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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
Uniform Electric Field: An electric field which has same magnitude & strength at every point in a region is called uniform electric field.
Non-Uniform Electric Field: An electric field which has different magnitude & different strengths at every point in a region is called non-uniform electric field.
ELECTRIC FIELD INTENSITY IN POSITION VECTOR FORM:
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
(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
(a) Farad
(b) Weber
(c) C2N-1 m-2
(d) C2N-1 m-2
(a) Farad
(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
(a) 6.25 × 1018
(b) 625 × 1018
(c) 6.023 × 1023
(d) None
Ans: (a)
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NOTES:
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