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प्रश्न
The line that draws power supply to your house from street has ______.
- zero average current.
- 220 V average voltage.
- voltage and current out of phase by 90°.
- voltage and current possibly differing in phase `phi` such that `|phi| < pi/2`.
विकल्प
b and c
a and d
b and d
a and c
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उत्तर
a and d
Explanation:
Alternating currents are used for household supplies, which are having zero average value over a cycle.
The line is having some resistance, so power factor `cos phi = R/Z ≠ 0`
So, `phi = pi/2 ⇒ phi < pi/2`
i.e., phase lies between 0 and `pi/2`.
Important point: The average value of alternating quantity for one complete cycle is zero.
The average value of ac over half cycle (t = 0 to T/2)
`I_(av) = (int_0^(T/2) i dt)/(int_0^(T/2) dt) = (2i_0)/pi = 0.637i_0` = 63.7% of i0
Similarly `V_(av) = (2V_0)/pi` = 0.637 V0 = 63.7% of V0.
APPEARS IN
संबंधित प्रश्न
State the principle on which transformer works.
Derive an expression for ratio of e.m.f.s and currents in terms of number of turns in primary and secondary coil.
Express the turn ratio in terms of voltages.
Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.
Name the transformer used in the power transmitting station of a power plant.
The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.
1) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.
2) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.
3) Write two values each shown by the teachers and Geeta.
How is the e.m.f. across primary and secondary coils of a transformer related with the number of turns of the coil in them?
The following diagram in Fig.10.42 shows a coil of several turns of copper wire connected to a sensitive centre zero galvanometer G near a magnet NS. The coil is free to move in the direction shown in the diagram.
(i) Describe the observation if the coil is rapidly moved.
(ii) How would the observation be altered if (a) the coil has twice as many turns (b the coil was made to move three times as fast?
Mention the two characteristic properties of the material suitable for making core of a transformer.
Describe briefly and two energy losses, giving the reasons for their occurrence in actual transformers ?
What is the turns ratio i.e., transformer ratio, ns: np, in an ideal transformer which in-creases ac voltage from 220 V to 33000 V?
Applying e.m. f to primary coil is 210 V. If the number of turns in primary coil is 200 turns and that of in secondary coil is 20 turns, then find out the output voltage. Name the type of transformer.
A transformer lowers e.m.f. from 220 V to 15 V. If 400 W power is given in primary, calculate (i) the current in primary coil and (ii) the current in secondary coil.
Describe the construction and working of a transformer with a neat labelled diagram.
A power transmission line feeds input power at 2300 V to a stepdown transformer with its primary windings having 4000 turns. What should be the number of turns in the secondary in order to get output power at 230 V?
Read the following paragraph and answer the question:
Long distance power transmissions
The large-scale transmission and distribution of electrical energy over long distances is done with the use of transformers. The voltage output of the generator is stepped up. It is then transmitted over long distances to an area sub-station near the consumers. There the voltage is stepped down. It is further stepped down at distributing sub-stations and utility poles before a power supply of 240 V reaches our homes.
If the secondary coil has a greater number of turns than the primary, ______.
Name one electrical device which works on Faraday's law of electromagnetic induction principle.
A transformer consisting of 300 turns in the primary and 150 turns in the secondary gives output power of 2.2 kW. If the current in the secondary coil is 10 A, then the input voltage and current in the primary coil are ______.
For what purpose are the transformers used?
