# Class 12 Physics Chapter 3 Important Questions Current Electricity

Q 1:- When does a current flow in a circuit? Ans:- An electric current flow in an electric circuit when some source of emf is present and the electric

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Current Electricity. The best solution of the problem is to practice as many Physics Class 12 Chapter 3 Important Questions as possible to clear the doubts.

Q 1:- When does a current flow in a circuit?
Ans:- An electric current flow in an electric circuit when some source of emf is present and the electric is complete.

Q 2:- Why is an electric current not possible without the source of emf?
Ans:- An electric current is established in a circuit only when current carries move in a particular direction through the circuit. To maintain steady flow of current carries, some energy is to be supplied for which a source of emf is essential.

Q 3:- A steady current passes through a metallic wire . Is there an electric field inside the conductor?
Ans:- Yes, a steady electric field is present along the length of a metallic wire when a steady current flows through it. A current will flow only when a potential difference V = V1-V2 is maintained between the two ends of a wire. If L be the length of the wire, then an electric field E = V/L is present inside the conducting wire parallel to its length.

Q 4:- What do you understand by the emf of a cell? Is it a force or energy?
Ans:- The emf of a cell is the amount of energy supplied by the cell for flow of unit positive charge once round the entire circuit, both outside as well as inside the cell. Alternately, it is equal to potential difference between the terminal of a cell, when the cell is in an open circuit, i.e., when no current is being drawn from the cell. The emf of a source is not a force, but it is an energy.

Q 5:- An electrochemical cell is used to maintain flow of electric current through a circuit. What is the real source of energy?
Ans:- An electrochemical cell utilises chemical energy stored in it. When a current flow in an electric circuit, the chemical energy is being transformed into electrical energy.

Q 6:- State the condition in which terminal voltage across a cell or battery is equal to its emf.
Ans:- The terminal potential difference across a cell or battery or source of emf is equal to its emf when no current is being drawn from the cell, i.e., the cell is in open circuit.

Q 7:- Define electrical conductivity of a conductor and give its SI unit.
Ans:- The conductivity of a conductor is the reciprocal of its resistivity. Alternately, conductance of a unit cube of the given material is known as its conductivity. The SI unit of conductivity is siemen per metre (S/m).

Q 8:- A wire of resistivity ρ is stretched so as to double its length. What will be its new resistivity?
Ans:- The resistivity remains unchanged as ρ because resistivity depends only on the material of wire and temperature but is independent of its length and cross-section area.

Q 9:- When do we say that Ohm's law is being followed by a substance?
Ans:- A substance is said to follow Ohm's law if its resistance is independent of the magnitude and polarity of the applied potential difference at a given temperature.

Q 10:- What is the shape of I-V graph for ohmic and nonohmic devices?
Ans:- For ohmic devices, I-V graph is a straight line graph passing through origin. However, for a nonohmic device, I-V graph may be either a curve other than straight line or a straight line which does not pass through the origin.

Q 11:- Is the relation R = V/I true for nonohmic resistors too?
Ans:- Yes, relation R = V/I is true for nonohmic resistors too. However, the numerical value of R does not remain constant and changes with change in value of applied potential difference V.

Q 12:- Name any one material having a small value of temperature coefficient of resistivity too?
Ans:- Manganin is a material whose temperature coefficient of resistivity is extremely small. Due to this property, manganin is used to prepare resistance coils of standard values.

Q 13:- Of metals and alloys, which have greater value of temperature coefficient of resistivity?
Ans:- Pure metals have greater value of temperature coefficient of resistivity as compared to the alloys.

Q 14:- Name two important parameters of a material which determine its resistivity.
Ans:- The resistivity of the material of a conductor is mainly determined by (a) the number density of electrons (n), and (b) the relaxation time (τ), i.e., the mean time between two successive collisions of an electron.

Q 15:- What is the relation between conductance and resistance?
Ans:- The conductance (G) of a substance is reciprocal of its resistance (R), i.e.,

G=\frac{1}{R}

Q 16:- Is the value of temperature coefficient of resistivity always positive?
Ans:- No, the value of temperature coefficient of resistivity (α) may be positive or negative. For metallic conductors, the value of α is positive but for semiconductors and insulators, the value of α is negative.

Q 17:- Connecting wires in electrical circuit are generally made from thick copper wire. Why?
Ans:- Copper is a good conductor having an extremely small value of resistivity (about 1.7×10-8 m). For a thick wire, resistance offered by copper wire become still lesser. Hence, we prefer to use a thick copper wire as the connecting wire.

