Alternating Current Physics - Alternating Current

Symbol of Inductance in electric circuit is-

With increase in frequency of an A.C. supply, the inductive reactance.

The line that draws power supply to your house from street has:

For two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false and R is also false.

Assertion: An alternating current shows magnetic effect.
Reason: Magnitude of alternating current varies with time.

For two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false and R is also false.

Assertion: Long distance transmission of A.C. is carried out at extremely high voltage.
Reason: For large distance, voltage has to be large.

At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. If she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work?

Name the logic gates marked P and Q in the given logic circuit.

Define the term relaxation time' in a conductor.

Two wires one of copper and other of manganin have same resistance and equal length. Which wire is thicker and why?

A small town with a demand of 800kW of electric power at 220V is situated 15km away from an electric plant generating power at 440V. The resistance of the two wire line carrying power is 0.5Ω per km. The town gets power from the line through a 4000-220V step-down transformer at a sub-station in the town.

1. Estimate the line power loss in the form of heat.
2. How much power must the plant supply, assuming there is negligible power loss due to leakage?
3. Characterise the step up transformer at the plant.

A light bulb and an open coil inductor are connected to an ac source through a key as shown in Fig. 7.9.

The switch is closed and after sometime, an iron rod is inserted into the interior of the inductor. The glow of the light bulb (a) increases; (b) decreases; (c) is unchanged, as the iron rod is inserted. Give your answer with reasons.

A lamp is connected in series with a capacitor. Predict your observations for dc and ac connections. What happens in each case if the capacitance of the capacitor is reduced?

A capacitor'C', a variable resistor 'R' and a bulb 'B' are connected in series to the ac mains in circuit as shown. The bulb glows with some brightness. How will the glow of the bulb change if (i) a dielectric slab is introduced between the plates of the capacitor, keeping resistance R to be the same; (ii) the resistance R is increased keeping the same capacitance?

The household supply of electricity is at 220V (rms value) and 50Hz. Find the peak voltage and the least possible time in which the voltage can change from the rms value to zero.

(a) For circuits used for transporting electric power, a low power factor implies large power loss in transmission. Explain.
(b) Power factor can often be improved by the use of a capacitor of appropriate capacitance in the circuit. Explain.

During a thunderstorm the ‘live' wire of the transmission line fell down on the ground. A group of boys passing through noticed it and some of them wanted to place the wire by the side. As they were approaching the wire and trying to lift it, Bari noticed it and immediately pushed them away to prevent them from touching the wire. Two of them got hunt in the process. Bari took them to a doctor to get medical aid.
Based on the above paragraph, answer the following:

1. Write two values which Bari displayed during the incident.
2. Why is it that a bird can sit over a suspended ‘live' wire without any harm whereas touching it on the ground can give a fatal shock?
3. The electric power from a power plant is set up to a very high voltage before transmitting it to distant consumers. Write the reason for it.

Hari is a student of Class X in a school near his village. His uncle gifted him a bicycle with a dynamo fitted in it. He was thrilled to find that while cycling during night, he could light the bulb and see the objects on the road clearly. He, however, did not know how this device works. He asked this question to his teacher. The teacher considered it an opportunity and explained the working of a dynamo to the whole class.
Answer the following questions:

1. State the principle and working of a dynamo.
2. Write two values each displayed by Hari and his Science teacher.

Shiv had a high tension tower erected on his farm land. He kept complaining to the authorities to remove it since it occupied a large portion of his land. His uncle, who was a teacher, explained to him the need for erecting these towers for efficient transmission of power. As Shiv got convinced and realised significance, he stopped complaining.
Based on the above paragraph, answer the following questions:

1. Why is it necessary to transport power at high voltages?
2. ‘A low power factor implies large power loss'. Explain.
3. Write the two values displayed by Shiv and his Uncle.