# Electricity and Magnetism: Physics Class 12 Short Questions and Answers

In this post, we have discussed Electricity and Magnetism Short Questions and Answers of Physics Class 12 where you will get a detailed explanation.

#### 1. What are the differences between a superconductor and a perfect conductor?

The differences between a superconductor and a perfect conductor are given:
(a) A superconductor is a substance that losses resistance below a critical temperature though it has significant resistance above t au temperature. However, a perfect conductor has zero resistance at temperatures.

(b) A superconductor has real existence whereas perfect conductors are just hypothetical approximations.

(c) Superconductors show several quantum mechanical effects such as the Meissner Effect but perfect conductor does not.

#### 2. What do you mean by the potential difference across a conductor?

The potential difference across a conductor is defined as the amount of work done per unit positive test charge in moving it from one point to the other without acceleration against the electrostatic force due to the electric field. Its unit is volt in the SI system of units.

#### 3. Why are resistances connected in series?

When the resistances are connected in series, the net resistance is equal to the sum of the individual resistances. It means the net resistance is always greater than the individual resistances. Hence to increase the value of resistance in the circuit, the resistances are connected in series.

#### 4. Is Ohm’s law universally applicable are all conducting materials?

No. Ohm’s law is applicable for conductors at a constant temperature, pressure, and mechanical strain. But it is not applicable for semiconductors, vacuum diodes, transistors, Zener diodes, etc.

#### 5. The electrons drift in a conductor arises due to the force experienced by electrons in the electric field. This force should cause the acceleration of electrons. But the electrons acquire a steady drift velocity. Why? [HSEB 2064]

Yes, the electrons should be accelerated but they acquire a steady velocity. It is because the electrons suffer a large number of collisions with the positive ions of the conductor during their movement. However, the electric field accelerates an electron between two collisions, it decelerates on collision. So the gain in speed between collision is lost in the next collision. Thus, the net acceleration averages out to zero, and the electron acquires a constant average drift speed.

#### 6. What do you mean by current density?

The current flowing through the unit area of a conductor is called the Current Density. Mathematically it is given by, I J = A Where I is the current passing through a conductor of cross-sectional area A. It is a vector quantity and its unit in the SI system is Am-2.

#### 7. Why is not the drift velocity of positive ions considered?

In the presence of an external electric field, the positive ions in the conductor also experience the electric force but because of having greater mass and tightly bounded, they have negligible drift velocity. Hence their drift velocity is not considered.

#### 8. Why current is regarded as a scalar quantity?

Current is regarded as scalar quantity due to the following reasons:

It does not obey all the rules of the vector. i.e. it can be added or subtracted by using simple rules of the algebra.

It is obtained by multiplying clearly the two vectors’ current density and the plane area. i.e. I = j. A.

The magnitude and direction of current do not depend upon the direction of the flow of charge.

It does not change with the change in cross-sections at different points along the length of the conductor.

#### 9. A wire is carrying current. Is it charged? Why?

A wire carrying current is not charged because the electric current is the flow of free electrons. When current flows through a conductor, at any instant, the number of electrons flowing into it is always equal to the number of electrons leaving it. This makes the net charge zero on the conductor leaving it uncharged.

#### 10. What do you mean by a shunt? OR Explain the action of a shunt.

A shunt is a small resistance connected in parallel to a high-value resistance (i.e. parallel to the coil of galvanometer) so that it provides a bypass to the major part of the current and it saves the instruments from damage. The shunt is used to convert a galvanometer into an ammeter. It is also used to decrease the total resistance in the circuit.

#### 11. Why is an ammeter connected in series in an electric circuit? [HSEB 2052, 064]

An ammeter has very low resistance. An ideal ammeter has zero internal resistance. Hence, when an ammeter is connected in series, the total resistance of the circuit does not change appreciably and the current which has to be measured will almost be the same as before connecting it. Therefore it reads the current with negligible error.

#### 12. What will happen when an ammeter be connected in parallel in a circuit?

We know that the internal resistance of an ammeter is very low. When it is connected in parallel in a circuit, it will reduce the total resistance of the circuit considerably and it increases the current in the circuit will flow through the arm, enter and it will read the current which flows through it only but not the total current in the circuit. So, an ammeter is connected in series in an electric circuit.

#### 13. why an ammeter should have low resistance as far as possible? Explain. OR The resistance of an ammeter must essentially be very small. Why? [HSEB 20671]

An ammeter is used to measure the current flowing in the circuit and is connected in series since it has very low internal resistance. If its internal resistance is not low, its connection in series will increase the total resistance of the circuit which will decrease the current that has to be read. Hence it will not read the actual current flowing in the circuit which has to be measured. For this reason, an ammeter should have low resistance.

#### 14. Does an ammeter read slightly less or more than the actual current in the original circuit? Explain.

An ammeter has some internal resistance. When it is connected in series in a circuit to measure the current, it increases the total resistance of the circuit which decreases the current. Hence an ammeter reads slightly less than the actual current in the circuit.

#### 15. Can we change the range of an ammeter?

We know the value of shunt (S) in an ammeter which has to be connected to convert a galvanometer into an ammeter is given by S = IgRg/(I-Ig)

where Ig, is the current which gives full deflection in galvanometer, I is the total current to be measured in the circuit and Rg is the resistance of the coil of the galvanometer.

