Class 12 Physics

NCERT Exemplar Solutions for CBSE Class 12 Physics, Chapter 7 (from question number 7.27 to 7.31) are given below:

Question 7.27:

Solution 7.27:

Question 7.28:

1MW 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

(i) power is transmitted at 220V. Comment on the feasibility of doing this.

(ii) a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transformer is used to bring voltage to 220 V.

(ρ_Cu = 1.7 × 10^–8 SI unit)

Solution 7.28:

Question 7.29:

Consider the LCR circuit shown in Fig 7.6. Find the net current i and the phase of i. Show that i = v/Z. Find the impedance Z for this circuit.

Solution 7.29:

Question 7.30:

For an LCR circuit driven at frequency ω, the equation reads

L (di/dt) + Ri + q/C = v_i = v_m sin ωt

(i) Multiply the equation by i and simplify where possible.

(ii) Interpret each term physically.

(iii) Cast the equation in the form of a conservation of energy statement.

(iv) Intergrate the equation over one cycle to find that the phase difference between v and i must be acute.

Solution 7.30:

Question 7.31:

In the LCR circuit shown in Fig 7.7, the ac driving voltage is v = v_m sin ωt.

(i) Write down the equation of motion for q (t).

(ii) At t = t₀, the voltage source stops and R is short circuited. Now write down how much energy is stored in each of L and C.

(iii) Describe subsequent motion of charges.

Solution 7.31