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**Q 34. An ideal capacitor of capacitance 0.2 µF is charged to a potential difference of 10 V. The charging battery is then disconnected. The capacitor is then connected to an ideal inductor of self-inductance 0.5 mH. The current at a time when the potential difference across the capacitor is 5 V, is: [2018]****(1) 0.34 A****(2) 0.25 A****(3) 0.15 A****(4) 0.17 AAns:** (4)

Q_{o} = 0.2 × 10 µC = 2µC

V_{o} = 10V

E_{i} of capacitor = 1/2 × 0.2 µf × (10V)^{2}

= 10µJ

E_{f} of capacitor = 1/2 × 0.2µf × (5V)^{2}

= 2.5 µj

E_{inductor} = 7.5 µJ = 1/2 Li^{2}

⇒ 7.5 × 10^{−6} = 1/2 × 0.5 × 10^{−3} × i^{2}

⇒ 30 × 10^{−3} = i^{2}

⇒ i = √3/10 = 0.17 A**Q 35. A power transmission line feeds input power at 2300V to a step down transformer with its primary windings having 4000 turns giving the output power at 230V. If the current in the primary coil of the transformer is 5A and its efficiency is 90% the output current would be: [2018](1) 45A(2) 50A(3) 20A(4) 25AAns: **A

Solution:

We know, formula

where, η is efficiency, P

denotes power of primary transmission

so, 0.9 = 230 x I

hence, I

therefore, output current would be 45A

(1)

**(2)**

**(3)**

**(4)Ans:** A

If E

direction of

(1)

**(2) (3) qωr ^{2}(4) Ans:** (2)

Ans:

Given, a coil of N turns having an area A, rotated with angular speed 'ω' in a uniform magnetic field 'B' connected to a resistor ' R'.

The flux linking to coil, φ = NBA sin (ωt)

Therefore,

Induced EMF,

When, sinωt = ±1, the induced emf value is maximum.

∴ the maximum EMF, E

(1) 250 Wb

(2) 275 Wb

(3) 200 Wb

(4) 225 Wb

Ans

Magnitude of change in flux = R × area under current vs time graph

(1)

(2)

(3)

(4)

Ans:

Magnetic field due to outer current loop

Induced emf in inner loop

(1) 10 Ω and tan

(2) 7 Ω and 45°

(3) 7 Ω and tan

(4) 10 Ω and tan

Ans:

e

x

Its magnetic field

Ans:

These materials are used to make magnets for electric generators, transformer core and electromagnet core. [2016]

Then it is proper to use :

(1) A for electric generators and transformers

(2) A for electromagnets and B for electric generators

(3) A for transformers and B for electric generators

(4) B for electromagnets and transformers

Ans:

Conceptual (Requires low retentivity and low coercivity)

(1) 80 H

(2) 0.08 H

(3) 0.044 H

(4) 0.065 H

Ans:

∴

A: Blue light

B: Yellow light

C: X-ray

D: Radiowave

(1) D, B, A, C

(2) A, B, D, C

(3) C, A, B, D

(4) B, A, D, C

Ans:

radio wave<yellow light<blue light<X-rays

(Increasing order of energy)

Current will be in the form of

I = I

Graph will be sinusoidal

Ans:

The small signal model of a BJT is shown in the figure. From this figure, R

Input resistance will be R

Hence

Since r

**(2) For an electromagnetic wave propagating in +y direction the electric field is and he magnetic field is ****(3) For an electromagnetic wave propagating in +x direction the electric field is and the magnetic field is ****(4) For an electromagnetic wave propagating in +x direction the electric field is and the magnetic field is ** ** [2016]****Ans: **(4)**Solution:**

If wave is propagating in x direction, must be functions of (x, t) & must be in y- z plane.

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