Cambridge International AS & A Level · 18 · structured

Define a field and explain beam deflection

Define a field and explain beam deflection · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand: demanding
Marks: 7
Topics: 2
Answer: Complete

structured7 marks

A uniform electric field points vertically upward between horizontal parallel plates. A small positive particle enters the field horizontally to the right with speed u; neglect gravity and interactions with other particles.

(a)
Explain what makes the region an electric field of force and define electric field strength, including direction and unit.
3 marks
(b)
Describe the positive particle's trajectory while between the plates and after it leaves. Explain the horizontal and vertical motion separately.
3 marks
(c)
How does the deflection direction change if an otherwise equivalent negative particle enters with the same initial velocity?
1 mark

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Answer overview(a) A charge can experience electric force there; E is force per unit positive test charge, in N C⁻¹, directed as force on a positive charge. (b) Constant horizontal speed with constant upward acceleration gives a parabola; outside the field it travels in a straight tangent. (c) It deflects downward, opposite to E.

(a)

3 marks

Explain what makes the region an electric field of force and define electric field strength, including direction and unit.

How to approach it

Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.

1
An electric field is a field of force acting on electric charges.
2
Define E=F/q for a small positive test charge.
3
Its direction is that positive charge's force direction; unit N C⁻¹.

(b)

3 marks

Describe the positive particle's trajectory while between the plates and after it leaves. Explain the horizontal and vertical motion separately.

How to approach it

Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.

1
Electric force is upward, so vertical acceleration qE/m is constant.
2
There is no horizontal force, so horizontal speed remains u; combining the motions gives a parabola.
3
After leaving the field no force acts in the model, so motion continues in a straight line along the exit velocity.

(c)

1 mark

How does the deflection direction change if an otherwise equivalent negative particle enters with the same initial velocity?

How to approach it

Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.

1
Force is qE; reversing charge sign reverses the electric force and the vertical acceleration.

Complete marking guidance

Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.

(a) An electric field is a field of force acting on electric charges.
(a) Define E=F/q for a small positive test charge.
(a) Its direction is that positive charge's force direction; unit N C⁻¹.
(b) Electric force is upward, so vertical acceleration qE/m is constant.
(b) There is no horizontal force, so horizontal speed remains u; combining the motions gives a parabola.
(b) After leaving the field no force acts in the model, so motion continues in a straight line along the exit velocity.
(c) Force is qE; reversing charge sign reverses the electric force and the vertical acceleration.

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