Uniform fields, parallel plates and potential difference
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Question
What is a uniform electric field?
Answer
A field with the **same strength and direction everywhere** — drawn as **evenly-spaced, parallel** lines. You get one in the gap between two parallel charged plates.
Question
How are the field lines drawn between parallel plates?
Answer
**Evenly-spaced parallel lines** running from the **+ plate** to the **− plate** (the direction a positive charge is pushed).
Question
Formula for the field between parallel plates?
Answer
$E = \dfrac{V}{d}$ — voltage across the plates ÷ the gap between them. Given in the data booklet. Unit: V m⁻¹.
Question
What is the unit of electric field strength E?
Answer
**Volts per metre (V m⁻¹)**, which is the same as **N C⁻¹** (newtons per coulomb).
Question
Halve the gap between the plates (same voltage) — what happens to E?
Answer
E **doubles** — field strength is inversely proportional to the separation d (E = V ÷ d).
Question
Force on a charge q in a field E?
Answer
$F = qE$ (rearranged from the data-booklet definition $E = \dfrac{F}{q}$). Bigger charge or stronger field → bigger force.
Question
Work done moving a charge q through a potential difference V?
Answer
$W = qV$ (in joules). This is the energy the charge gains — and for a charge from rest, its kinetic energy. Not in the booklet — memorise it.
Question
What is an electronvolt (eV)?
Answer
The energy a charge of **e** (1.6 × 10⁻¹⁹ C) gains moving through **1 V**: 1 eV = 1.6 × 10⁻¹⁹ J. A charge e through V volts gains V eV.
Question
Convert 250 eV into joules.
Answer
Multiply by 1.6 × 10⁻¹⁹: 250 × 1.6 × 10⁻¹⁹ = 4.0 × 10⁻¹⁷ J.
Question
Plates 0.020 m apart at 600 V — find the field.
Answer
E = V ÷ d = 600 ÷ 0.020 = 3.0 × 10⁴ V m⁻¹.
Question
Which way do the field lines between plates point?
Answer
From the **+ plate to the − plate** — the direction a **positive** charge would be pushed.
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Topic 4.2 hub
Electric and magnetic fields
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