1 Marks Question

Question 11 Mark

Choose the correct alternative:
Alloys usually have much (lower/ higher) temperature coefficients of resistance than pure metals.

Answer

Alloys usually have lower temperature coefficients of resistance than pure metals.

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Question 21 Mark

Choose the correct alternative:
Alloys of metals usually have (greater/ less) resistivity than that of their constituent metals.

Answer

Alloys of metals usually have greater resistivity than that of their constituent metals.

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Question 31 Mark

Answer the following questions:
Is Ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ohm’s law.

Answer

No, Ohm's law is not universally applicable for all conducting elements. Vacuum diode semi-conductor is a non-ohmic conductor. Ohm's law is not valid for it.

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Question 41 Mark

Choose the correct alternative:
The resistivity of a typical insulator (e.g., amber) is greater than that of a metal by a factor of the order of (10²²/10³).

Answer

The resistivity of a typical insulator is greater than that of a metal by a factor of the order of 10²².

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Question 51 Mark

Choose the correct alternative:
The resistivity of the alloy manganin is nearly independent of/ increases rapidly with increase of temperature.

Answer

The resistivity of the alloy, manganin, is nearly independent of increase of temperature.

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Question 61 Mark

The resistance of the platinum wire of a platinum resistance thermometer at the ice point is $5 \Omega$ and at steam point is $5.23 \Omega$. When the thermometer is inserted in a hot bath, the resistance of the platinum wire is $5.795 \Omega$. Calculate the temperature of the bath.

Answer

$R_0=5 \Omega, R_{100}=5.23 \Omega$ and $R_t=5.795 \Omega$
Now,
$
\begin{array}{l}
t=\frac{R_t-R_0}{R_{100}-R_0} \times 100, \quad R_t=R_0(1+\alpha t) \\
=\frac{5.795-5}{5.23-5} \times 100 \\
=\frac{0.795}{0.23} \times 100=345.65^{\circ} C
\end{array}
$

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Question 71 Mark

The plot of the variation of potential different across a combination of three identical cells in series, versus current is shown below. What is the emf and internal resistance of each cell?

Answer

E = 2V
$\text{r}= 2 \Omega$.

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Question 81 Mark

V – I graph for a metallic wire at two different temperatures T₁ and T₂ is as shown in the figure. Which of the two temperatures is higher and why?

Answer

T₁ > T₂Slope of T₁ is higher than that of T₂. (or Resistance, at T₁, is higher than that of T₂).

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Question 91 Mark

Nichrome and copper wires of same length and same radius are connected in series. Current I is passed through them. Which wire gets heated up more? Justify your answer.

Answer

Nichrome
RNi > RCu (or ResistivityNi > ResistivityCu)

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Question 101 Mark

Two identical cells, each of emf E, having negligible internal resistance, are connected in parallel with each other across an external resistance R. What is the current through this resistance?

Answer

Current, $\text{I} = \frac{\text{E}}{\text{R}}$

Concept:

emf of combination of two (or more cells) remain same.

Internal resistance is negligible i.e., zero.

So, $\text{I} = \frac{\varepsilon_{eq}}{\text{R} + \text{r}_{eq}} = \frac{\varepsilon}{\text{R}}$

(r_eq = 0).

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Question 111 Mark

Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?

Answer

Manganin.

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Question 121 Mark

Two conducting wires X and Y of same diameter but different materials are joined in series across a battery. If the number density of electrons in X is twice that in Y, find the ratio of drift velocity of electrons in the two wires.

Answer

$\text{I} = \text{n}_{x}\text{eAv}_{x} = \text{n}_{y}\text{eAv}_{y}$
$\Rightarrow\frac{\text{v}_{X}}{\text{v}_{Y}} = \frac{\text{n}_{Y}}{\text{n}_{X}} = \frac{\text{n}_{Y}}{2\text{n}_{Y}} = \frac{1}{2}.$

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Question 131 Mark

Define the term 'drift velocity' of charge carriers in a conductor and write its relationship with the current flowing through it.

Answer

Drift velocity $v_d$ is the directed velocity acquired by the electrons for a unit value of the applied Electric field.

Current, $I=neAv_d$

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Question 141 Mark

Plot a graph showing variation of current versus voltage for the material GaAs.

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Question 151 Mark

Show variation of resistivity of Si with temperature in a graph.

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Question 161 Mark

Define the term 'electrical conductivity' of a metallic wire. Write its S.I unit.

