹. Particles with this speed pass straight through; others curve towards one plate. A strong answer earns marks for these points. States that the electric and magnetic forces on the particle act in opposite directions in a velocity selector. Uses the balance condition QE = BQv to derive v = E/B. Explains that only particles with speed v = E/B pass through undeflected, independent of charge and mass. Applies v = E/B to calculate the selected speed from given field values. A common slip: Thinking the selected speed depends on the particle's charge or mass. Instead, Q cancels in QE = BQv, so v = E/B is the same for all charged particles. Exam tip: Write the force balance QE = BQv first, then cancel Q to obtain v = E/B.
Key words
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Crossed electric and magnetic fields
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Force balance on a moving charge
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Velocity selection
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Independence of selected speed from charge and mass
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See every definition
- Crossed electric and magnetic fields
- A region occupied by both electric and magnetic fields, through which a charged particle moves in a velocity selector.
- Force balance on a moving charge
- The electric force QE and magnetic force BQv act in opposite directions, and when they balance QE = BQv.
- Velocity selection
- Only particles with speed v = E/B travel undeflected, while slower or faster particles are deflected one way or the other.
- Independence of selected speed from charge and mass
- The selected speed is independent of charge and mass, which is why the device can pick a chosen velocity from a mixed beam.
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