Electricity

Exemplar Problems

Long Answer Questions

Part 1

Question 29. Three incandescent bulbs of 100 W 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.

Answer: No the bulb in the two circuits will not glow with the same brightness. Because the resistance of the parallel circuit will be less than the series circuit, consequently electric current through the series circuit will be less than the parallel circuit. Thus, bulbs in the parallel circuit will glow with more brightness.

(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: Break down of any one of the components in a series circuit results in break in the circuit. But it does not happen in case of parallel circuit. So, the bulb will not glow in series circuit, but rest of the bulbs will continue to glow in parallel circuit.

Question 30. State Ohm's law? How can it be verified experimentally? Does it hold good under all conditions? Comment.

Answer: Ohm’s Law states that potential difference across the given conductor is directly proportional to the electric current flowing through it at constant temperature.

In other words, electric current increases with increase in the potential difference or voltage and vice versa.

Let the electric current I is drawn through an electric circuit and potential difference between two ends of the electric circuit is equal to V.

Therefore,according to Ohm's Law V ∝ I

Or, V = IR

Where, R is a constant. This is known as resistance of conductor.

Ohm’s Law holds good in normal conditions but it is not obeyed in some circumstances.

Limitation of Ohm’s Law

This law cannot be applied for unilateral network: The network consisting of unilateral element like, diode, transistor etc, which do not have same voltage current relation for both direction of current.
Ohm’s law also not applicable for non – linear elements: Non – linear elements are those which do not give current exactly proportional to applied vltage, that means resistance value of those element changes for different values of voltage and current. Examples of non – linear elements are thyristors, electric arc etc.

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: Resistivity quantifies the property of a conducting material due to which it opposes the flow of current. Electrical resistivity is also known as resistivity, specific electrical resistance, or volume resistivity.

The the SI unit of resistivity is Ωm.

Take an ammeter, a cell, plug key, nichrome wire, wires of other metals, etc. Complete the cirucuit; as shown in the diagram.
Use nichrome wire of a particular length in the first case. Replace this with a nichrome wire of different length. Repeat the step with a third nichrome wire of different thickness. Finally, use a copper wire instead of nichrome wire.
Now replace the wire by a thicker nichrome wire, of the same length l [marked (3)]. A thicker wire has a larger cross-sectional area. Again note down the current through the circuit.
It is observed that resistance varies when the conducting material is changed. Resistance of the nichrome wire increases when its length is increased. Resistance of nichrome wire decreases when its thickness, i.e. area of cross-section is increased.
It can be said that resistance depends on the material, length and thickness of the conducting wire.

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: Take three resistors R1, R2 and R3, a battery, an ammeter, a voltmeter and some wires. Complete the circuit; as shown in the diagram.

Measure the electric current, using the ammeter as shown in the circuit diagram.
Now, connect the ammeter between R1 and R2 and measure the electric current.
Now, connect the ammeter between R2 and R3 and measure the electric current.
It is observed that the ammeter reading is same in all the three cases. This shows, that the same current flows through every part of a circuit in which resistances are connected in series.