ELECTRIC FIELD

---

INTRODUCTION

Electric field is an important concept in Physics specially for class 12 and competitive exam JEE and NEET in which questions are asked frequently from the concept of electric field.
Here, in this article we will learn about electric field in simple language what is electric field, what is its formula, units , dimensions and where is used in our lives.

WHAT IS ELECTRIC FIELD ?

Electric field is the region where the effect of electric charge can be experienced.

In another simple words,

The space around an electric charge (positive or negative) where the charge exerts electrical force on another charge, is termed as the electric field.

Furthermore,

An electric field is said to exist at a point if an electrical force is exerted on a stationary charge placed at that point.

Quantitatively,

Electric field is defined as the electrostatic force per unit test charge acting on a vanishingly small positive test charge placed at the given point.

Mathematically,

In simple form,

and in general, we can write as

Electric field is a vector quantity whose direction is same as that of the force exerted on a positive test charge.

UNITS

The units of electric field are given generally in three forms :

(A) newton per coulomb

Using the expression, electric field = Force/ charge,

As, SI unit of force is newton (N) and that of charge is coulomb (C), hence, the SI unit of electric field is N/C.

(B) volt per metre

Electric field is equal to the negative potential gradient,
i.e., electric field = potential/ length
SI unit of potential is volt (V) and that of length is metre (m).
Hence, SI unit of electric field is V/m.

(C) kg m s⁻³A⁻¹

Let us express the units of electric field in terms of base units:
N/C= kg ms⁻²/As = kg ms⁻³A⁻¹
V/m = JC⁻¹ /m = kgm²s⁻²(As)⁻¹/m = kg ms⁻³A⁻¹
Thus, in terms of base units , unit of electric field is kg ms⁻³A⁻¹.

DIMENSIONS

Dimensions of force = [MLT⁻²]

Dimensions of charge = [AT]

Now, [Electric Field] = [Force]/ [Charge]
= [MLT⁻²]/[AT]
= [MLT⁻³A⁻¹]

What is the physical significance of electric field

By knowing electric field at any point, the force on a charge placed at that point can be determined.

Suppose that Q is a source charge situated at point O ,the origin. It produces electric field around it in the vicinity. Select a point P in its electric field.Let vector r be the position vector of the point P,its electric field strength is E(r)

Now suppose a charge q₀ is placed at the point P,then definitely it will experience the coulombic force F. This force is given as

Thus an electric field plays an intermediary role in the forces between two charges.
Charge⇌ Electric field⇌ Charge

Electric field due to a point charge

Consider a point source charge +Q placed at the origin O. There is a point P at the distance of r from O. A test charge q₀ is placed at point P. The test charge q₀ will experience the Coulombic force.

Using the Coulomb's law this force will be as

F= (1/4πε₀)(Qq₀/r²)

The direction of this force is from O to P.

Electric field at point P will be

E = F/q₀ = (1/4πε₀)(Q/r²)

Its direction is same as the direction of force i.e., from O to P.

☛ Clearly, E ∝ 1/r².

This means that at all points on the spherical surface drawn around the point charge, the magnitude of E is same and does not depend on the direction of r. Such a field is called spherically symmetric or radial field.

APPLICATIONS OF ELECTRIC FIELD

Here are the Applications of Electric Field  with clear, exam-ready explanations 

 Electric field is not just a theory—it is widely used in real-life devices and technology.

1. Electrostatic Precipitator (Air Purifier)

Used in factories to remove smoke and dust particles from air.

🔍 Explanation:

1. A strong electric field is created between charged plates.
2. Dust/smoke particles get charged when they pass through the field.
3. These charged particles are attracted towards oppositely charged plates.
4. Clean air comes out.

 Application: Pollution control in industries 🔹

2. Photocopiers / Laser Printers

Used in xerox machines and printers.

🔍 Explanation:

1. A charged drum creates an electric field.
2. Light falls on certain regions, removing charge selectively.
3. Ink (toner), which is charged, sticks to charged areas.
4. Paper gets the image due to electric field attraction.

Application: Printing and document copying 🔹

3. Cathode Ray Tube (CRT) / Old TVs

(Still important for exams)

🔍 Explanation:

1. Electrons move in a controlled electric field.
2. The field deflects electrons, controlling where they hit the screen. This creates images.

