Answer the questions.[Phy-2M]

Question 12 Marks

You are supplied with a number of $100 Ω$ How could you combine some of these resisters to make a $250 Ω$ resistor?

Answer

For obtaining 250 ohms, connect two 100 ohm in series with a parallel combination of two 100 ohms.

Question 22 Marks

Why is a series arrangement not used for connecting domestic electrical appliances in a circuit?

Answer

A series arrangement is not used for connecting domestic electrical appliances in a circuit because if one electrical appliance stops working due to some defect, then all other appliance also stop working as the whole circuit is broken.

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Question 32 Marks

Why is an electric light bulb not filled with air? Explain why argon or nitrogen is filled in an electric bulb.

Answer

If air is filled in an electric bulb, then the extremely hot tungsten filament would burn up quickly in the oxygen of air. So, the electric bulb is filled with a chemically unreactive gas like argon or nitrogen. Thes gases do not react with the hot tungsten filament and hence prolong the life of the filament of the bulb.

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Question 42 Marks

Why does the connecting cord of an electric heater not glow hot while the heating element does?

Answer

The connecting cord of the heater made of copper does not glow because negligible heat is produced in it by passing current (because of its extremely low resistance); but the heating element made of nichrome glows because it becomes red-hot due to the large amount of heat produced on passing current (because of its high resistance).

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Question 52 Marks

Why are the coils of electric irons and electric toasters made of an alloy rather than a pure metal?

Answer

Because,

Resistivity of an alloy is much higher than that of a pure metal.
An alloy does not undergo oxidation (or burn) easily even at high temperature.

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Question 62 Marks

Which of the following resistor arrangement, A or B, has the lower combined resistance?

Answer

Resistance of arrangement A is 10 ohm.
Combined resistance of arrangement B is caculated as follows:
$\frac{1}{\text{R}}=\frac{1}{10}+\frac{1}{1000}=\frac{(100+1)}{1000}$
$\text{R}=\frac{1000}{101}=9.9\ \text{ohm}$

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Question 72 Marks

Which of the following are conductors and which are insulators? Sulphur, Silver, Copper, Cotton, Aluminium, Air, Nichrome, Graphite, Paper, Porcelain, Mercury, Mica, Bakelite, Polythene, Manganin.

Answer

Conductor: Silver, Copper, Aluminum, Nichrome, Graphite, Mercury, Manganin.

Insulators: Sulphur, Cotton, Air, Paper, Porcelain, Mica, Bakelite, Polythene.

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Question 82 Marks

Which is the better way to connect lights and other electrical appliances in domestic wiring: series circuits or parallel circuits? Why?

Answer

Parallel circuits – Because if one electrical appliance stops working due to some defect, then all other appliances in the circuit will keep working properly.

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Question 92 Marks

Which has more resistance:

A long piece of nichrome wire or a short one?
A thick piece of nichrome wire or a thin piece?

Answer

A long piece of nichrome wire.
A thin piece of nichrome wire.

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Question 102 Marks

When a 12V battery is connected across an unknown resistor, there is a current of 2.5mA in the circuit. Calculate the value of the resistance of the resistor.

Answer

V = 12 volt, I = 2.5 × 10 - 3A
We know that
V = IR
$\text{R}=\frac{\text{V}}{\text{I}}$
$\text{R}=\frac{12}{(2.5\times10^{-3})}$
R = 4.8 × 103 ohm = 4800 ohm.

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Question 112 Marks

What p.d. is needed to send a current of 6A through an electrical appliance having a resistance of $40\Omega$?

Answer

I = 6 amp, R = 40 ohm
We know that
V=IR
V = 6 × 40 = 240V.

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Question 122 Marks

What is the potential difference between the terminals of a battery if 250 joules of work is required to transfer 20 coulombs of charge from one terminal of battery to the other?

Answer

Given: Work done = 250J, Charge moved = 20C.

we know that,Potential difference $= \frac{\text{Work done}}{\text{Charge\ moved}}$

$=\frac{250}{20}=12.5$

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Question 132 Marks

What is the maximum power in kilowatts of the appliance that can be connected safely to a 13A ; 230V mains socket?

