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ELECTRIC FIELD:

The region around a charged body within which its influence can be realized by other charges is called electric field.

Source Charge: The point charge which produces the electric field is known as source charge.

Test Charge: The small charge which experiences the electrostatic force due to the source charge is known as test charge.

Note: Source charge is the creator of field & test charge is the tester of the field.

ELECTRIC FIELD INTENSITY (E):
The electric field intensity due to a source charge is defined as the force acting per unit positive  test charge placed at that point.

q is a source charge produces electric field. A test charge q0 is placed at point P. If F is the force between both the charges , then field intensity is defined as,

👉 Field intensity is a vector quantity because it has both magnitude & direction. The direction is along the direction in which the test charge move.
👉 SI unit of filed intensity is: N/C or newton/coulomb
👉 The Dimensional formula is:

ELECTRIC FIELD INTENSITY DUE TO A POINT CHARGE: Let us consider a source charge +q produces electric field. A test charge q0 is placed at point P at a distance 'r'.

So the force between q& q0 is given by

Now the field intensity is given as,

Uniform Electric Field: An electric field which has same magnitude & strength at every point in a region is called uniform electric field.
Let us consider two point charges q & q0 placed at points A & P having position vector r1 & r2 respectively.
Now Coulomb's force between both charges is given by
ELECTRIC FIELD INTENSITY FOR A SYSTEM OF CHARGES (SUPERPOSITION PRINCIPLE):
According to superposition principle, net electric strength at a point due to a group of charges is equal to the vector sum of all the field strengths produced by the individual charges at that point.
Let there are n point charges q1, q2, q3, ....................qn having their position vector r1, r2, r3, ..............rn. Let q0 is the test charge placed at point P where the electric field due to all other charges to be determined.
Eleectric field at P due to point charges q1, q2, q3, ....................., qn are given by

This is the superposition principle.
MCQ:
Q1: A soap bubble is given negative charge. Its radius will _____________
(a) Increase (b) Decrease (c) Remain unchanged (d) Fluctuate
Ans: (a) due to repulsive forces

Q2: If the radius of a solid and hollow copper spheres are same which one can hold greater charge
(a) Solid sphere (b) Hollow sphere (c) Both will hold equal charge (d) None of these
Ans: (c) Charge resides on the surface of conductor, since both the sphere having similar surface area so they will hold equal charge.

Q3: The force between two charges 0.06m apart is 5 N. If each charge is moved towards the other by 0.01m, then the force between them will become
(a) 7.20 N (b) 11.25 N (c) 22.50 N (d) 45.00 N
Ans: (b)

Q4: Three equal charges each +Q, placed at the corners of on equilateral triangle of side a what will be the force on any charge
Ans: (d)

Q5: Equal charges Q are placed at the four corners A, B, C, D of a square of length a. The magnitude of the force on the charge at B will be
Ans: (c)

Q6:  Five point charges each of value +Q are placed on five vertices of a regular hexagon of side L. What is the magnitude of the force on a point charge of value – q placed at the centre of the hexagon
Ans: (a)

Q7: The force per unit charge is known as
(a) electric flux
(b) electric field
(c) electric potential
(d) electric current
Ans: (b)

Q8: Electric field lines provide information about
(a) field strength
(b) direction
(c) nature of charge
(d) all of these
Ans: (d)

Q9: SI unit of permittivity of free space is
(b) Weber
(c) C2N-1 m-2
(d) C2N-1 m-2

Ans: (c)

Q10: The number of electron-taken out from a body to produce 1 coulomb of charge will be :
(a) 6.25 × 1018
(b) 625 × 1018
(c) 6.023 × 1023
(d) None
Ans: (a)

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