Science (English Medium) कक्षा १२ - CBSE Question Bank Solutions for Physics

A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different numbers of turns and wound on the same soft iron core.

The number of turns in the primary and secondary coils of an ideal transformer is 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

The value of current in primary coil is ______.

A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different numbers of turns and wound on the same soft iron core.

The number of turns in the primary and secondary coils of an ideal transformer is 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

Power in primary coil is ______.

Two charges –q each are fixed separated by distance 2d. A third charge q of mass m placed at the mid-point is displaced slightly by x(x << d) perpendicular to the line joining the two fixed charged as shown in figure. Show that q will perform simple harmonic oscillation of time period.

`T = [(8pi^3 ε_0 md^3)/q^2]^(1/2)`

A room has AC run for 5 hours a day at a voltage of 220V. The wiring of the room consists of Cu of 1 mm radius and a length of 10 m. Power consumption per day is 10 commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of aluminium of the same dimensions?

`[ρ_(Cu) = 1.7 xx 10_(Ωm)^-8, ρ_(Al) = 2.7 xx 10^-8 Ωm]`

Let the magnetic field on earth be modelled by that of a point magnetic dipole at the centre of earth. The angle of dip at a point on the geographical equator ______.

1. is always zero.
2. can be zero at specific points.
3. can be positive or negative.
4. is bounded.

Assume the dipole model for earth’s magnetic field B which is given by B_V = vertical component of magnetic field = `mu_0/(4pi) (2m cos theta)/r^3` B_H = Horizontal component of magnetic field = `mu_0/(4pi) (sin theta m)/r^3` θ = 90° – lattitude as measured from magnetic equator. Find loci of points for which (i) |B| is minimum; (ii) dip angle is zero and (iii) dip angle is ± 45°.

Consider the plane S formed by the dipole axis and the axis of earth. Let P be point on the magnetic equator and in S. Let Q be the point of intersection of the geographical and magnetic equators. Obtain the declination and dip angles at P and Q.

Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?

1. For a given power level, there is a lower current.
2. Lower current implies less power loss.
3. Transmission lines can be made thinner.
4. It is easy to reduce the voltage at the receiving end using step-down transformers.

For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I²Z cos φ.

1. Here, the power factor cos φ ≥ 0, P ≥ 0.
2. The driving force can give no energy to the oscillator (P = 0) in some cases.
3. The driving force cannot syphon out (P < 0) the energy out of oscillator.
4. The driving force can take away energy out of the oscillator.

The line that draws power supply to your house from street has ______.

1. zero average current.
2. 220 V average voltage.
3. voltage and current out of phase by 90°.
4. voltage and current possibly differing in phase `phi` such that `|phi| < pi/2`.

If a LC circuit is considered analogous to a harmonically oscillating spring block system, which energy of the LC circuit would be analogous to potential energy and which one analogous to kinetic energy?

Draw the effective equivalent circuit of the circuit shown in figure, at very high frequencies and find the effective impedance.

A 60 W load is connected to the secondary of a transformer whose primary draws line voltage. If a current of 0.54 A flows in the load, what is the current in the primary coil? Comment on the type of transformer being used.

1 MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to power transmitted if

1. power is transmitted at 220 V. Comment on the feasibility of doing this.
2. a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transfomer is used to bring voltage to 220 V. (ρ_Cu = 1.7 × 10^–8 SI unit)

Fusion processes, like combining two deuterons to form a He nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact ______.

1. nuclear forces have short range.
2. nuclei are positively charged.
3. the original nuclei must be completely ionized before fusion can take place.
4. the original nuclei must first break up before combining with each other.

The electric potential V as a function of distance X is shown in the figure.

The graph of the magnitude of electric field intensity E as a function of X is ______.

(i) A ray of light incident on face AB of an equilateral glass prism, shows minimum deviation of 30°. Calculate the speed of light through the prism.

(ii) Find the angle of incidence at face AB so that the emergent ray grazes along the face AC.

1. Calculate the energy and momentum of a photon in a monochromatic beam of wavelength 331.5 nm.
2. How fast should a hydrogen atom travel in order to have the same momentum as that of the photon in part (a)?

1. Calculate the frequency of a photon of energy 6.5 × 10⁻¹⁹ J.
2. Can this photon cause the emission of an electron from the surface of Cs of work function 2.14 eV? If yes, what will be the maximum kinetic energy of the photoelectron?

A conductor of 10 Ω is connected across a 6 V ideal source. The power supplied by the source to the conductor is ______.