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प्रश्न
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.
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उत्तर
1) The transformer is a device used for changing the alternating voltage to a higher or lower value. Formation of eddy currents in the iron core of the transformer is one of the causes of power dissipation in this device.
2) In the long-distance transmission of energy through transmission lines, the power loss is mainly due to resistive loss. Now, this resistive loss is proportional to the square of current flowing through the lines i.e
`H prop I^2`
So, to minimise this loss, we need to minimise the current flowing in the wire. In case of dc, if we decrease the current flowing in the wire, though the power losses will decrease but simultaneous power received at the receiving station will also decrease. So, to achieve the required amount of power at the receiving station, we transmit the ac signal by stepping up the voltage with the lower value of current as ac gives the flexibility of stepping up or down the voltage. In this way, the power losses are reduced while maintaining the required amount of power at the receiving station.
3)
Two values shown by the teachers are:
- Believer of giving practical knowledge
- Good explainer
Two values shown by Geeta are:
- Curiosity of learning
- Good listener
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संबंधित प्रश्न
Derive an expression for ratio of e.m.f.s and currents in terms of number of turns in primary and secondary coil.
Draw a labeled diagram of a step-down transformer.
A power transmission line feeds input power at 2200 V to a step-down transformer with its primary windings having 300 turns. Find the number of turns in the secondary to get the power output at 220 V.
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 power supply to the primary coil of a transformer is 200 W. Find
(i) Current in primary coil if the e.m.f. supply to it is equal to 220V.
(ii) The number of turns in the primary coil is equal to 80 and that in secondary is 800. What is the transformation ratio?
(iii) Name the type of transformer.
(iv) What will be the output voltage?
(v) What is the current in the secondary coil for an ideal transformer?
(vi) What is the output power?
(vii) Is output and input power equal?
(viii) Compare the current flowing in a secondary coil and in a primary coil.
A transformer having efficiency of 80% is working on 200 V and 6 kW power supply. If the current in the secondary coil is 6 A, the voltage across the secondary coil and the current in the primary coil respectively are ____________.
Two coils P and Q are kept near each other. When no current flows through coil P and current increase in coil Q at the rate 10A/s, the emf in coil P is 15mV. When coil Q carries no current and current of 1. 8A flows through coil P, the magnetic flux linked with the coil Q is ______.
An iron rod is placed parallel to magnetic field of intensity 2000 Am-1. The magnetic flux through the rod is 6 × 10−4 Wb and its cross-sectional area is 3 cm2. The magnetic permeability of the rod in Wb A-1m-1 is ______.
The self-inductance of a closely wound coil of 200 turns is 10 mH. Determine the value of magnetic flux through the cross-section of the coil when the current passing through the coil is 4 mA.
How do the input and output powers in a transformer compare? State the assumption made.