Answer

Electrical conductivity is defined as current density per unit electric field. (Alternatively, Reciprocal of resistivity)
SI Unit : ohm^-1 m^-1

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Question 171 Mark

Define the term 'Mobility' of charge carriers in a conductor. Write its S.I. unit.

Answer

Magnitude of the drift velocity of charge carrier per unit Electric field is called mobility.

Alternate Answer

$\mu = \frac{|\text{V}_{d}|}{\text{E}}\text{ or } \frac{\text{e}\tau}{\text{m}}$

SI unit = m² / (volt second) or ms^-1N^-1C.

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Question 181 Mark

Show variation of resistivity of copper as a function of temperature in a graph.

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Question 191 Mark

The emf of a cell always greater than its terminal voltage. Why? Give reason.

Answer

Because the cell has some finite internal resistance./Emf is determined when the cell is in open circuit and no current is drawn.

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Question 201 Mark

A 10 V battery of negligible internal resistance is connected across a 200 V battery and a resistance of 38 $\Omega$ as shown in the figure. Find the value of the current in circuit.

Answer

$\text{i} = \frac{\text{V}}{\text{R}} = \frac{190}{38} = 5 \text{A}.$

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Question 211 Mark

Show on a graph, the variation of resistivity with temperature for a typical semiconductor.

Answer

[If the graph is correct but the axis of the graph are not labelled, or labelled in an incorrect manner,]

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Question 221 Mark

A resistance R is connected across a cell of emf ε and internal resistance r. A potentiometer now measures the potential difference between the terminals of the cell as V. Write the expression for 'r' in terms of ε, V and R.

Answer

$\text{r} = \text{R}\left\{\frac{\text{E}}{\text{V}} - 1 \right\}.$

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Question 231 Mark

A wire of resistance 8R is bent in the form of a circle. What is the effective resistance between the ends of a diameter AB?

Answer

R_effective = 2R.

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Question 241 Mark

The plot of the variation of a potential difference across a combination of three identical cells in series, versus current is as shown below. What is the emf of each cell?

Answer

2 volts

Alternate Answer

When I = 0 , E = V

$\therefore \text{Emf of each cell} = \frac{6\text{V}}{3} = 2\text{ volts}.$

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Question 251 Mark

Sketch a graph showing variation of resistivity of carbon with temperature.

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Question 261 Mark

A source with e.m.f. '$\varepsilon$'and internal resistance 'r' is combined with external resistance 'R'. Write the value of voltage on resistance R.

Answer

Value of voltage on resistance $R$ is: $V=\frac{\varepsilon R}{R+r}$ or $V=\varepsilon-I r$

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Question 271 Mark

The voltage V and current I graph for a conductor at two different temperatures $T _1$ and $T _2$ are shown in the figure. Which of the highest value of temperature $T _1$ and $T _2$ is more and why?

Answer

$T_2>T_1$($T_2$ is higher than $T_1$.

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Question 281 Mark

Is work done by a battery always equal to the thermal energy developed in electrical circuit? What happens if a capacitor is connected in the circuit?

Answer

No, the work done by a battery is not always equal to the thermal energy developed in the electrical circuit. In case of a non-ideal battery, the work done by the battery is the sum of the thermal energy developed in the electric circuit and the thermal energy developed in the internal resistance of the battery. 12 C V² of energy is lost in the form of heat and electromagnetic radiation.

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Question 291 Mark

When a current is established in a wire, the free electrons drift in the direction opposite to the current. Does the number of free electrons in the wire continuously decrease?

Answer

No the complete circuit (which is the only condition to flow current) restores number of electrons equal to the number of electrons passed through wire.

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Question 301 Mark

In a TV tube, electrons are accelerated from the rear to the front. What is the direction of the current?

Answer

Direction of current is in opposite direction of electron moving thus the current is from front to rear.

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Question 311 Mark

Does a conductor become charged when a current is passed through it?

Answer

Yes as charge passes through it is is charged. For example we use electrical magnets in some equipment.

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Question 321 Mark

When is a Wheatstone’s bridge most sensitive?

Answer

The Wheatstone’s bridge is most sensitive when all the four resistances of the bridge are equal.

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Question 331 Mark

Sometimes it is said that "heat is developed" in a resistance when there is an electric current in it. Recall that heat is defined as the energy being transferred due to temperature difference. Is the statement in quotes technically correct?

Answer

Yes both statements are correct.

