# Electricity and Magnetism: Short Questions and Answers Physics Part 2

#### 1. When a motor car is started, its light becomes slightly dim. Why? [HSEB 2065]

When a motor car is started, current flows through the circuit which will be very high. Then there will be a large potential drop across the internal resistance of the cell and reduces the terminal p.d. of the battery. Due to this reason, the light of the motor car becomes dim.

#### 2. When a cell is used for a long time, its power decreases because its emf decreases. Is it correct?

No, it is incorrect. When a cell is used for a long time, its internal resistance is increased which decreases the current and hence the power,

#### 3. Differentiate between emf and terminal p.d. of a cell.

The followings are the differences between emf and terminal p.d.

#### 4. At what condition, the maximum power is transferred from a cell in an electric circuit?

According to the maximum power transfer theorem, a cell (or a battery) transfers maximum power to the circuit of the external resistance in the circuit is equal to the internal resistance of the cell. If a cell of emf E and internal resistance r sends current through external resistance R as shown in the figure, the maximum power across the resistance R is given by Pout (max) = E^2/4r, and the maximum efficiency of the circuit is 50%.

#### 5. What is a fuse? What characteristics should the material of the fuse wire have?

A fuse is a device to break a circuit, usually by melting when the current exceeds a certain value. Therefore, a fuse wire should have a low melting point and low resistivity. Generally, it is made of tin-lead alloy (63% tin and 37% lead).

#### 6. How can we use rheostat as a variable resistance in an electric circuit?

In a rheostat, a very long resistance wire is wound on a porcelain cylinder, each turn is well insulated from the other. The two ends of the wire act as the base terminals B1 and B2 as shown in the figure. The sliding contact T acts as a variable terminal. If we use one terminal as the sliding contact and one base terminal in the circuit, then as the sliding contact is moved, the resistance and hence the current in the circuit will vary. Thus a rheostat can be used as a variable resistance.

#### 7. Why is it easier to start a motor engine on a hot day than on a cold day?

During hot days, the viscosity of the electrolytes present in the battery is less. So, an active chemical reaction takes place and current is produced inefficient way. The internal resistance of a battery is inversely proportional to the temperature. So the internal resistance of the battery Of the motor engine is less on a hot day than on a cold day. As a result, a large current is drawn from the battery on a hot day due to the smaller internal resistance of the battery. A larger current starts the engine more easily than a smaller current. Therefore, it is easier to start a motor engine on a hot day than on a cold day.

#### 8. What is a thermistor?

A thermistor is a heat-sensitive device usually made of a semiconductor material whose resistance changes very rapidly with a change of temperature. Thermistors are made from semiconductor oxides of iron, nickel, and cobalt. A thermistor has the following properties: (a) Its temperature coefficient is very high. (b) Its temperature coefficient can be both positive and negative. Thermistors are used for voltage stabilization, temperature control, remote sensing, etc.

#### 9. What are the uses of superconductors?

Few uses of superconductors are as given below:

(a) It can be used in making superconductors.

(b) It will offer the possibility of loss-free transmission of electric power.

(c) Superconducting magnets may be constructed which would be 10 times stronger than the best normal electromagnets.

#### 10. What do you mean by an overloaded circuit? What precaution is taken to protect an electric circuit from overloading?

The circuit in which there is more current than the conductors can carry safely is called an Overloaded Circuit. To protect an electric circuit from overloading, a fuse is inserted in series with the circuit. When the current in the circuit exceeds the safe value, the heat produced melts the fuse wire. Consequently, the circuit is broken. This saves the circuit from damage.

#### 11. Two bulbs of coil resistances 55 Ω and 220 Ω are connected in a room. Which will glow brighter? If one of the bulbs is turned off, what will be the effect on illumination?

In household wiring, all domestic appliances are connected in parallel so that voltage across each bulb is the same. The rate of dissipation of 2 1 V energy in an appliance is P = R. i.e. P cc. This means, the smaller the resistance, the greater is the rate of dissipation of electric energy. Therefore, the bulb of smaller resistance will glow brighter. When one of the bulbs is switched off, there will be no effect on the illumination of the other because the operation of each appliance is independent of the other in parallel combination. But, the total light (brightness) in the room will decrease.

#### 12. Two bulbs of 40W and 100W, having the same voltage rating are connected in series across the supply in a room. Which bulb will glow brighter? If one of the bulbs is switched off, what will be the effect on illumination?

