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Students are often asked:
Q1. Define an electric field. State its SI unit.
Q2. What does the direction of an electric field indicate?
Let’s answer this cleanly and exam-ready:
Exam-Ready Answer
An electric field is the region around a charged object where another charge experiences a force.
The electric field at a point is defined as the force experienced by a unit positive test charge placed at that point.
\( E = \frac{F}{q} \)
SI unit: N/C (newton per coulomb) or V/m (volt per metre).
Direction: It is the direction of force on a positive test charge.
Extra Tip to Score Full Marks
Always mention “unit positive test charge.”
Do NOT say “any charge.”
Keep the formula and unit together - this adds precision.
This is one of the most common Board and competitive exam questions:
Q3. Calculate the electric field at a point 20 cm from a charge of ( +4 \mu C ).
Step-by-Step Solution
Formula:
\(E = k \frac{Q}{r^2}\)
where
\(( k = 9 \times 10^9 \text{ N m}^2 \text{/C}^2 )\)
\(( Q = 4 \times 10^{-6} \text{ C} )\)
\((r=0.2\text{ m})\)
\(E = 9 \times 10^9 \times \frac{4 \times 10^{-6}}{(0.2)^2}\)
\(=9\times10^9\times\frac{4\times10^{-6}}{0.04}\)
\(E = 9 \times 10^9 \times 1 \times 10^{-4}\)
\(=9\times10^5\text{ N/C}\)
Final Answer:
\(\boxed{9 \times 10^5 \text{ N/C}}\)
Real-Life Link
Electric fields from mobile chargers, electric fences, photocopiers, and touchscreens are based on the same principle - a point charge creates a field around it.
A common question:
Q4. A positive charge is placed at the origin. In which direction does the electric field at point (2, 0) lie?
How to Think:
Electric field lines move away from a positive charge.
Point (2,0) is on the positive x-axis, so the field is also along +x direction.
Exam-Ready Answer:
Electric field at (2,0) is along the positive x-direction because electric field lines radiate outward from a positive charge.
This is a guaranteed 2–3 mark question.
Q5. Draw the electric field lines for:
(a) Two like charges
(b) Two unlike charges
What to Include for Full Marks
Lines begin on positive and end on negative.
Lines never intersect.
Like charges - repulsion (lines push away).
Unlike charges - attraction (lines join).
Common Mistake (PAS logic applied):
Problem: Students often draw field lines crossing each other.
Agitate: This shows “two possible directions,” which is physically impossible.
Solution: Always draw lines smooth, continuous, and never intersecting.
Q6. Explain why electric field lines never intersect each other.
If two lines intersected, the electric field at that point would have two different directions, which is not possible. A charge cannot experience two forces at the same time at the same point.
Boards love this concept because it appears in capacitors, charges, and uniform fields.
Q7. Explain the electric field between two parallel plates. Why is it uniform?
Between large parallel plates, field lines are:
straight
equally spaced
parallel
This creates a uniform electric field, meaning the force on a charge is the same everywhere between the plates.
TV and computer CRT displays
Particle accelerators
Inkjet printers
Photocopiers
Q8. Two parallel plates are 4 mm apart and have 200 V potential difference. Calculate the field between them.
\(E = \frac{V}{d}\)
\(E = \frac{200}{0.004}\)
\(=50000\text{ V/m}\)
Final Answer:
\(\boxed{5 \times 10^4 \text{ V/m}}\)
Q9. Two charges +Q and -Q are placed 10 cm apart. What is the electric field at the midpoint?
Midpoint distance from each charge = 5 cm
Directions:
Positive charge - field away
Negative charge - field towards
At the midpoint, both fields are in same direction, so they add up.
Students wrongly subtract fields.
They forget that both fields point toward the −Q charge.
Q10. What is an electric dipole? Explain its electric field pattern.
Two equal and opposite charges separated by a small fixed distance form a dipole.
Lines go from + to -
Symmetrical curve
Stronger near charges, weaker far away
Students often ignore application-based questions, but these are easy marks.
Q11. State two applications of electric fields in daily life.
Van de Graaff generator (used in research labs)
Photocopiers (electric field attracts toner to paper)
Electric fences
Touchscreens (capacitive sensing)
Pollution control (electrostatic precipitators)
Q12. Explain why the electric field inside a conductor is zero.
Charges in a conductor move freely. When external electric field is applied:
charges rearrange
they cancel the external field
making the net field inside = zero
This is why:
humans are safe inside a car during lightning
metal shielding is used in cables
Q13. A spherical conductor has charge Q. What is the electric field at its surface?
\(E = k \frac{Q}{R^2}\)
A charged sphere behaves like a point charge at its center.
Q14. Write the properties of electric field lines.
Begin on + charge and end on - charge
Never intersect
Number of lines ∝ magnitude of charge
Closer lines - stronger field
Perpendicular to conductor surface
Do not form closed loops (exceptions in changing magnetic fields)
Boards have been adding case-study questions. Here’s an example:
Q15. A company designs an air purifier that charges dust particles using electric fields.
Answer the following:
Why are dust particles charged?
How does electric field help in removing them?
What principle is used here?
Particles gain charge through friction or corona discharge.
Charged particles get attracted to oppositely charged plates inside purifier.
Principle: Electric field exerts force on charged particles.
Electrostatic precipitators in industries remove 99% smoke particles using the same method.
Q16. Two charges +3Q and +Q are placed 10 cm apart. Where is electric field zero?
Both charges repel - zero point lies between them
Closer to smaller charge (because it creates weaker field)
\(k\frac{3Q}{x^2} = k\frac{Q}{(10 - x)^2}\)
\(\frac{3}{x^2} = \frac{1}{(10 - x)^2}\)
\(\sqrt{3}(10 - x) = x\)
Solve for x to get the exact point.
Q17. Use electric field concept to explain lightning rod action.
A lightning rod creates a region of high electric field at its sharp tip.
This helps:
discharge clouds gradually
redirect lightning safely to the ground
Q18. Students often get confused between electric field and electric force.
Differentiate them with examples.
Problem: Students think field and force mean same thing.
Agitate: This leads to wrong formulas or units in numericals.
Solution: Clarify with a table.
| Concept | Electric Field | Electric Force |
|---|---|---|
| Meaning | Influence per unit charge | Actual push/pull on charge |
| Symbol | E | F |
| Formula | F/q | qE |
| Unit | N/C | N |
Q19. A charge of 4 nC is placed in a field of ( 2 \times 10^4 , \text{N/C} ). Find force on it.
\(F = qE\)
\(= 4 \times 10^{-9} \times 2 \times 10^4 = 8 \times 10^{-5} \text{ N}\)
\(\boxed{8 \times 10^{-5} \text{ N}}\)
(a) zero
(b) infinite
(c) equal to surface charge
(d) constant
Correct Answer: (a)
(a) negative charges
(b) both
(c) positive charges
(d) none
Correct Answer: (c)
(a) far apart
(b) closer
(c) curved
(d) not present
Correct Answer: (b)
To score 5-mark questions:
Define clearly
Add formula
Add diagram
Give explanation
Mention applications or examples
This structure fetches full marks consistently.
Electric field is force per unit charge.
Field direction depends on sign of charge.
Superposition helps solve complex problems.
Field between plates is uniform.
Inside a conductor, field = zero.
Field lines never cross.
Real-life applications appear often in exams.
If you want to practice this topic, you can take a quiz in Curious Corner for better practice.
*Note: You must register yourself to access the quizzes.*
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