Edexcel A-Level Physics 9PH0
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Electric and Magnetic Fields
Uniform fields between parallel plates, and the potential near a point charge
- 1 thing to know
- Papers 1 and 3
Overview
Recall questions on E = V/d, the field between charged plates and where it stops being uniform, and the potential a distance r from a point charge.
What you need to know
Answer each one before you reveal it.
Questions from this subtopic
Each one with the model answer underneath.
- 1
Two parallel plates 5.0 mm apart have a potential difference of 250 V across them. What is the field strength between them?
Model answer
E = V/d = 250 ÷ (5.0 × 10⁻³) = 5.0 × 10⁴ V m⁻¹.
- 2
A charge of 2.0 nC is placed in a uniform field of strength 5.0 × 10⁴ V m⁻¹. What force acts on it?
Model answer
F = EQ = 5.0 × 10⁴ × 2.0 × 10⁻⁹ = 1.0 × 10⁻⁴ N.
- 3
What happens to the field strength between parallel plates if the plate separation is halved at constant potential difference?
Model answer
The field strength doubles, because E is inversely proportional to d.
- 4
State the equation for the potential at a distance r from a point charge:
Model answer
V = Q/(4πε₀r), where Q is the point charge.
- 5
How does the potential due to a point charge vary with distance?
Model answer
It is inversely proportional to the distance, falling as 1/r and tending to zero at infinity.
- 6
What is the sign of the potential at a point near an isolated negative charge?
Model answer
It is negative, because work must be done against the attraction to remove a positive charge to infinity.
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