Definition
Electric field
Force per unit positive test charge at a point: E = F/q. SI unit: N/C or V/m. Field due to point charge: E = (1/4πε₀) q/r² along the radial direction.
-- NCERT Class 12 Physics, Ch. 1, p. 15Electric Field Lines
8 MCQs9-step worked example
Source: NCERT ElectrostaticsOfficial key: NTA-verifiedLast updated: 21 Sep 2026
Electric Field Lines, explained for NEET
The trap: reading field strength from the number of lines in a diagram. NCERT Class 12 Physics Part I, Chapter 1 (pages 19–20) says the number of lines is not important — another person may draw more, and infinitely many can be drawn. What matters is the relative density: the number of lines per unit cross-sectional area placed perpendicular to the lines. Lines crowd where the field is strong and spread apart where it is weak.
What a field line is. A field line is a curve drawn so that the tangent to it at each point is in the direction of the net electric field at that point; an arrow on the curve picks one of the two directions a tangent allows (NCERT Chapter 1, Section 1.8, page 21). Faraday, who invented the picture, called them "lines of force"; NCERT prefers "field lines" (page 20).
General properties (NCERT page 21).
- Field lines start from positive charges and end at negative charges. With a single charge, they may start or end at infinity.
- In a charge-free region they are continuous curves without breaks.
- Two field lines never cross — the field at the crossing point would not have a unique direction.
- Electrostatic field lines do not form closed loops, because the electrostatic field is conservative (taken up in Chapter 2).
Standard pictures. A single positive charge: lines radially outward. A single negative charge: radially inward. Two equal positive charges: the lines show mutual repulsion. Equal and opposite charges: the lines show attraction. In a region of constant field, the lines are uniformly spaced parallel straight lines (NCERT Chapter 1 Summary, page 38).
Bridge to NEET. Questions either test a property (why no crossing, why no loops, where lines start and end) or ask you to compare the field at two points from a diagram. For the comparison, divide lines by the perpendicular area at each point before comparing.
Watch out: equal areas placed perpendicular to the lines are needed for a fair comparison. Comparing raw line counts through unequal areas gives the wrong ratio.
Can you answer these Electric Field Lines MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
For an electrostatic field line, what does the tangent drawn to the line at a point give?
Show answer and why every option is right or wrong
Answer: B. NCERT Class 12 Physics Part I, Chapter 1, Section 1.8 (page 21) defines a field line as a curve whose tangent at each point is in the direction of the net field at that point.
Why A is wrong: A is wrong because it turns the definition through 90°; the tangent lies along the net field, not at right angles to it.
Why C is wrong: C is wrong because a single tangent carries direction only; the strength of the field is read from how closely the lines are packed, not from the tangent.
Why D is wrong: D is wrong because the field direction is the direction of force on a positive charge; a negative charge feels a force opposite to the tangent's arrow.
MCQ 2Easy RecallPractice
Why can two electric field lines never cross each other?
Show answer and why every option is right or wrong
Answer: C. NCERT Class 12 Physics Part I, Chapter 1 (page 21): if two lines crossed, the field at the point of intersection would not have a unique direction, which is absurd.
Why A is wrong: A is wrong because field lines are in general curves (for example around two equal positive charges); straightness is not the reason they cannot cross.
Why B is wrong: B is wrong because field lines are a drawing that maps the field; they are not charged objects and exert no force on each other.
Why D is wrong: D is wrong because the reason NCERT gives concerns direction, not magnitude; a crossing would give two directions at one point.
MCQ 3Easy RecallPractice
Which statement matches one of NCERT's general properties of electrostatic field lines?
Show answer and why every option is right or wrong
Answer: B. NCERT Class 12 Physics Part I, Chapter 1 (page 21), property (i): field lines start from positive charges and end at negative charges, and for a single charge they may start or end at infinity.
Why A is wrong: A is wrong because electrostatic field lines do not form closed loops; NCERT links this to the conservative nature of the electrostatic field.
Why C is wrong: C is wrong because it reverses the direction: lines leave positive charges and arrive at negative charges.
Why D is wrong: D is wrong because in a charge-free region field lines are continuous curves without any breaks.
MCQ 4Direct ApplicationPractice
In a field-line diagram, 10 lines cross a small area of 1.0 cm² placed perpendicular to the lines at point X, and 5 lines cross a small area of 2.5 cm² placed perpendicular to the lines at point Y. The ratio of field strengths E_X : E_Y is:
Show answer and why every option is right or wrong
Answer: D. Line density at X = 10/1.0 = 10 per cm²; at Y = 5/2.5 = 2.0 per cm². Field strength is proportional to line density, so E_X/E_Y = 10/2.0 = 5.0 (NCERT Class 12 Physics Part I, Chapter 1, page 20).
Why A is wrong: A is wrong because 2.0 = 10/5 compares raw line counts and ignores that the two areas are different; strength follows lines per unit area.
Why B is wrong: B is wrong because 0.40 = 1.0/2.5 compares the areas themselves instead of the lines per unit area.
Why C is wrong: C is wrong because 0.20 = 2.0/10 is the density ratio inverted (E_Y/E_X instead of E_X/E_Y).
MCQ 5Direct ApplicationPractice
In a certain region, the electric field lines are drawn as parallel straight lines with equal spacing. What can be said about the electric field in that region?
Show answer and why every option is right or wrong
Answer: D. NCERT Class 12 Physics Part I, Chapter 1 Summary (page 38): in regions of constant electric field, the field lines are uniformly spaced parallel straight lines. Equal spacing means equal strength; parallel lines mean the same direction.
Why A is wrong: A is wrong because lines are drawn only where a field exists; uniformly spaced lines show a constant non-zero field, not a zero field.
