Field lines and equipotentials Edexcel International A Level Physics revision
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In plain words
A map can show a hill in two ways: with arrows showing which way a ball would roll, or with contour lines joining places at the same height. Electric fields can be drawn both ways too. Field lines are the arrows, and equipotentials are the contours.
5 things to know
- An equipotential joins points that are at the same potential. No work is done in moving a charge along one.
- Equipotentials always cross field lines at right angles.
- In a radial field, round a point charge, the field lines are straight lines out from the charge, and the equipotentials are circles (really spheres) centred on it, getting further apart further out.
- In a uniform field, between parallel plates, the field lines are parallel and evenly spaced, and the equipotentials are evenly spaced lines parallel to the plates.
- Where equipotentials are close together the field is strong: E = the potential difference between two of them ÷ the distance between them.
Worked example
Two parallel plates are 0.040 m apart with a p.d. of 200 V across them. Find the field strength, and how far apart the equipotentials are if one is drawn for every 50 V.
- E = V ÷ d = 200 ÷ 0.040 = 5000 V/m.
- The field is uniform, so the potential changes evenly: 50 V takes a quarter of the gap.
- Spacing = 0.040 ÷ 4 = 0.010 m.
Tips and tricks
- Field lines point from high potential to low potential, like a ball rolling downhill.
- Draw equipotentials as dashed lines, so they can't be mistaken for field lines.
It lands in your notebook with its questions as flashcards.
Field lines and equipotentials: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the angle between an equipotential and the field lines it crosses?
If it were anything else, there would be a force along the equipotential.
How much work is done in moving a charge along an equipotential?
There is no change in potential.
What do the equipotentials look like between two parallel charged plates?
The potential changes evenly across a uniform field.
What does it mean when equipotentials are close together?
The potential changes a lot over a short distance.
What shape are the equipotentials round a point charge?
In three dimensions they are spheres.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Field lines and equipotentials
Edexcel International A Level Physics WPH · 6 marks · papermunch.org
Name ______________________________ Date ______________
Describe the equipotentials round a point charge, and how they are related to the field lines.[2]
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They are circles (spheres) centred on the charge, getting further apart further from the charge. They cross the field lines at right angles.
A charge of 2.0 µC moves from an equipotential at 100 V to one at 40 V. Find the energy transferred.[2]
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1.2 × 10⁻⁴ J. 2.0 × 10⁻⁶ × 60.
Explain why no work is done when a charge is moved along an equipotential.[2]
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Every point on it is at the same potential, so there is no potential difference, and work done = charge × potential difference = 0. (The force on the charge is always at right angles to its motion.)
Answers: Field lines and equipotentials
- 1. They are circles (spheres) centred on the charge, getting further apart further from the charge. They cross the field lines at right angles.
- 2. 1.2 × 10⁻⁴ J. 2.0 × 10⁻⁶ × 60.
- 3. Every point on it is at the same potential, so there is no potential difference, and work done = charge × potential difference = 0. (The force on the charge is always at right angles to its motion.)