 Application: TVs, oscilloscopes 🔹

4. Capacitors (Energy Storage)

🔍 Explanation:

1. Electric field is created between two plates.
2. Energy is stored in the electric field between plates.
3. Used to store and release electrical energy.

 Application: Circuits, power supply, flash cameras 🔹

5. Inkjet Printers

🔍 Explanation:

1. Ink droplets are charged.
2. Electric field deflects droplets to precise positions.
3. This forms text/images on paper

 Application: High-quality printing 🔹

6. Electrostatic Painting / Spray Painting

🔍 Explanation:

1. Paint particles are given electric charge.
2. Object is oppositely charged.
3. Electric field ensures uniform coating (less wastage).

 Application: Car painting, industrial coating 🔹

7. Lightning (Natural Application) 

🔍 Explanation:

1. Strong electric field develops between cloud and ground.
2. When it becomes very high, air breaks down.
3. Sudden discharge occurs → lightning.

 Application: Understanding weather & safety 🔹

8. Deflection of Charged Particles

🔍 Explanation:</ h4>

1. Electric field exerts force on charged particles.
2. This changes their path or direction.

 Application: Particle accelerators, physics experiments 🔹

9. Electrostatic Separation

🔍 Explanation:

1. Different materials get charged differently.
2. Electric field separates them based on charge/mass.

 Application: Mining, recycling 🔹

10. Medical Applications (Electrotherapy)

🔍 Explanation:

1. Controlled electric fields are used in treatments.
2. Helps in nerve stimulation and muscle therapy.

 Application: Physiotherapy

Exam Tip (Important Line)

 “Electric field is used to exert force on charged particles without physical contact, enabling control, separation, motion, and energy storage.”

MCQ

Here are 50 MCQs on Electric Field (Class 12 Physics) covering definition, formula, applications, numericals, and concepts:

1–10: Basic Concepts

1. Electric field is defined as:

A) Force per unit charge
B) Work per unit charge
C) Energy per unit charge
D) Charge per unit force

2. SI unit of electric field is:

A) Volt
B) Newton/Coulomb
C) Joule
D) Ampere

3. Electric field is a:

A) Scalar
B) Vector
C) Tensor
D) Constant

4. Direction of electric field is:

A) Direction of force on negative charge
B) Direction of force on positive charge
C) Opposite to force
D) Random

