Skip to content

As shown in the… (Class 10 Science SQP 2024-25, Q36B)

questionbuddy.in/q/cbse-10-science-2024-25-sqp-q36b-lamps-lamp-rated-lamp-rated

ScienceCBSE Class 10 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 10 Science sample paper 2024-25: question 36(B)

From the official CBSE sample question paper for 2024-25, section D, question 36(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q36(B) · 5 marks · Long answerElectricity

As shown in the figure above A and B are two lamps. Lamp A is rated at 12 V, 24W. Lamp B is rated at 6.0 V. When lamp B operates at its rated voltage, the current in it is 3.0 A. The values of and are chosen so that both lamps operate at their rated voltages.
Based on the information given, answer the following.
(i) Calculate the current in Lamp A.
(ii) State and give reason for the reading of the Voltmeter.
(iii) Calculate the resistance of .
(iv) Find the value of the resistance .

(For visually impaired students: B. (i) State Joules law of heating and write its mathematical expression. (ii) Two resistors of resistances 2Ω and 4Ω are connected in a) series b) parallel with a battery of given potential difference. Compute the ratio of total quantity of heat produced in the combination in the two cases if the total voltage and time are kept the same for both.)

Figure in the paper: 15 V battery in series with R1; then lamp A in parallel with lamp B in series with R2, and a voltmeter V connected across the same parallel combination. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

Power, voltage and current are linked by P = VI.

Hint 2

Both lamps work at their rated voltages. Lamp A is directly across the parallel part, so what p.d. must that part have?

Hint 3

R₁ carries the sum of the currents in the two branches and takes up whatever is left of the 15 V.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) Use , so .

    ½ mark

  2. Lamp A works at its rated values of 12 V and 24 W:

    ½ mark

  3. (ii) The voltmeter reads 12 V.

    ½ mark

  4. Reason: The voltmeter is connected across the parallel combination. One branch of this combination is lamp A, which runs at its rated voltage of 12 V. Branches in parallel have the same p.d., so the voltmeter (and the branch containing B and ) shows 12 V.

    ½ mark

  5. (iii) In the branch containing lamp B and (in series):

    ½ mark

  6. Lamp B works at its rated 6.0 V, so:

    ½ mark

  7. is in series with lamp B, so the current through is the same as the current through B, which is 3.0 A.

    ½ mark

  8. ½ mark

  9. (iv) is in the main line, so it carries the total current:

    P.d. across

    ½ mark

  10. ½ mark

Answer: (i) 2 A; (ii) 12 V, because the voltmeter is across the parallel part and lamp A runs at 12 V; (iii) R₂ = 2 Ω; (iv) R₁ = 0.6 Ω

Where the marks are

  • Uses P = VI (0.5 mark)
  • Obtains I_A = 2 A (0.5 mark)
  • States that the voltmeter reads 12 V (0.5 mark)
  • Gives the reason: parallel arms have equal p.d. and lamp A runs at 12 V (0.5 mark)
  • Writes the p.d. relation in the B–R₂ branch (0.5 mark)
  • Finds the p.d. across R₂ = 6 V (0.5 mark)
  • Notes that the current in R₂ = 3 A (series with B) (0.5 mark)
  • Obtains R₂ = 2 Ω (0.5 mark)
  • Finds the total current 5 A and the p.d. across R₁ = 3 V (0.5 mark)
  • Obtains R₁ = 0.6 Ω (0.5 mark)

Revise the chapter

Practise this chapterBoard-style questions from Electricity, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.