I = (IgRg/S)- Ig

Since Ig and Rg have fixed value for a given galvanometer to change the range of the value of I the value of S has to be changed. Hence the range of an ammeter can be changed by changing the value of shunt.

#### 16. Does a voltmeter read slightly less or more than the original voltage drop across the conductor in the circuit? Explain.

A voltmeter reads slightly less potential drop than the original p.d. across the conductor in the circuit. It is so because it draws a small part of current from that part of the circuit across which it is connected and the current through that part is decreased, reducing the p.d. across it.

#### 17. Compare an ammeter and a voltmeter. OR Write down the differences between an ammeter and a voltmeter.

An ammeter and a voltmeter have the following differences.

#### 18. Why is a voltmeter connected in parallel across a circuit element? [HSEB 2059]

A voltmeter has very high internal resistance. When it is connected across a circuit element, the net resistance of that parallel combination will be almost the same as that of a given element. So the presence of a voltmeter does not affect the resistance in the circuit. Also, it draws a very small current through it which does not make an appreciable change in p.d. across that element, and p.d. across the voltmeter gives almost accurate p.d. across that element.

#### 19. What happens when a voltmeter is connected in series in a circuit?

When a voltmeter is connected in series in a circuit, the total resistance of the circuit will be increased to a high value since its internal resistance is very high. It reduces the current into the circuit to a very low value which is an undesired effect.

#### 20. How can you convert a galvanometer into a voltmeter?

A galvanometer can be converted into a voltmeter by connecting a high resistance in series with the resistance of the coil of the galvanometer which makes the total resistance of the voltmeter very high. The value of the resistance which has to be connected is given by DC circuits 77 V R = -g- -G Where V is the maximum value of voltmeter reading, I, is the current which makes the galvanometer show full deflection and G be the resistance of the galvanometer.

#### 21. What is ohmmeter?

An ohmmeter is an arrangement which is used for measuring resistance directly in ohm. It consists of a meter a resistor and a source of emf. In this meter, larger currents correspond to smaller resistances, to this stale reads backward compared to the scale showing the current.

#### 22. What is an electric cell?

A device, which supplies the energy necessary to maintain the flow of electric charge in the circuit or can maintain a constant p.d. across its terminals without depending on external factors, is called a Cell. Until the cell is active it acts as a source of p.d. and when it is discharged, it simply acts as a normal resistor or capacitor.

#### 23. What do you mean by emf of a cell?

Emf of a cell is the total work is done or energy used to move a unit positive test charge around a circuit joined to it. It is denoted by E. In an open circuit, it is the p.d. between the two terminals of the cell. It is the characteristic property of a cell and depends on the material used as electrodes and the electrolyte inside it. Its unit is volt in the SI system of units.

#### 24. What do you mean by the internal resistance of a cell?

The resistance offered by the electrolyte of a cell when the electric current flows through it is called Internal Resistance. For freshly prepared cells the value of internal resistance is very low but after more and more use of the cell, its internal resistance increases. It is shown externally in series to the cell as shown in the figure. Internal resistance depends upon the distance between two electrodes, the nature of electrolytes and electrodes. Internal resistance appears in series to the external resistance in the circuit.

#### 25. On what factors does the internal resistance of a cell depend?

The internal resistance of a cell depends upon the following factors:

(a) It depends upon the distance between the electrodes. When this distance is increased, the internal resistance also increases.

(b) It depends upon the nature of electrodes and electrolytes.

(c) It depends upon the area of electrodes immersed in an electrolyte.

(d) It depends upon the temperature of the electrolyte.

#### 26. When the ends of a wire are connected to a battery, initially the current is slightly larger, but soon it decreases slowly and becomes steady at a lower value although the emf of the battery remains unchanged. Explain.

When current begins to flow through the wire, its temperature increases which increase the resistance. The increment in resistance decreases the current. So, in the beginning, the current will be maximum but when the current starts flowing, it decreases. when the temperature of the wire increases, it increases the heat radiation also. When the rate of heat radiation will be equal to the rate of dissipation of electrical energy, the temperature of the wire becomes constant and the current remains unchanged.

#### 27. Define the term terminal potential difference of a cell.

The terminal potential difference of a cell is defined as the potential difference between the two terminals of the cell when the cell sends a current in an external circuit. It is denoted by V. Its unit is volt in the SI system of units.

#### 28. What is the difference between an emf and a potential difference? Under what circumstances are the potential difference between the terminals of a battery and the emf of the battery equal to each other?

The total energy supplied by a cell to a positive unit test charge to circulate in an electrical closed loop ie from the positive terminal of the cell through an external circuit to the same positive terminal of the cell is called the emf of the cell whereas the energy supplied by the cell to positive unit test charge to cross any component i.e resistor of the circuit is called the potential difference across the resistor. Hence we have

Emf = sum of all p.d across the resistors

or, Emf = sum of all p.d across external resistors + p.d across an internal resistor of the cell,

Therefore, when the cell has no internal resistance, the emf of the cell will be equal to the p.d. between the terminals of the battery.

#### 29. Explain, why heat is produced in a conductor when current flows through it. [HSEB 2064]

When current flows through a conductor, electrons collide against other electrons and also against the ions in the conductor. During each collision, a part of their kinetic energy is transferred to the ions. As the ions get energy, their kinetic energy increases thereby increasing the total kinetic energy of the particles inside the conductor. As heat is the sum of kinetic energy, the heat content of the conductor increases. Thus heat is produced in a conductor when current flows through it.