5. Electric field due to point charge depends on:

A) Distance
B) Charge
C) Medium
D) All of these

6. Electric field lines never:

A) Intersect
B) Bend
C) Start
D) End

7. Field lines originate from:

A) Negative charge
B) Positive charge
C) Neutral body
D) Ground

8. Field lines terminate at:

A) Positive charge
B) Negative charge
C) Neutral body
D) Infinity only

9. Electric field inside a conductor is:

A) Maximum
B) Zero
C) Infinite
D) Constant

10. Electric field at infinity is:

A) Maximum
B) Zero
C) Infinite
D) Undefined

11–20: Formula Based

11. Electric field due to point charge is:

A) kq/r²
B) kq²/r
C) kr/q
D) q/r

12. Electric field is given by:

A) F/q
B) q/F
C) F×q
D) q²

13. Unit of k (Coulomb constant):

A) Nm²/C²
B) N/C
C) Joule
D) Volt

14. Value of k in vacuum is:

A) 9×10⁹ Nm²/C²
B) 10⁹
C) 8×10⁹
D) 7×10⁹

15. Electric field is inversely proportional to:

A) r
B) r²
C) r³
D) constant

16. Electric field increases when:

A) Distance increases
B) Charge increases
C) Both decrease
D) None

17. Electric field due to negative charge is:

A) Outward
B) Inward
C) Circular
D) Zero

18. Force on charge q in field E:

A) q/E
B) qE
C) E/q
D) q²

19. Electric field is independent of:

A) Test charge value
B) Source charge
C) Distance
D) Medium

20. Electric field lines density shows:

A) Charge
B) Force
C) Strength of field
D) Energy

21–30: Conceptual

21. Parallel electric field lines indicate:

A) Uniform field
B) Non-uniform field
C) Zero field
D) Infinite field

22. Electric field at center of ring:

A) Zero
B) Maximum
C) Infinite
D) Constant

23. Electric field inside hollow sphere:

A) Zero
B) Maximum
C) Infinite
D) Constant

24. Field lines are closer where field is:

A) Weak
B) Strong
C) Zero
D) Infinite

25. Electric field outside charged conductor is:

A) Tangential
B) Perpendicular
C) Parallel
D) Circular

26. Electric field inside dielectric:

A) Zero
B) Reduced
C) Increased
D) Infinite

27. Electric dipole consists of:

A) Two equal charges
B) Two opposite charges
C) One charge
D) Three charges

28. Electric field of dipole decreases as:

A) 1/r
B) 1/r²
C) 1/r³
D) Constant

29. Electric field at equatorial line of dipole:

A) Along dipole
B) Opposite to dipole moment
C) Zero
D) Infinite

30. Electric field at axial line is:

A) Along dipole moment
B) Opposite
C) Zero
D) Circular

31–40: Numerical Based

31. If charge doubles, electric field:

A) Doubles
B) Halves
C) Same
D) Zero

32. If distance doubles, field becomes:

A) 1/2
B) 1/4
C) 2
D) 4

33. Electric field at 2m vs 1m:

A) 1/2
B) 1/4
C) 2
D) 4

34. Field due to 0 charge:

A) Infinite
B) Zero
C) Constant
D) Undefined

35. Force on charge increases with:

A) Field strength
B) Distance
C) Time
D) None

36. Electric field unit can also be:

A) V/m
B) J/m
C) N/m
D) C/m

37. Superposition principle means:

A) Add fields vectorially
B) Multiply
C) Subtract only
D) Ignore

38. Net electric field is:

A) Scalar sum
B) Vector sum
C) Product
D) Ratio

39. Electric field near sharp edge is:

A) Low
B) High
C) Zero
D) Constant

40. Electric field in capacitor is:

A) Uniform
B) Non-uniform
C) Zero
D) Infinite

41–50: Advanced & Assertion Type

41.Assertion: Field lines intersect.

Reason: Field has multiple directions.

A) Both true
B) Both false
C) A true, R false
D) A false, R true

42. Assertion: Field inside conductor is zero.

Reason: Charges are at rest.

A) Both true
B) Both false
C) A true, R false
D) A false, R true

43. Assertion: Field is vector.

Reason: It has magnitude and direction.

A) Both true
B) Both false
C) A true, R false
D) A false, R true

44. Assertion: Field lines form closed loops.

Reason: They are circular.

A) Both true
B) Both false
C) A true, R false
D) A false, R true

45. Assertion: Electric field depends on medium.

Reason: Permittivity affects it.

A) Both true
B) Both false
C) A true, R false
D) A false, R true

46. Electric field between plates is:

A) Non-uniform
B) Uniform
C) Zero
D) Infinite

47. Field at midpoint of equal charges:

A) Zero
B) Infinite
C) Depends
D) Zero (if same sign)

48. Electric field direction is from:

A) − to +
B) + to −
C) Both
D) None

49. Electric field due to infinite plane sheet is:

A) Constant
B) Zero
C) Infinite
D) Variable

50. Electric field is strongest where:

A) Lines are far apart
B) Lines are dense
C) Lines vanish
D) Lines curve

CASE STUDY

Case Study 1: Electric Field due to Point Charge

A point charge Q is placed in air. Points A, B, and C are located at distances 1 m, 2 m, and 3 m respectively from the charge.

Questions:

1. The electric field at a distance r is given by:

A) kQ/r
B) kQ/r²
C) kr/Q
D) Q/r

2. Electric field at point B compared to A is:

A) Same
B) Half
C) One-fourth
D) Double

3. Direction of electric field at point C is:

A) Towards charge
B) Away from charge
C) Circular
D) Zero

4. If charge becomes negative, electric field at A will:

A) Increase
B) Reverse direction
C) Become zero
D) Remain same

5. Which point has minimum electric field?

A) A
B) B
C) C
D) Same at all

Case Study 2: Superposition of Electric Fields

Two charges Q and q are placed at a fixed distance. A point P lies on the line joining them.

Questions:

1. Electric field at point P is:

A) Scalar sum
B) Vector sum
C) Product
D) Zero always

2.If P is midpoint, net field is:

A) Zero
B) Maximum
C) Infinite
D) Constant

3. Direction of net field at midpoint is:

A) From − to +
B) From + to −
C) Circular
D) Zero

4. Superposition principle applies to:

A) Only force
B) Only field
C) Both force and field
D) None

5. If both charges are positive, field at midpoint is:

A) Zero
B) Maximum
C) Infinite
D) Constant

Case Study 3: Electric Field Lines

A diagram shows electric field lines around a single positive charge.