Answer

I = 13 amp, V = 230V
Power = VI
= 230 × 13
= 2990W
P = 2.99KW

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Question 142 Marks

What is the conventional direction of the flow of electric current? How does it differ from the direction of flow of electrons?

Answer

Conventional direction of flow of electric current is from positive terminal of a battery to the negative terminal, through the outer circuit. The direction of flow of electrons is opposite to the direction of conventional current, i.e. from negative terminal to positive terminal.

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Question 152 Marks

What is meant by the “resistance of a conductor”? Write the relation between resistance, potential difference and current.

Answer

The property of a conductor due to which it opposes the flow of current through it is called resistance of the conductor.
$\text{R}=\frac{\text{V}}{\text{I}}$

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Question 162 Marks

What is meant by the heating effect of current? Give two applications of the heating effect of current.

Answer

When an electric current is passed through a high resistance wire, the wire becomes very hot and produces heat. This effect is knows as heating effect of current. This effect is used in room heaters and electric ovens.

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Question 172 Marks

What is meant by “electric power”? Write the formula for electric power in terms of potential difference and current.

Answer

The rate at which electrical work is done or the rate at which electrical energy is consumed is known as electrical power.
It is given by P = VI = Watt
P = VI = Watt

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Question 182 Marks

What is meant by conductors and insulators? Give two examples of conductors and two of insulators.

Answer

Conductors: Those substances through which electricity can flow are known as conductors.

E.g., Copper, silver etc.

Insulators: Those substances through which electricity cannot flow are known as insulators.

E.g., Plastic, cotton etc.

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Question 192 Marks

What is an electric current? What makes an electric current flow in a wire?

Answer

An electric currrent is a flow of electric charges (electrons) through a conductor. Potential difference between the ends of the wire makes electric current to flow in the wire.

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Question 202 Marks

What do you understand by the term “electric potential”? (or potential) at a point? What is the unit of electric potential?

Answer

The electric potential (or potential) at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point.

Unit of electric potential is volt.

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Question 212 Marks

What do you understand by the “resistivity” of a substance?

Answer

Resistivity is the characteristic property of a substance which depends on the nature of the substance and its temperature. It is numerically equal to the resistance between the opposite faces of a 1m cube of the substance.

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Question 222 Marks

What current will be taken by a 920W appliance if the supply voltage is 230V?

Answer

P = 920W, V = 230V, I = ?
We know that
P = V × I,
920 = 230 x II = $\frac{920}{230}$ = 4amp

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Question 232 Marks

The resistance of a metal wire of length 1m is $26 Ω$ at 20°C. If the diameter of the wire is 0.3mm, what will be the resistivity of the metal at that temperature?

Answer

I = 1m
R = 26 ohm
$\text{r}=\frac{\text{d}}{2}=\frac{0.3}{2}\text{mm}=0.15\text{mm}=0.15\times10^{-3}\text{m}$
$\rho=\frac{\text{R}\times\text{A}}{\text{I}}=\frac{\text{R}\times\pi\text{r}^2}{\text{I}}$
$\frac{26\times3.14\times(0.15\times10^{-3})^2}{1}$
$-1.83\times10^{-6}\Omega\text{m}$

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Question 242 Marks

The potential difference between the terminals of an electric iron is 240V and the current is 5.0A. What is the resistance of the electric iron?

Answer

V = 240 volt, I = 5 amp
We know that
V = IR
240 = 5 x R
$\text{R}=\frac{240}{5}=48\text{ohm}.$

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Question 252 Marks

The figure below shows a variable resistor in a dimmer switch.

How would you turn the switch to make the lights: (a) brighter, and(b) dimmer? Explain your answer.

Answer

Turn the switch to right side so as the resistance decreases.
Turn the switch to the left side so as the resistace increases.

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Question 262 Marks

The diagram below shows a circuit containing a lamp L, a voltmeter and an ammeter. The voltmeter reading is 3V and the ammeter reading is 0.5A

What is the resistance of the lamp?
What is the power of the lamp?