Why B is wrong: B is wrong because strength follows line density, and the spacing here does not change along the lines, so the strength does not change either.
Why C is wrong: C is wrong because the total number of lines is arbitrary but the relative density is not; equal spacing everywhere still tells you the field is equally strong everywhere.
MCQ 6Direct ApplicationPractice
Consider the field lines around two equal positive point charges (q, q) and no other charges. Where do these field lines end?
Show answer and why every option is right or wrong
Answer: A. Field lines start from positive charges and end at negative charges; with no negative charge present, the lines leaving the two positive charges go out to infinity (NCERT Class 12 Physics Part I, Chapter 1, page 21). The picture shows their mutual repulsion.
Why B is wrong: B is wrong because field lines end only on negative charges; a line from one positive charge cannot end on another positive charge.
Why C is wrong: C is wrong because electrostatic field lines do not form closed loops.
Why D is wrong: D is wrong because two field lines can never cross; a crossing would give the field two directions at one point.
MCQ 7CalculationPractice
Field lines are drawn radially from an isolated point charge. Compare the number of lines crossing unit area (placed perpendicular to the lines) at a distance of 3.0 cm from the charge with that at 1.0 cm from the charge. The ratio (density at 3.0 cm) : (density at 1.0 cm) is:
Show answer and why every option is right or wrong
Answer: C. The same number of lines n passes through a given solid angle ΔΩ, but the area it covers is r²ΔΩ. Density = n/(r²ΔΩ), so the ratio is (1.0/3.0)² = 1/9 (NCERT Class 12 Physics Part I, Chapter 1, page 20). The lines thin out as 1/r², matching a field that weakens as 1/r².
Why A is wrong: A is wrong because it uses the fact that the same number of lines crosses both enclosing areas, but ignores that the area at 3.0 cm is 9 times larger, so lines per unit area fall.
Why B is wrong: B is wrong because 1/3 scales the area with r instead of r²; the area subtending a given solid angle grows as the square of the distance.
Why D is wrong: D is wrong because 9 is the ratio inverted; the lines are denser close to the charge, not farther away.
MCQ 8Concept TrapPractice
Four sketches are proposed as electrostatic field-line pictures in a region. Which one can be a correct picture?
Show answer and why every option is right or wrong
Answer: A. Lines starting on the positive charge and ending on the negative charge obey property (i), and crowding near the charges shows the stronger field there (NCERT Class 12 Physics Part I, Chapter 1, pages 19–21). This is the attraction picture NCERT describes for equal and opposite charges.
Why B is wrong: B is wrong because electrostatic field lines do not form closed loops, as the electrostatic field is conservative.
Why C is wrong: C is wrong because in a charge-free region field lines are continuous curves without breaks; a line can end only on a negative charge (or at infinity).
Why D is wrong: D is wrong because two field lines can never cross; the field at the crossing would have no unique direction.
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How do you solve a Electric Field Lines question? A worked example
- 1
Given
In a field-line diagram of some charge configuration, small area elements are placed perpendicular to the lines at two points:• At P: 18 lines cross an area of 3.0 mm².• At Q: 8 lines cross an area of 4.0 mm².
The electric field at Q has magnitude 2.0 × 10³ N/C. - 2
Required
(a) The line density at P and at Q. (b) The ratio E_P / E_Q. (c) The magnitude of the field at P.
- 3
Concept
The number of field lines crossing unit area placed perpendicular to the lines is proportional to the field strength at that point. The total number of lines drawn is arbitrary; only the relative density carries meaning (NCERT Class 12 Physics Part I, Chapter 1, pages 19–20).
- 4
Formula
Line density = (number of lines) / (perpendicular area)
E_P / E_Q = (density at P) / (density at Q) - 5
Substitution
(a) Density at P = 18 / 3.0 mm²; density at Q = 8 / 4.0 mm²
(b) E_P / E_Q = (18/3.0) / (8/4.0)
(c) E_P = (E_P / E_Q) × 2.0 × 10³ N/C - 6
Calculation
(a) Density at P = 6.0 lines per mm²; density at Q = 2.0 lines per mm²
(b) E_P / E_Q = 6.0 / 2.0 = 3.0
(c) E_P = 3.0 × 2.0 × 10³ = 6.0 × 10³ N/C
The line counts 18 and 8 are exact counting numbers and do not limit the significant figures; the areas and the given field have two significant figures, so the answers are reported to two significant figures. - 7
Final answer
Densities: 6.0 per mm² at P, 2.0 per mm² at Q; E_P / E_Q = 3.0; E_P = 6.0 × 10³ N/C
- 8
Common trap
Comparing raw line counts: 18/8 = 2.25 would suggest E_P ≈ 4.5 × 10³ N/C. That ignores the unequal areas. Always convert to lines per unit perpendicular area first; the field is stronger where the lines are more closely packed, not where more lines happen to be counted.
- 9
Similar NEET-style question
"In a field-line diagram, 15 lines cross an area of 5.0 mm² placed perpendicular to the lines at point A, and 9 lines cross an area of 1.5 mm² placed perpendicular to the lines at point B. Find E_A / E_B."
Strategy: Density at A = 15/5.0 = 3.0 per mm²; at B = 9/1.5 = 6.0 per mm². E_A / E_B = 3.0/6.0 = 0.50. Point B has fewer lines counted but a stronger field. Do not use 15/9 ≈ 1.7 (raw counts).
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What to remember before solving Electric Field Lines questions
More in Electrostatics: 3 exam traps and mistakes · 11 formulas · 6 question patterns from its other lessons.
Electric Field Lines questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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