CBSE Class 12 Physics Solved Question Paper 2014

Kirchhoff’s First Law or Junction Rule states that “The sum of the currents flowing towards a junction is equal to the sum of currents leaving the junction.”

This is in accordance with the conservation of charge which is the basis of Kirchhoff’s current rule. 

Here, I₁, I₂ I₃, and I₄ are the currents flowing through the respective wires.

Convention: The current flowing towards the junction is taken as positive and the current flowing away from the junction is taken as negative. 

I₃ + (− I₁) + (− I₂) + (− I₄) = 0 

Kirchhoff’s Second Law or Loop Rule states that In a closed loop, the algebraic sum of the emfs is equal to the algebraic sum of the products of the resistances and the currents flowing through them. 

 OR

“The algebraic sum of all the potential drops and emfs along any closed path in a network is zero.”

For the closed loop BACB:

E₁ − E₂ = I₁R₁ + I₂R₂ − I₃R₃

For the closed loop CADC:

E₂ = I₃R₃ + I₄R₄ + I₅R₅

This law is based on the law of conservation of energy.

Lorentz magnetic force is given by:

; q is the magnitude of the moving charge. 
The direction of the magnetic force is perpendicular to the plane containing the velocity vector and the magnetic field vector. 

Work done (W) in moving a test charge along an equipotential surface is zero.

Work done is given by, 


F is the electric force and s is the magnitude of displacement.

For non-zero displacement, this is possible only when cos   is equal to 0.
 
         = 90° 

Thus, the force acting on the point charge is perpendicular to the equipotential surface.

Electric field lines give us the direction of electric force on a charge.

Thus, for any charge configuration, equipotential surface through a point is normal to the electric field.

(i) When a dielectric slab is introduced between the plates of the capacitor, its capacitance will increase. Hence, the potential drop across the capacitor will decrease (V= Q/C  ) thereby decreasing the potential drop across the bulb because, both the bulb and capacitor is connected in series. So, brightness of the bulb will increase.

(ii) When resistance (R) is increased keeping the capacitance same, the potential drop across the resistor will increases. Therefore, the potential drop across the bulb will decrease because both are connected in series. So, brightness of the bulb will decrease. 

The graph below shows variation of resistivity of copper with temperature. The graph is parabolic in nature.
'

Drift velocity per unit applied electric field is known as the mobility of charge carriers in a conductor.

Mathematically, 

Mobility, 

S.I. unit of mobility:  or  

The glass bob would reach the ground earlier. Glass bob which is non-conducting in nature will only experience Earth’s gravitational pull unlike the metallic bob which is conducting.

Since the metallic bob is conducting in nature, eddy current is induced as it falls through the magnetic field of the Earth. As per Lenz’s law, current is induced in a direction opposite to the motion of the metallic bob. Hence, there is a delay. 

Given, Electric flux,  

We have, Electric flux, ϕ = E.A =  

                                         = 50 Weber

When the plane makes a 30° angle with the x-axis, the area vector makes 60° with the x-axis

                ϕ = E. A

 ⇒              ϕ=EA cos θ

 ⇒              ϕ= (5×10³)(10⁻²) cos 60°

 ⇒              ϕ=  

 ⇒             ϕ = 25 Weber 

A is a paramagnetic material because its permeability is greater than unity and its susceptibility is positive. The relative permeability of a paramagnetic material is between   and susceptibility is

For a diamagnetic material, the relative permeability lies between 0 ≤ μ_r < 1 and its susceptibility lies between −1<  χ<  0.

Therefore, 'B' is a diamagnetic material and its susceptibility is negative.

This is because its relative permeability is less than unity.

The generalized equation of a carrier wave is given by, 


The generalized equation of a modulating signal is given by, 

 

On comparing the given equation of carrier wave with the generalized equation, we get,

Amplitude of modulating signal, A_m = 1 V

Amplitude of carrier wave, A_c = 2 V

Modulation index is the ratio of the amplitude of modulating signal to the amplitude of carrier wave .

It is denoted by,

So,