12TH PHYSICS UNIT 1 IMPORTANT QUESTIONS WITH ANSWERS
2 Marks:
1. Electrostatics
The branch of physics which deals with stationary charges is called Electrostatics.
2. Triboelectric charging:
Charging the objects through rubbing is called triboelectric charging.
3. Coulomb's law:
Coulomb's law states that the electrostatic force between two point charges is directly proportional to the product of the magnitude of the two point charges and is inversely proportional to the square of the distance between the two point charges.
4. Superposition principle:
The total force acting on a given charge is equal to the vector sum of forces exerted on it by all the other charges.
5. Electric field:
Electric field is defined as the force experienced by a unit charge placed at that
point.
6. Electric dipole:
Two equal and opposite charges separated by a very small distance constitute an electric dipole.
7. Electric dipole moment:
The magnitude of the electric dipole moment is equal to the product of the
magnitude of one of the charges and the distance between them. p = 2qa
8. Electric potential:
The electric potential at a point is equal to the work done by an external force to bring a unit positive charge with constant velocity from infinity to that point in the region of the external electric field.
9.Electric flux:
The number of electric field lines crossing a given area kept normal to the electric field lines is called electric flux. It is usually denoted by the letter ΦE. Its unit is N m2 C-1. Electric flux is a scalar quantity and it can be positive or negative.
10.Gauss law:
Gauss’s law states that the total electric flux ΦE through the closed surface is equal to 1/ε0 times the net charge enclosed within that closed surface.
Φ = Qenclosed / €o
Here Qencldenotes the charges inside the closed surface.
11. Capacitance:
The capacitance C of a capacitor is defined as the ratio of the magnitude of charge on either of the conductor plates to the potential difference existing between the conductors.
C = Q / V
The SI unit of capacitance is coulomb per volt or farad (F)
12. Applications of capacitors:
(a)The flash which comes from the camera is due to the energy released from the capacitor, called a flash capacitor.
(b) During cardiac arrest, a device called heart defibrillator is used to give a sudden surge of a large amount of electrical energy to the patient’s chest to retrieve the normal heart function. This defibrillator uses a capacitor of 175 μF.
(c) Capacitors are used in the ignition system of automobile engines to eliminate
sparking.
(d) Capacitors are used to reduce power fluctuations and to increase the efficiency
of power transmission.
13. Disadvantage of capacitor:
Even after the battery or power supply is removed, the capacitor stores charges and energy for some time. For example if the TV is switched off, it is always advisable to not touch the back side of the TV panel.
14. Action of points or corona discharge:
Leakage of charges from the sharp ends of a charged conductor.
15. Principle and Uses of Van de Graaff generator:
Principle: Action of points and Electrostatic induction.
Uses: (i) Used to produce a large potential difference of 107 V.
(ii) Used to accelerate positive ions (protons and deuterons) for nuclear disintegrations.
3 Marks:
1. Basic properties of charges
(i) Electric charge
Materials are made up of atoms. Atoms are made up of protons, neutrons and electrons.
S.I unit of charge is coulomb.
(ii) Conservation of charges
Charges are neither created nor destroyed. They can be transferred from one
object to other. Net change in the charge will always be zero. This is called
conservation of charges.
(iii) Quantisation of charges
The charge q of any system is equal to the integral multiple of the fundamental
unit of charge e.
q = ne.
Here n is any integer (0, ±1, ±2, ±3, ±4............. ). This is called quantisation of
electric charge.
The value of e = 1.6 × 10-19C. The charge of an electron is −1.6 × 10-19C and the charge of the proton is +1.6 × 10-19C. Charge of any system is quantised. It can take only discrete set of values.
2. Important aspects of coulomb’s law:
(i) Coulomb's law:
Coulomb's law states that the electrostatic force between two-point charges is directly proportional to the product of the magnitude of the two-point charges and is inversely proportional to the square of the distance between the two point charges.
(ii) Direction of coulomb’s force always act along the line joining the two charges.
(iii) In SI units, k = 1/4πε0 and its value is 9 × 109 N m2 C-².
Here εo is the permittivity of free space or vacuum and the value of εo = 8.85 x 10 -12 N-1 m-²C2
(iv) One coulomb:
One coulomb is the charge which when placed at a distance of 1m in air from an equal and similar charge will experience a force of 9 × 109 N
(v) The force between two-point charges in a medium is always less than that in vacuum.
The relative permittivity εr = ε / εo.
For vacuum or air, εr = 1 and for all other media εr > 1.
3. Comparison of Coulomb force with Gravitational force:
Coulomb force:
(i) Coulomb force between two charges can be attractive or repulsive.
(ii) k = 9 × 109 N m2 C
-2
(iii) k > G
(iv) Coulomb force depends on the medium
(v) Coulomb force is true only for point charges.
(vi) When the charges are in motion, in addition to Coulomb force, Lorentz force also comes into play.
Gravitational force:
(i) Force between two masses is always attractive.
(ii) G = 6.67 x 10 -11 N m2kg-2
(iii) G < k
(iv) Gravitational force does not depend on the medium
(v) Gravitational force is obeyed even in the case of larger mass.
(vi) Gravitational force between two masses is the same whether the masses are at rest or motion.
4. Important aspects of electric field:
(i) For a positive source charge, the electric field points radially outward.
(ii) For a negative source charge, the electric field points radially inward.
(iii) The force experienced by a test charge q0 is given by, F = q0E
(iv)Electric field depends only on the source charge.
(v) As distance increases, electric field decreases.
(vi)There are two kinds of electric field: uniform and non-uniform electric field.
(vii)Uniform electric field will have the same direction and same magnitude at all points.
(viii)Non uniform electric field will have different directions or different magnitude or both different at all points.
5. Derive an expression for the electric field due to a point charge:
• Consider a source point charge q located at a point O.
• Another test charge qo is placed at a point P.
• It is at a distance r from the charge q. (OP =r )
• The electrostatic force experienced by the charge qo due to q is given by
Coulomb’s law.
F=k qqo/r2
But E= F/qo
+q -q
----------------------------->
O P
So, E= kq/r²
E= q/4π€or²
Electric field acts along OP if q is positive and acts along PO if q is negative.
6. Explain the working of microwave oven.
• Microwave works on the principle of torque acting on a dipole.
• When the microwave is switched on, it produces oscillating
electromagnetic fields.
• This produces torque on the water molecules present in the food items.
• Due to this torque, the water molecules rotate very fast.
• Hence, heat energy is released.
• This heat is used to cook the food.
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