The 40W bulb has more resistance (R =,n than that of the 100W bulb. When two bulbs are connected in series across the supply, the same current I passes through each of them. Then the rate of dissipation of energy (P = I2R) is more in that bulb which has resulted, 40W bulb will glow bri larger resistance. As a circuit resistance fighter. If one of the bulbs is switched off, decrease thereby increasing the current. As a g result, the rate of dissipation of energy of the other bulb increases.

#### 13. The wires supplying current to an electric bulb are not heated whereas the filament of the bulb becomes hot. Explain.

The resistance per unit length of the wires supplying negligible as compared to that of the filament of the bulb. From Joule’s current is the law of heating, the rate of production of heat is directly proportional to the resistance per unit length. Therefore, the rate of production of heat m the connecting wires is so small that it cannot cause a noticeable rise in temperature. On the other hand, the rate of production of heat in the bulb is very high. Consequently, the filament of the bulb becomes hot.

#### 14. There is an impression among many people that a person touching a high power line gets shocked. Is it true? Explain.

No. if a person touches a high power line, the current in it disorganizes our nervous system. It makes the person lose temporarily his ability to exercise his nervous control to get himself free from the high power line. But, if We touch to ground with a power line at the same time, the current will pass through our body to the ground and produce a harmful effect.

#### 15. When is a 1.5 V AAA battery not actually a 1.5V battery?

That is, when do its terminals provide a p.d. of less than 1.5 V? When a battery is operated in a closed circuit, the terminals provide a p.d. of less than 1.5 V. In this case, the internal resistance of a battery comes to play so that some energy is lost in the form of heat. The relation between emf and terminal p.d. is given as E = V + Ir. This is why in normal cases, emf is always greater than terminal p.d.

#### 16. How can be the range of a voltmeter increased or decreased?

For a galvanometer of resistance G, the required high resistance to being connected in series to it to convert into a voltmeter of range (0 – V) volt is

R=V/Ig – G where Ig is the current for full-scale deflection in the galvanometer. or V = Ig(R + G)

Ig & G are constants for a given galvanometer. So to increase the range of voltmeter, a suitable high resistance can be connected in series. Similarly to decrease its range, the resistance can be reduced. This can be done by putting a suitable resistance in parallel to the voltmeter.

#### 17. Why is it possible for a bird to sit on a high voltage wire without being electrocuted?

The bird is resting on a wire of a fixed potential. In order to be electrocuted, a p.d. is required. There is no potential difference between the two feet of the bird. So a bird can sit on a high voltage wire without being electrocuted.

#### 18. Would a fuse work successfully if it were placed in parallel with the device it is supposed to protect?

Fuses are placed in series with the device they are to protect so that when the fuse burns out, the devices are turned off. This would not happen if the fuse were in parallel. So a fuse would not work successfully if it were placed in parallel.

#### 19. Batteries are always labeled with their emf. Would it also be appropriate to put a label on batteries stating how much current they provide? Why or Why not? [HSEB 2069]

No, it would not be appropriate to put a label on batteries stating how much current they provide because the magnitude of current a battery provides to a resistor depends upon the resistance of the resistor. It means the same battery can provide different values of current to different resistors.

#### 20. Can we use Kirchhoff’s laws to both a.c. and d.c.?

Yes, we can use Kirchhoff’s laws to both a.c. and d.c. It is because the conservation laws of charge and energy are followed by both a.c. and d.c. Kirchhoff’s first law concludes the conservation of change and the second law concludes the conservation of energy.

#### 21. On what conservation laws are Kirchhoff’s laws related?

Kirchhoff’s laws are related to the law of conservation of charge and the law of conservation of energy. The first Kirchhoff’s law is based on the law of conservation of charge and the second law is based on the law of conservation of energy.

#### 22. Explain why are Kirchhoff’s laws preferred over Ohm’s law.

Ohm’s law can be used to analyze the simple electric circuits only but it cannot be used to analyze the complicated electric circuits containing many components and loops. It can be done by using Kirchhoff’s laws. So, Kirchhoff’s laws are preferred over ohm’s law in an electrical circuit because Kirchhoff’s laws can be used to analyze the circuits containing many sources of emf.

#### 23. Write down the significance of Kirchhoff’s law.

When the circuit contains more than one source of emf and the current is divided into many branches, the circuit will be very complicated. Then we can not analyze the circuit by using only ohm’s law. But in this case, Kirchhoff’s laws simplify it to analyze the circuit.

