Answer the questions.[Phy-2M]

Question 12 Marks

The rest position of the needles in a milliammeter and voltmeter, not in use, are as shown in Figure A. When a student uses these instruments in his experiment, the readings of the needles are in the positions shown in Figure B. Determine the correct values of current and voltage the student should use in his calculations.

Answer

The correct values of current and voltage the student should use in his calculations are $38 \mathrm{~mA}, 3.2 \mathrm{~V}$

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Question 22 Marks

In the circuit given below, the resistors $\mathrm{R}_{1}, \mathrm{R}_{2}$ and $\mathrm{R}_{3}$ have the values $10 \Omega, 20 \Omega$ and $30 \Omega$ respectively, which have been connected to a battery of 12 V . Calculate
a. the current through each resistor,
b. the total circuit resistance, and
c. the total current in the circuit.

Answer

Given: $\mathrm{R}_{1}=10 \Omega ; \mathrm{R}_{2}=20 \Omega ; \mathrm{R}_{3}=30 \Omega$According to Ohm's law,
$\mathrm{V}=\mathrm{IR}$
Given $\mathrm{V}=12 \mathrm{~V}$
a. Current through resistor $\mathrm{R}_{1}$ :
$\mathrm{I}_{1}=\frac{\mathrm{V}}{\mathrm{R}_{1}}=\frac{12}{10}=1.2 \mathrm{~A}$
Current through resistor $\mathrm{R}_{2}$ :
$\mathrm{I}_{2}=\frac{\mathrm{V}}{\mathrm{R}_{2}}=\frac{12}{20}=0.6 \mathrm{~A}$
Current through resistor $\mathrm{R}_{3}$ :
$\mathrm{I}_{3}=\frac{\mathrm{V}}{\mathrm{R}_{3}}=\frac{12}{30}=0.4 \mathrm{~A}$
b. Total circuit resistance, R
$\frac{1}{\mathrm{R}}=\frac{1}{\mathrm{R}_{1}}+\frac{1}{\mathrm{R}_{2}}+\frac{1}{\mathrm{R}_{3}}$
$\frac{1}{R}=\frac{1}{10}+\frac{1}{20}+\frac{1}{30}$
$\frac{1}{R}=\frac{11}{60}$
$\mathrm{R}=\frac{60}{11}=5.45 \Omega$
c. The total current in the circuit is $\mathrm{I}=\mathrm{I}_{1}+\mathrm{I}_{2}+\mathrm{I}_{3}$
$=1.2+0.6+0.4=2.2 \mathrm{~A}$

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Question 32 Marks

A student wound an insulated copper wire around a soft iron rod. He then connected one end to the rheostat and the other free end to the battery via a key. He closed the key and observes the deflection in the magnetic needle placed nearby. Now he altered the current using by reversing the connections of the battery and again noted the change in the deflection of the needle.
i. Why do the student performed this activity?
ii. What did the student observe?
iii. Comment on the statement a material in the middle of a current carrying coil gets magnetised.

Answer

i. The student conducted this activity to make an electromagnet.
ii. The electrical current flowing through a coil will create a uniform magnetic field. This magnetic field causes the needle to turn. Reversing, the connections to the battery, reverses the irection of the current flow and the needle will point in the opposite direction.
iii. When an iron rod is placed along the axis of a current carrying coil, it gets magnetised under the influence of the magnetic field produced by the coil through induction. But this magnetism lasts as long as the current supply is not withdrawn.

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Science Answer the questions.[Phy-2M]

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