Q 18:- What happens to the power dissipation if the electric current passing through a conductor of constant resistance is doubled?
Ans:- We know that power dissipation P = I2R. If for the constant resistance R, the current passing through the conductor is doubled, then as per above relation, power dissipated will become four times of its previous value.

Q 19:- On which basic principles of physics, are Kirchhoff's first and second rules based upon?
Ans:- Kirchhoff's first rule (the current rule) is based on the principle of conservation of electric charge. Kirchhoff's second rule (the voltage rule) is based on the principle of conservation of energy.

Q 20:- What is the principle of a metre bridge? Can it be called a slide wire bridge?
Ans:- A metre bridge is based on wheatstone bridge principle. Yes, it can be called a slide wire bridge too because in it, the jockey slides over the bridge wire.

Q 21:- Why are the metal strips used in a metre bridge made broad and thick?
Ans:- We prefer to broad and thick metal strips in a metre bridge arrangement to ensure that the resistance offered by the strips is extremely small and hence negligible.

Q 22:- Name the device used for measuring the emf of a cell.
Ans:- A potentiometer is used to measure the emf of a cell.

Q 23:- Of which material, is a potentiometer wire normally made, and why?
Ans:- Potentiometer wire is generally made of alloys like manganin or constantan because resistivity of these alloys is comparatively high and temperature coefficients of resistivity are extremely small and negligible.

Q 24:- What type of a cell should be used in the auxiliary circuit of a potentiometer, and why?
Ans:- A cell of constant and high emf should be used as a driver cell in the auxiliary circuit of a potentiometer so as to maintain a constant flow of current through potentiometer wire. Moreover, emf of a driver cell must be greater than that of the cell whose emf is to be determined, otherwise one will not get a balance point on the potentiometer wire.

Q 25:- What is conventional current? What is its direction?
Ans:- The flow of positive charge through a conductor is said to conventional current. Conventional current flows from positive terminal of voltage source to negative terminal of voltage source through the external circuit. As in metallic conductor, current is established due to drift motion of negatively charged electrons, hence conventional current flows in a direction opposite to that of flow of electrons.

Q 26:- The electron drift arises due to the force experienced by electrons in the electric field inside the conductor. But force should cause acceleration. Why then do the electrons acquire a steady average drift speed?
Ans:- Under the influence of force due to electric field, electrons are accelerated increasing their drift speed until they collide with positive ions or atoms of the metal. Electrons lose their drift speed after collision, but are again accelerated and so on. Therefore, on an average, electrons acquire only a steady average drift Speed.

Q 27:- When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?
Ans:- No, all the free electrons of a metal do not move in the same direction. The drift velocity of free electrons is supposed over the large random velocities of electrons.

Q 28:- Why is there no electric current through a metallic conductor under normal conditions when it possesses a large number of free electrons?
Ans:- Under normal condition, free electrons inside a metallic conductor move randomly with different speeds in different directions so that net average velocity of electrons is zero. As there is no definite drift motion of electrons in a particular direction, no electric current flows through the conductor unless some external electric field is applied across it.

Q 29:- Explain why power is dissipated in a conductor when an electric current is passed through it.
Ans:- When an electric current flows through a conductor, in fact, free electrons are drifting along the conductor. During their drift motion, elctrons frequently collide with ions and atoms present in the conductor. At each and atoms present in the conductor. At each collision, electrons lose some of their energy. Due to this, electrical power is continuously being dissipated, which reapears in the form of heat.

Q 30:- A piece of aluminium and another of germanium are cooled from 300 K to 100 K. What will be the effect on their conductivities, and why?
Ans:- We know that aluminium is a good conductor. As its temperature is lowered, its resistivity decreases and consequently, conductivity rises. On the other hand, germanium is a typical semiconductor. As its temperature is lowered, its resistivity increases and conductivity decreases. So, on cooling from 300 K to 100 K, electrical conductivity of aluminium increase but that of germanium decreases.

Q 31:- Why is Wheatstone bridge arrangement called a bridge?
Ans:- In Wheatstone arrangement, four resistances P, Q, R and S form a quadrilateral and both cell and the galvanometer are joined between diagonally opposite junctions of that quadrilateral. Hence, it is known as a bridge.