#### 24. What is Wheatstone’s bridge?

Wheatstone’s bridge is an electrical circuit comprising of a network of four resistances which is used to measure the low resistance of a conductor accurately. It can also be used to compare the resistance of the two resistors. In the balanced condition of the Wheatstone bridge circuit, the ratio of resistances of the corresponding arms is the same. The Wheatstone bridge circuit is as shown in the figure. Wheatstone Bridge Circuit

#### 25. When is the Wheatstone’s bridge more sensitive?

Wheatstone’s bridge will be more sensitive when all the four resistances and the galvanometer resistance will be of the same order.

#### 26. Why should we not use the Wheatstone bridge for measuring very high resistance?

We should not use Wheatstone bridge for measuring high resistance because in this case, we have to use a galvanometer of high internal resistance. It will make low current flow through the galvanometer which will make it less sensitive.

#### 27. why should we not use the Wheatstone bridge for measuring very low resistance?

When we use a Wheatstone bridge for measuring very low resistance, all the other resistances should also be low to make the circuit sensitive. But a low resistance galvanometer is not so much sense. Load resistance and contact resistances will also make some error in the measurement.

#### 28. A slide wire bridge is also called a meter bridge. Why?

A slide wire bridge is called a meter bridge because in a slide wire, wire of one meter is used to form Wheatstone bridge and it works on the Wheatstone bridge principle.

#### 29. A meter bridge is also called a slide wire bridge. Why?

A meter bridge is also called a slide wire bridge because the jockey is slid over the wire of one-meter length in the bridge while doing the experiment.

#### 30. What is the principle used in a meter bridge?

A meter bridge is based on the Wheatstone bridge principle. It states that, in the balanced condition of the Wheatstone bridge circuit, the ratio of the resistances of the corresponding arms is the same. i.e. P/Q=X/R is satisfied in equilibrium condition.

#### 31. It is better to get a null point in the middle of the wire in a meter bridge. Why?

It is better to get a null point at the middle of the wire in a meter bridge because in the middle, the resistances of the ratio arms are nearly equal and the bridge is more sensitive. Then the metal strips at the end of the slide wire makes less effect on the results and the measurements will be more accurate.

#### 32. Why should we use the thick metal strips in a meter bridge?

When we use the thick metal strips in a meter bridge, their resistances will be less and they can be safely ignored.

#### 33. Why is a potentiometer named potentiometer?

The potentiometer is named as potentiometer because it is used to measure the potential difference.

#### 34. What is the principle used in a potentiometer?

The principle used in potentiometer is when a constant current is passed through a wire of uniform cross-sectional area, the potential drop across any portion of the wire is proportional to the length of the wire. If V is the p.d. across a section of wire of length l, when a steady current passes through it, then V ∝ l.

#### 35. Why do we prefer a potentiometer with a longer bridge wire?

To make a potentiometer more sensitive the potential drop per unit length of its wire should be as small as possible. So, to make p.d. per unit length small, we prefer a potentiometer with a longer bridge wire.

#### 36. How can you make a potentiometer more sensitive?

The sensitivity of a potentiometer indicates the smallest potential difference that can be measured with it. We can make a potentiometer as small as possible. To do this, resistance can be connected in series to more sensitive by decreasing the potential difference per unit length as the wire of the potentiometer. It decreases the current and makes the potentiometer more sensitive.

#### 37. A potentiometer is preferred than a voltmeter to measure the emf of a cell. Why? [HSEB 2069] OR We prefer a potentiometer to measure the emf of a cell rather than a voltmeter. Why? [HSEB 2070 Set D, HSEB 2072]

When a voltmeter is connected across a cell, it draws a small current from the cell because of having high internal resistance. It only reads the terminal potential difference across the cell. On the other hand, when a potentiometer is used to measure the emf of a cell, it draws no current from the cell. Hence it measures the emf of the cell. For this reason, a potentiometer is preferred over a voltmeter for measuring the emf of a cell.

#### 38. The wire of the potentiometer should be of the uniform area of cross-section or a uniform thickness. Why?

If the wire of a potentiometer is made of uniform cross-sectional area, the potential drop will be proportional to the length of the wire only and we can increase the sensitivity of the potentiometer only by increasing the length of the wire. But, if the wire has not uniform thickness, the principle of the potentiometer cannot be applied.