Electric field of point charges: Class 12 practice
The field of a point charge is E = kq/r², pointing away from positive and towards negative charges. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Coulomb's law
- Vectors
The idea behind electric field of point charges
The field of a point charge is E = kq/r², pointing away from positive and towards negative charges. Fields from several charges add as vectors; on a line, take signs carefully.
Step-by-step method
- Find each field magnitude with E = kq/r².
- Give each a direction.
- Add as vectors (on a line: with signs).
Worked example 1 (Basic)
Find the electric field 0.5 m from a 1 μC charge.
- E = 9 × 10⁹ × 1 × 10⁻⁶/0.25
- = 3.600e+4 N/C
- Answer: 3.600e+4 N/C
Worked example 2 (Standard)
Charges +4 μC and +7 μC are 0.4 m apart. Find the field at the midpoint.
- E₁ = 9.00e+5, E₂ = 1.57e+6 N/C (opposite directions)
- Net = 6.750e+5 N/C
- Answer: 6.750e+5 N/C
Worked example 3 (Challenge)
Two positive charges 1 μC and 8 μC are 0.4 m apart. Where between them is the field zero?
- kq₁/x² = kq₂/(0.4 − x)²
- x = 0.104 m from the 1 μC charge
- Answer: 0.104 m from the first charge
Common mistakes and misconceptions
- Not squaring r
- Adding fields that point in opposite directions
Practice questions
- (Basic) Find the electric field 0.4 m from a 3 μC charge.
- (Basic) Find the electric field 0.3 m from a 7 μC charge.
- (Standard) Charges +2 μC and +9 μC are 0.6 m apart. Find the field at the midpoint.
- (Standard) Charges +5 μC and +4 μC are 0.2 m apart. Find the field at the midpoint.
- (Challenge) Two positive charges 4 μC and 6 μC are 0.8 m apart. Where between them is the field zero?
- (Challenge) Two positive charges 5 μC and 2 μC are 1 m apart. Where between them is the field zero?
Answer key with full working
- Q1: E = 9 × 10⁹ × 3 × 10⁻⁶/0.16 → = 1.687e+5 N/C → Answer: 1.687e+5 N/C
- Q2: E = 9 × 10⁹ × 7 × 10⁻⁶/0.09 → = 7.000e+5 N/C → Answer: 7.000e+5 N/C
- Q3: E₁ = 2.00e+5, E₂ = 9.00e+5 N/C (opposite directions) → Net = 7.000e+5 N/C → Answer: 7.000e+5 N/C
- Q4: E₁ = 4.50e+6, E₂ = 3.60e+6 N/C (opposite directions) → Net = 9.000e+5 N/C → Answer: 9.000e+5 N/C
- Q5: kq₁/x² = kq₂/(0.8 − x)² → x = 0.36 m from the 4 μC charge → Answer: 0.36 m from the first charge
- Q6: kq₁/x² = kq₂/(1 − x)² → x = 0.613 m from the 5 μC charge → Answer: 0.613 m from the first charge
Syllabus note
This skill is listed under "Electric Charges and Fields" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is electric field of point charges in?
"Electric Charges and Fields", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.