Answer

Given: V = 3V, I = 0.5 amp

R = ?

we know that V = IR

3 = 0.5R

R = 6 ohms

Power of lamps = VI

3 × 0.5

= 1.5 watt

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Question 272 Marks

The atoms of copper contain electrons and the atoms of rubber also contain electrons. Then why does copper conduct electricity but rubber does not conduct electricity?

Answer

Copper has free electrons that are loosely held by the nuclei of the atoms. These free electrons result in conduction of electricity. The electrons present in rubber are strongly held by the nuclei of its atoms. So, rubber does not have free electrons to conduct electricity.

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Question 282 Marks

State the law of combination of resistances in parallel.

Answer

According to the law of combination of resistance in parallel, the reciprocal of the combined resistance of a number of resistances connected in parallel is equal to the sum of the reciprocals of all the individual resistances.

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Question 292 Marks

On what factors does the resistance of a conductor depend?

Answer

Resistance of a conductor depends on the following factors:-
Length of the conductor, area of cross section of the conductor, nature of material of the conductor and temperature of the conductor.

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Question 302 Marks

Name two factors on which the resistivity of a substance depends and two factors on which it does not depend.

Answer

Resistivity of a substance on the nature of the substance and its temper. it does not depend the length or thickness of the conductor.

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Question 312 Marks

Name the material which is the best conductor of electricity.

Answer

Silver metal.

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Question 322 Marks

Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?

Answer

V = R
$\text{I}=\frac{\text{V}}{\text{R}}$
keeping V constant, $\text{I}\propto\frac{1}{\text{R}}$
So, when R is double I becomes half.

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Question 332 Marks

If you were going to connect two light bulbs to one battery, would you use a series or a parallel arrangement? Why? Which arrangement takes more current from the battery?

Answer

Parallel arrangement because if one electrical bulb stops glowing due to some defect the other will keep glowing. Parallel arrangement takes more current from the battery due to its lesser equivalent resistance.

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Question 342 Marks

If the lamps are both the same in the figure given below and if A₁ , reads 0.50A, what do A₂ , A₃ , A₄ and A₅ read?

Answer

The total current of 0.5A flowing in the circuit distribution equally in the two arms having lamps (since the lamps have same resistances). so the current through each of these arms is 0.25A. Hence A₂, A₃, A₄ and A₅ all will read 0.25A.

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Question 352 Marks

If five resistances, each of value 0.2 ohm, are connected in series, what will be the resultant resistance?

Answer

As per the law of combination of resistances in series,
R = R₁ + R₂ + R₃ + R₄ + R₅
R = 0.2 + 0.2 + 0.2 + 0.2 + 0.2 = 1ohm.

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Question 362 Marks

If current flows through two lamps arranged: (a) in series, (b) in parallel, and the filament of one lamps breaks, what happens to the other lamp ? Explain your answer.

Answer

In case of series connection, if filament of one lamp breaks, the other wil stop glowing.
In case of parallel connection, if filament of one lamp breaks, the other will keep glowing.

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Question 372 Marks

If 3 resistances of 3 ohm each are connected in parallel, what will be their total resistance?

Answer

$\text{R}_1=\text{R}_2=\text{R}_3=3\Omega$
$\frac{1}{\text{R}}=\frac{1}{\text{R}_1}+\frac{1}{\text{R}_2}+\frac{1}{\text{R}_3}$
$=\frac{1}{3}+\frac{1}{3}+\frac{1}{3}=\frac{3}{3}=1$
$\therefore\text{R}=1\Omega$

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Question 382 Marks

How will you connect three resistors of $2 Ω, 3 Ω$ and $5 Ω$ respectively so as to obtain a resultant resistance of $2.5 Ω$ ? Draw the diagram to show the arrangement.

Answer

Given: Three resistance of 2 ohm, 3 ohms, 5 ohms.
Their resultant, R = 2.5 ohm

Resistance of first line = 2 + 3 = 5 ohm
So, $\frac{1}{\text{R}}=\frac{1}{5}+\frac{1}{5}$
On solving we get that
R = 2.5 ohms.