Q 32:- What are the parameters on which resistivity of a metallic conductor depends?
Ans:- The resistivity of a metallic conductor depends upon

• (a). the number density (n) of conduction electrons present in the conductor and
• (b). the relaxation time (τ).

In fact, respectively,

ρ=frac{m}{ne^2τ}

Where m = mass of an electron and e = charge of an elctron

Q 33:- In a meter bridge experiment, why is it considered important to obtain the balance point near the mid-point of bridge wire?
Ans:- The sensitivity of a Wheatatone bridge is maximum when all the four resistance P, Q, R and S are equal or nearly equal. For this purpose, we consider it important that the null point is obtained near the mid-point of bridge wire so that lengths l and (100-l) and consequently, their resistance P and Q are almost equal.

Q 34:- Is a current carrying conductor charged? Give reason.
Ans:- In a current carrying conductor, charge carries flow such that net inflow rate is exactly equal to net outflow rate at any cross-section. Consequently as a whole, there is no net charge on the conductor.

Q 35:- Does electric current change with change in its cross-section for a conductor of variable cross-section?
Ans:- No, for a given conductor, current does not change with change in cross-section. It is because electric charge is conserved and electric current is the rate of flow of charge. The charge entering per unit time at one end of conductor is exactly equal to the charge leaving per unit time at the other end.

Q 36:- The electron drift speed is estimated to be only a few mm/s for currents in the range of a few amperes. How then is current established almost the instant a circuit is closed?
Ans:- As soon as a circuit is closed, an electric field is established throughout the circuit almost instantly causing at every point a local electron drift. Establishment of a current does not have to wait for electrons from one end of conductor to travel up the other end.

Q 37:- Two heating coils, one of fine wire and other of thick wire, made of same material and same length are connected one by one to a source of electricity. Which one of the two coils will dissipate electric power at a greater rate?
Ans:- When a heating coil of residence R is connected to a source of electricity of voltage V, the power dissipated,

P=frac{V^2}{R}

As thicker wire has less resistance than a thin wire, power dissipated in thicker wire will be more than in a fine wire.

Q 38:- A metallic conductor is initially at a temperature T1. Then, its temperature is increased to T2. Will the product of its resistivity and conductivity change?
Ans:- The product of resistivity (ρ) and conductivity (σ) of a metallic conductor remains constant at all temperatures having a value ρ.σ = 1. As temperature of a metallic conductor is raised, its resistivity rises and in turn, conductivity falls but their product remains unchanged at 1.

Q 39:- Does the value of resistance of a conductor depend upon the potential difference applied across it or the current passed through it?
Ans:- No, the resistance of a conductor does not depend upon either on the potential difference applied across it or the current being passed through it. If the potential difference applied across a conductor is also proportionally increased such that their ratio V/I, i.e. the resistance R of the conductor remains constant.

Q 39:- A high voltage supply (H.T.) must have a very high internal resistance. Why?
Ans:- If the internal resistance of a high voltage supply is high enough, then the current will not exceed a safe limit even if, by chance, the H. T. supply is short-circuited.

Q 40:- In what respect, does a nearby discharged electrochemical cell differ mainly from a freshly prepared cell in its emf or in its internal resistance?
Ans:- The cell mainly differs in its internlal resistance. A freshly prepared cell has an extremely low internal resistance of the order of a fraction of one ohm. However, a nearly discharge cell has a high internal resistance of few hundred ohms.

Q 41:- Bends in a pipe slow down the flow of a liquid through that pipe. Do bends in a conductor increase its electrical resistance? Give reason.
Ans:- No, the bends in a conductor do not increase its electrical resistance because free electrons are of extremely small dimensions as compared to bends and hence, they can easily adjust their flow direction.

Q 42:- It is easier to start a car engine on a warm day than on a chilly day. Why?
Ans:- On a warm day, room temperature is higher and with rise in temperature, the internal resistance of car battery decreases. As a result, car battery can provide a greater current and car engine will start easily.

Q 43:- Light from a bathroom bulb gets dimmer for a moment when the geyser is switched on. Explain why?
Ans:- When the geyser is switched on, it draws a large current. Although light bulb and geyser are in parallel, but for a moment, voltage of the circuit is lowered on account of large current drawn by geyser. Consequently, the bathroom bulb gets dimmer. However, within a short time, the voltage is stabilised by the transmission grid and so, the bulb starts glowing fully at its normal rate.