NCERT Class X Science Class: Chapter –12. Electricity – Part-7

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Long Answer Questions

Question 29:

Three incandescent bulbs of each are connected in series in an electric circuit. In another circuit another set of three bulbs of the same wattage are connected in parallel to the same source.

(a) Will the bulb in the two circuits glow with the same brightness? Justify your answer.

(b) Now let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit? Give reason.

Answer:

Image Three incandescent bulbs a

Image Three Incandescent Bulbs A

Image Three incandescent bulbs a

No. The resistance of the bulbs in series will be three times the resistance of single bulb. Therefore, the current in the series combination will be one-third compared to current in each bulb in parallel combination. The parallel combination bulbs will glow more brightly.

Image Three incandescent bulbs

Image Three Incandescent Bulbs

Image Three incandescent bulbs

The bulbs in series combination will stop glowing as the circuits broken and current is zero. However the bulbs in parallel combination shall continue to glow with the same brightness.

Question 30:

State Ohm’s law? How can it be verified experimentally? Does it hold good under all conditions? Comment.

Answer:

Ohm’s law: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

Experiment to verify ohm’s law

Procedure:

1. Connect the various components as.

2. Close the sky, so that current begins to flow in the circuit.

3. Note down the potential difference (V) across the conductor PQ of resistance R shown by the voltmeter and the corresponding current (I) shown by the ammeter.

Image State Ohm’s law

Image State Ohm’S Law

Image State Ohm’s law

4. Now move the knob of rheostat so that the current in the circuit increases.

5. Again note down the potential difference (V) across the conductor R in the voltmeter and current in the circuit shown by ammeter.

6. Repeat the experiment five times by increasing the current in the circuit by moving the knob of the rheostat in steps.

Observations:

Potential Difference (V)
Title: Potential Difference (V)

S.N

Potential difference (V)

Current (I)

V/I

1.

2.

3.

4.

5.

Mean value of

Plot a graph between V and I by taking V along X- axis along. We get a straight line passing through origin.

Conclusion: From the graph between V and I, we conclude that , which is Ohm’s law. Hence Ohm’s law is verified experimentally.

Ohm’s law does not hold under all conditions. Ohm’s law does not hold for non-ohmic material such as electrolyte.

Question 31:

What is electrical resistivity of a material? What is its unit? Describe an experiment to study the factors on which the resistance of conducting wire depends.

Answer:

The electrical resistivity of a material is defined as the resistance of a conductor made of that material of unit length and unit cross-sectional area. Its Sl unit is Ohm meter.

Consider an electric circuit consisting of a cell, an ammeter, a nichrome wire of length L(marked 1 to 4) and a plug key as shown below:

Image electrical resistivity of a material

Image Electrical Resistivity of a Material

Image electrical resistivity of a material

At point plug the key and note the current ammeter. Replace the nichrome wire by another nichrome wire of same thickness but twice the length that is 2l at point 2. Again note the reading. Now, replace the wire by a thicker nichrome wire of same length (marked 3). A thicker wire has a larger cross-sectional area.

Again note down the current through the circuit. Replace nichrome wire with copper wire of same length and same area of cross-section at point 4, Note the value of current.

Notice the difference in the current in all cases. When the length of the wire is doubled, then the ammeter reading decreases to half its previous value i.e., current through the wire is halved. Since, resistance of the wire , then R is doubled which implies .

When the nichrome wire is replaced by a thicker one of same material and length, the current in the wire increases which means that the resistance of the thicker wire (3) is less than that of the thinner wire (1). This implies When the nichrome wire is replaced by a copper wire (4) of the same length and same cross-sectional area, then the current recorded by the ammeter is more.

This means that the resistance of copper wire is less than that of the nichrome wire of the same dimensions i.e., the resistance of the wire depends on the nature of its material.

Question 32:

How will you infer with the help of an experiment that the same current flows through every part of the circuit containing three resistances in series connected to a battery?

Answer:

Let the experimental set up comprises of three resistors and of three different values such as and 3 Ω which are connected in series. Connect them with a battery of 6 V, an ammeter and plug a key, as shown in figure,

Image Three resistors R 1,R 2and R 3 of

Image Three Resistors R 1,R 2and R 3 Of

Image Three resistors R 1,R 2and R 3 of

The key K is closed and the ammeter reading is recorded. Now, the position of ammeter is changed to anywhere in between the resistors again, the ammeter reading is recorded each time. It’s observed that there was identical reading each time, which shows that same current flows through every part of the circuit containing three resistances in series connected to a battery.

Question 33:

How will you conclude that the same potential difference (voltage) exists across three resistors connected in a parallel arrangement to a battery?

Answer:

The experimental set-up comprises of three resistors which are joined in parallel combination and connecting them with a battery, an ammeter (A), a voltmeter (V) and a plug key K, as shown in figure (a). The key K is closed and the voltmeter and ammeter readings are recorded.

K is switch voltmenter and ammeter are read

K is Switch Voltmenter and Ammeter Are Read

K is switch voltmenter and ammeter are read

The key K is open and removing the ammeter and voltmeter from the circuit and insert the voltmeter V in parallel with and ammeter in series with the resistor , as shown in fig. (b)Again, the voltmeter and ammeter readings are recorded.

Image Voltmeter and ammeter readings are recorded

Image Voltmeter and Ammeter Readings Are Recorded

Image Voltmeter and ammeter readings are recorded

Similarly, measuring the potential differences across resistances and . It is found that voltmeter gives identical reading which leads to conclude that the voltage or potential difference across each resistors is same and equal to the potential difference across the combination.

Question 34:

What is Joule’s heating effect? How can it be demonstrated experimentally? List its four applications in daily life.

Answer:

The heating effect of current is defined by Joules law of heating. It states that the heat H produced by a resistor of resistance P, due to current flowing through it for time? Is

It is also called ohmic heating and resistive heating. In a conductor when an electric field is applied across its ends, the free electrons available in it start drifting opposite to the direction of the electric field. These electrons collide with the atoms which have lost the electrons.

As a result of these collisions some energy of the electrons is transferred to the atoms which vibrate violently as they gain energy. Thus, heat is developed in the conductor. Greater the current , greater will be the rate of collision and so greater will be the heat produced.

The four applications using heating effect of current are as follows:

  1. room heater

  2. (ii) electric bulb

  3. (iii) electric iron

  4. (iv) electric fuse

Question 35:

Find out the following in the electric circuit given in Figure:

Image Electric circuit

Image Electric Circuit

Image Electric circuit

(a) Effective resistance of two resistors in the combination

(b) Current flowing through resistor

(c) Potential difference across resistance

(d) Power dissipated in resistor

(e) Difference in ammeter readings, if any.

Answer:

Since, to resisror are in parallel , then their effective resistance is given by

Total resistance in the circuit:

Current through the circuit,

Thus, current through 4 Ω resistor is 1A as 4 Ω and are in series and same current flow through them.

Potential difference across 4 Ω resistor is potential drop by the 4 Ω resistors.

i.e.

Power dissipate in 4 Ω resistor,

There is no difference in the reading of ammeters as same current flows through all elements in a series current.