The big idea: Hold a charged balloon near your arm and the fine hairs rise and lean toward it — they feel a pull across empty space. That region of force around a charge is an electric field.
Electric field strength E
- It measures how strong the field is: the force per unit charge.
- That is the force on a tiny test charge ÷ the size of that charge.
- It is a vector (it has a direction), and its unit is N C⁻¹ (newtons per coulomb).
Field of a positive point charge. The arrows point OUT of a +charge (a small positive test charge is pushed away). For a −charge the arrows point IN instead.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Which way does the field point?: The field points the way a small positive test charge would be pushed.
So field lines point OUT of a positive charge and IN to a negative charge.
Field-line patterns
A charged metal sphere: The charge sits on the outside surface. Inside, the field is zero.
Outside, the field is the same as for a point charge at the centre, E = kQ/r², and the lines leave the surface at 90°.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Electric field strength is the force on a test charge divided by the size of that charge:
- electric field strength (N C⁻¹)
- force on the test charge (N)
- size of the small test charge (C)
E = F ÷ q, so F = qE. Cover the one you want: two letters side by side → multiply (F = q E); one above the other → divide (E = F ÷ q).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
The field of a point charge: A single point charge Q makes a field that gets weaker with distance. Combining the data-booklet equations gives:
E = kQ ÷ r²
Double the distance r and the field drops to a quarter (inverse-square). Here k is the Coulomb constant, 8.99 × 10⁹ N m² C⁻².
- electric field strength (N C⁻¹)
- Coulomb constant, 8.99 × 10⁹ N m² C⁻² (given)
- size of the charge making the field (C)
- distance from that charge to the point (m)
A point charge of +2.0 × 10⁻⁶ C sits in a vacuum. Find the electric field strength at a point 0.30 m away. (k = 8.99 × 10⁹ N m² C⁻².)
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Practice with exam-style questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
How this is tested — field strength and superposition are the core skill here:
Paper 1A
- Find the resultant field between two charges — work out each with E = kQ ÷ r², then add them as vectors (mind the directions).
Paper 2
- Locate the zero-field (null) point between two like charges, where the two fields are equal and opposite so they cancel.
The classic trap: Adding the two field magnitudes without checking direction. Between two like charges the fields point opposite ways, so you subtract; between two opposite charges they point the same way, so you add.
Superposition — add fields as vectors: The total field at a point is the vector sum of the field from each charge.
Work out each one with E = kQ ÷ r², then combine with directions: same direction → add the sizes; opposite directions → subtract them.
Two charges sit on a line 0.40 m apart: a +3.0 × 10⁻⁹ C charge on the left and a +3.0 × 10⁻⁹ C charge on the right. (a) Find the field strength each charge produces at the midpoint, 0.20 m from each. (b) Find the resultant field there. (k = 8.99 × 10⁹ N m² C⁻².)
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.