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Question 392 Marks

How much heat will an instrument of 12W produce in one minute if it is connected to a battery of 12V?

Answer

Given: P = 12W, V = 12V, t = 60 Sec
P = VI
$\text{I}=\frac{\text{P}}{\text{V}}=\frac{12}{12}=1\text{A}$
V = IR
$\text{R}=\frac{\text{V}}{\text{I}}=\frac{12}{1}=12\Omega$
H = I²Rt
H = 1² × 12 × 60
H = 720J

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Question 402 Marks

How much energy is transferred by a 12V power supply to each coulomb of charge which it moves around a circuit?

Answer

Given: Potential difference = 12V, Charge moved = 1C

We know that,

Work done = Potential difference × charge moved

= 12 × 1 = 12 joules

Since work done on each coulomb of charge is 12 joules, the energy given to each coulomb of charge is also 12 joules.

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Question 412 Marks

How many electrons are flowing per second past a point in a circuit in which there is a current of 5 amp?

Answer

I = 5A
t = 1s
$\text{I}=\frac{\text{Q}}{\text{t}}$
Q = 1 × t = 5 × 1 = 5C
No. of electrons comparing 1.6 × 10^-19 C - 1
No. of electrons comparing $5\text{C}=\frac{5}{1.6\times10^{-19}}=31.25\times10^{19}$

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Question 422 Marks

How does the resistance of a wire vary with its:

Area of cross-section?
Diameter?

Answer

By increasing the area of cross section, the resistance will decrease.
By increasing the diameter, the resistance will decrease.

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Question 432 Marks

Give two reasons why nichrome alloy is used for making the heating elements of electrical appliances.

Answer

Nichrome alloy is used for making the heating elements of electrical appliances because:

Nichrome has very high resistivity.
Nichrome does not undergo oxidation (or burn) easily even at high temperature.

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Question 442 Marks

Give two examples of substances which are good conductors of electricity. Why do you think they are good conductors of electricity?

Answer

Silver and copper are good conductors of electricity because they have free electrons available for conduction.

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Question 452 Marks

Give the law of combination of resistances in series.

Answer

According to the law of combination of resistances in series, the combined resistance of any number of resistances connected in series is equal to the sum of the individual resistances.

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Question 462 Marks

Give one example to show how the resistance depends on the nature of material of the conductor.

Answer

If we take two similar wires of same length and same diameter, one of copper metal and other of nichrome alloy, we will find that the resistance of nichrome wire is about 60 times more than that of the copper wire. This shows that the resistance depends on the nature of material of the conductor.

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Question 472 Marks

Four resistances of 16 ohms each are connected in parallel. Four such combinations are connected in series. What is the total resistance?

Answer

Resultant resistance of a parallel combination of four 16 ohm resistance is
$\frac{1}{\text{R}}=\frac{1}{16}+\frac{1}{16}+\frac{1}{16}+\frac{1}{16}=\frac{4}{16}$
R = 4 ohm
For such combination are connected in series, so total resistance = 4 + 4 + 4 + 4 = 16 ohm.

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Question 482 Marks

Find the current in each resistor in the circuit shown below:

Answer

R₁ = 6 ohm, R₂ = 4 ohm V = 24V
The two resistances are connected in parallel.
Current across $\text{R}_1=\text{I}_1=\frac{\text{V}}{\text{R}_1}=\frac{24}{6}=4\text{amp}$
Current across $\text{R}_2=\text{I}_2=\frac{\text{V}}{\text{R}_2}=\frac{24}{4}=6\ \text{amp}$

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Question 492 Marks

Explain why, tungsten is used for making the filaments of electric bulbs.

Answer

Tungsten is used for making the filaments of electric bulbs because it has a very high melting point. Due to its very high melting point, the tungsten filament can be kept white hot without melting away. Also, tungsten has high flexibility and low rate of evaporation at high temperature.

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Question 502 Marks

Explain why, the current that makes the heater element very hot, only slightly warms the connecting wires leading to the heater.

Answer

The connecting wires of the heater get only slightly warm because they have extremely low resistance due to which negligible heat is produced in them by passing current.

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