Questions:

1.Field lines are:

A) Circular
B) Radial outward
C) Random
D) Parallel

2. Density of lines indicates:

A) Charge
B) Force
C) Field strength
D) Energy

3. Field lines never intersect because:

A) Field is scalar
B) Field has unique direction at a point
C) Charges repel
D) Lines are imaginary

4. Electric field at a point is:

A) Tangent to field line
B) Perpendicular
C) Circular
D) Zero

5. At large distance, field lines become:

A) Dense
B) Parallel
C) Sparse
D) Circular

Case Study 4: Uniform Electric Field

A uniform electric field is created between two parallel plates separated by a distance.

Questions:

1. Electric field is uniform when:

A) Magnitude changes
B) Direction changes
C) Magnitude & direction constant
D) Zero

2. Field lines in uniform field are:

A) Diverging
B) Converging
C) Parallel and equally spaced

D) Circular

3. Force on charge in uniform field is:

A) Variable
B) Constant
C) Zero
D) Infinite

4. Electric field is given by:

A) V/d
B) V×d
C) d/V
D) V²

5. Work done moving charge in uniform field depends on:

A) Path
B) Distance
C) Potential difference
D) Time

Case Study 5: Electric Field in Conductors

A solid metal conductor is placed in an external electric field. Charges redistribute on its surface.

Questions:

1. Electric field inside conductor becomes:

A) Maximum
B) Zero
C) Infinite
D) Constant

2. Charges accumulate on:

A) Inside
B) Surface
C) Center
D) Random

3. Electric field just outside surface is:

A) Parallel
B) Perpendicular
C) Circular
D) Zero

4. Reason for zero field inside is:

A) No charge
B) Charge motion cancels field
C) Infinite resistance
D) External field weak

5. Electric field is strongest at:

A) Flat region
B) Sharp edges
C) Center
D) Inside

ASSERTION- REASON

Use this code:

A) Both Assertion (A) and Reason (R) are true and R is correct explanation

B) Both A and R are true but R is not correct explanation

C) A is true, R is false

D) A is false, R is true

Assertion–Reason (Electric Field)

1

A: Electric field is a vector quantity.

R: It has both magnitude and direction.

2

A: Electric field due to a point charge decreases with distance.

R: It follows inverse square law.

3

A: Electric field lines intersect each other.

R: At intersection, field has two directions.

4

A: Electric field inside a conductor is zero.

R: Charges rearrange to cancel internal field.

5

A: Electric field depends on test charge.

R: Electric field is force per unit charge.

l

6

A: Electric field is independent of test charge.

R: It depends only on source charge.

7

A: Electric field lines start from positive charge.

R: Positive charges repel field lines.

8

A: Electric field lines never form closed loops.

R: Electric field is conservative.

9

A: Electric field is maximum near sharp edges.

R: Charge density is higher at sharp points.

10

A: Electric field at infinity is zero.

R: Force becomes negligible at large distance.

11

A: Electric field due to negative charge is inward.

R: Field direction is opposite to force on positive charge.

12

A: Electric field lines are closer where field is strong.

R: Density of lines represents field strength.

13

A: Electric field inside hollow charged sphere is zero.

R: Charges reside on outer surface.

14

A: Electric field is scalar.

R: It has only magnitude.

15

A: Electric field is same at all points in uniform field.

R: Magnitude and direction remain constant.

16

A: Electric field is zero at midpoint of two equal positive charges.

R: Fields cancel each other.

17

A: Electric field is zero at midpoint of +Q and −Q.

R: Fields cancel each other.

18

A: Electric field obeys superposition principle.

R: Fields add vectorially.

19

A: Electric field is perpendicular to conductor surface.

R: Charges move until tangential component becomes zero.

20

A: Electric field lines diverge from positive charge.

R: Positive charge repels test charge.

21

A: Electric field is stronger where lines are dense.

R: Field strength proportional to line density.

22

A: Electric field inside dielectric decreases.

R: Polarization reduces net field.

23

A: Electric field is zero at center of ring.

R: Symmetry cancels field.

24

A: Electric field depends on medium.

R: Permittivity affects field.

25

A: Electric field direction is from + to −.

R: It is direction of force on positive charge.