After using for some time, big transformers get heated up. This is due to the fact that 1. current produces heat in the transformers 2. hysteresis loss occurs in the transformers 3. liquid used for cooling gets heated Select the correct answer using the code given below.
- A.1 only
- B.2 and 3 only
- C.1 and 2 only
- D.1, 2 and 3
Correct answer
Explanation
The correct answer is C, 1 and 2 only. A transformer heats up because of energy lost inside it: heat from current in the windings and hysteresis loss in the iron core.
When current flows through the copper windings, their resistance produces heat in proportion to I²R; this is called copper loss. The iron core is magnetised and demagnetised again and again by the alternating current, and the energy spent in each cycle appears as heat; this is hysteresis loss. Eddy currents induced in the core add further heat, which is why cores are made of thin laminated sheets of soft iron or silicon steel. Big transformers are filled with oil to carry this heat away.
A is incomplete because it leaves out hysteresis loss. B and D include statement 3, which reverses cause and effect: the cooling oil gets heated because the transformer is hot, it is not the reason the transformer heats up.
Exam tip: transformer losses are copper (I²R), hysteresis and eddy current; laminated soft-iron cores cut the last two.
Practice Questions
View allWith reference to polyethylene terephthalate, the use of which is so widespread in our daily lives, consider the following statements :
1. Its fibres can be blended with wool and cotton fibres to reinforce their properties.
2. Containers made of it can be used to store any alcoholic beverage.
3. Bottles made of it can be recycled into other products.
4. Articles made of it can be easily disposed of by incineration without causing greenhouse gas emissions.
Which of the statements given above are correct?
- A.1 and 3
- B.2 and 4
- C.1 and 4
- D.2 and 3
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Objects in space : Description
1. Cepheids : Giant clouds of dust and gas in space
2. Nebulae : Stars which brighten and dim periodically
3. Pulsars : Neutron stars that are formed when massive stars run out of fuel and collapse
How many of the above pairs are correctly matched?
- A.Only one
- B.Only two
- C.All three
- D.None
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- A.Observation and understanding of the Universe
- B.Study of the solar and the lunar eclipses
- C.Placing satellites in the orbit of the Earth
- D.Origin and evolution of living organisms on the Earth
Show answer
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- A.F is maximum when v and B are parallel to each other.
- B.F is maximum when v and B are anti-parallel to each other.
- C.F is maximum when v and B are perpendicular to each other.
- D.The force F is independent of the angle between v and B.
Show answer
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Read the full explanation →In Sun-Earth system, the Sun, the Earth and the Lagrange point L4 form :
- A.an isosceles triangle
- B.an equilateral triangle
- C.a straight line
- D.a scalene triangle
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L4 lies on Earth's orbit 60° ahead of the Earth, so it is as far from the Sun as from the Earth.
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