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Question 341 Mark

What happens to the power dissipation if the value of electric current passing through a conductor of constant resistance is doubled?

Answer

$\text{Power P}=\text{I}^2\text{Rt}\propto\text{I}^2$
Clearly if current is doubled, the power dissipated becomes 4 times.

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Question 351 Mark

A fan with copper winding in its motor consumes less power as compared to an otherwise similar fan aluminium winding. Explain.

Answer

As the copper has higher conductance than aluminum so there is low power loss thus it consumes lesser power.

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Question 361 Mark

A wire of resistivity $\rho$ is stretched to double its length. What will be its new resistivity?

Answer

New resistivity will be $\rho$ (unchanged) because resistivity is independent of dimensions of conductor.

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Question 371 Mark

Specific resistance of copper, silver and constantan are 1.18 × 10^–6, 1 × 10^–6, 45 × 10⁶ ohm cm respectively. Which is the best electrical conductor and why?

Answer

Smaller the resistivity of a substance, larger is its conductivity. The resistivity of silver is least so silver is the best conductor.

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Question 381 Mark

The potential difference applied across a given resistance is altered so that heat produced per second increases by a factor of 16. By what factor does the applied pd change?

Answer

Power $\text{P}=\frac{\text{V}^2}{\text{R}}\Rightarrow\text{V}\propto\sqrt{\text{P}}$ for given resistance.
Hence, for making power 16-times, voltage should be made 4-times.

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Question 391 Mark

Is current density a scalar or a vector quantity?

Answer

Current density is a vector quantity.

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Question 401 Mark

Suppose you have three resistors, each of value $30\Omega.$ List all the different resistances you can obtain using them.

Answer

All in series.
All in parallel.
Two parallel and one in series.

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Question 411 Mark

Name the device used for measuring the internal resistance of a secondary cell.

Answer

Potentiometer.

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Question 421 Mark

A uniform wire of resistance $50\Omega$ is cut into 5 equal parts. These parts are now connected in parallel. What is the value of equivalent resistance of the combination?

Answer

2 ohmsa.

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Question 431 Mark

The thermal energy developed in a current-carrying resistor is given by U = i² Rt and also by U = Vit. Should we say that U is proportional to i² or i?

Answer

As the simplest form is = i²Rt
So, U is directly proportional to i².

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Question 441 Mark

One of your friends argues that he has read in the previous chapters that there can be no electric field inside a conductor. And hence there can be no current through it. What is the fallacy of this argument?

Answer

If there can be no current inside a conductor there can be current on its surface so "there can be no current through it" is wrong.

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Question 451 Mark

The metallic conductor is at temperature $\theta_1.$ The temperature of metallic conductor is increased to $\theta_2.$ How will the product of its resistivity and conductivity change?

Answer

$\text{Product}\ \rho\ \sigma=\rho.\frac{1}{\rho}\Big(\sin\text{ce }\sigma=\frac{1}{\rho}\Big)$
⇒ Product is independent of temperature.

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Question 461 Mark

As the temperature of a conductor increases, both its resistivity as well as conductivity change. Will the ratio of its resistivity and conductivity increase, decrease or remain the same.

Answer

Increases.

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Question 471 Mark

A carbon resistor is marked in colour bands of red, black, orange and silver. What is the resistance and tolerance value of the resistor?

Answer

From colour-code table:

Red	Black	Orange	Silver
$\downarrow$	$\downarrow$	$\downarrow$	$\downarrow$
$2$	$0$	$3$	$\pm10\%$

$\text{R}=20\times10^3\Omega\pm10\%=20\ \text{k}\Omega\pm10\%$

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Question 481 Mark

Is it possible that the terminal potential difference across the cell be zero? If yes, state the condition.

Answer

Yes, terminal potential difference V = IR. If external resistance R = 0, V = 0; i.e. terminal potential difference is zero, when cell is short circuited.

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Question 491 Mark

The given circuit represents a balanced Wheatstone’s bridge. Calculate the value of resistance x.

Answer

In the balanced condition,
$\frac{\text{P}}{\text{Q}}=\frac{\text{R}}{\text{S}}$
$\therefore\frac{4+4}{1}=\frac{4\text{+}\text{x}}{2}$
$\Rightarrow\text{x}=12\Omega$

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Question 501 Mark

What is the value of I in the given current distribution?

Answer

I + 4 + 2 = 5 + 3
I = 2A

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