1. Which element is the best conductor of electricity?
(a) Silver
(b) Copper
(c) Aluminium
(d) Iron
[Chhattisgarh P.C.S. (Pre) 2011, M.P.P.C.S. (Pre) 1990]
Ans. (a) Silver
- Electric conductivity tells us how much electricity a material can carry.
- Silver is the best at conducting electricity, followed by copper, aluminium and iron.
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2. Which one of the following is the best conductor of electricity?
(a) Mica
(b) Copper
(c) Gold
(d) Silver
[U.P.P.C.S. (Mains) 2015]
Ans. (d) Silver
- Silver is the most efficient material for carrying electricity, but copper is what is usually used to make electrical wires.
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3. Assertion (A): Copper rods are generally preferred to iron rods for making lightning conductors.
Reason (R): Copper is a better conductor of electricity than iron and is not easily oxidized under atmospheric conditions.
Code :
(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
[U.P.R.O./A.R.O. (Mains) 2013]
Ans. (a) Both (A) and (R) are true and (R) is the correct explanation of (A).
- Copper rods are chosen as the material for lightning conductors because :
- Copper conducts electricity better than iron.
- Copper does not rust in oxygen and water, but iron does.
- Using a copper rod uses less energy than an iron rod because copper doesn’t conduct heat as well as iron.
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4. The characteristic of a superconductor is –
(a) High permeability
(b) Low permeability
(c) Zero permeability
(d) Infinite permeability
[39th B.P.S.C. (Pre) 1994]
Ans. (c) Zero permeability
- Permeability is a term used in electromagnetism to describe a material’s ability to create a magnetic field inside itself.
- It is symbolized by the Greek letter µ (Mu) and the permeability and resistance of a superconductor is zero.
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5. At which temperature superconductivity can be of tremendous economic interest saving billions of rupees?
(a) At very low temperature
(b) At a temperature when semiconductor becomes a superconductor
(c) at room temperature
(d) At a very high temperature
[U.P.P.C.S. (Pre) 2000]
Ans. (c) at room temperature
- If we could make electricity go through wires without any loss at regular temperatures, we could save a lot of money.
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6. The highest temperature attained by a superconductor is :
(a) 24 K
(b) 133 K
(c) 150K
(d) 300K
[U.P. Lower Sub. (Pre) 2013]
Ans. (*)
- When superconductivity was first found in 1911, it only worked at very cold temperatures close to absolute zero (– 273.15°C).
- But since then, scientists have been able to find materials that work at higher temperatures.
- Recently, they have been studying materials with hydrogen in them under high pressure, and have made some major discoveries.
- In 2019, it was found that a material called lanthanum hydride became a superconductor at around 250-260 degrees K under 170 gigapascals of pressure.
- In 2020, a research paper in Nature magazine reported a superconductor working at 15°C (288 K) that was made from hydrogen, carbon and sulphur and under 270 gigapascals of pressure.
- This is the highest temperature at which any material has ever shown superconductivity.
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7. The newly discovered high-temperature superconductors are –
(a) Metal alloys
(b) Pure rare earth metals
(c) Ceramic oxides
(d) Inorganic polymers
[Uttarakhand P.C.S. (Pre) 2006, U.P.P.C.S. (Pre) 2000]
Ans. (c)
- Ceramic oxides are extremely important when it comes to studying superconductivity.
- Ceramic superconductors contain –
- Mercury (Hg)/Thallium (Tl)
- Barium (Ba)
- Calcium (Ca)
- Copper oxide (CO)
- This was first reported back in 1993 and has a transition temperature of 94 K to 135 K.
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8. The conductivity of a semiconductor at zero Kelvin is–
(a) 105 ohm
(b) 10-1 ohm
(c) 10-5 ohm
(d) zero
[R.A.S./R.T.S. (Pre) 1999-2000]
Ans. (d) zero
- Germanium and Silicon are materials that have properties between conducting electricity and being an insulator.
- At the coldest temperatures, a semiconductor acts like an insulator.
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9. The resistance of a semiconductor on heating :
(a) Remains constant
(b) Decreases
(c) Increases
(d) None of the above
[U.P.P.C.S. (Mains) 2015]
Ans. (b) Decreases
- As temperature increases, so does the conductivity of semiconductor materials.
- This is because the outermost electrons gain energy, allowing them to leave the atom’s shell.
- This means that more carriers are present in the material, leading to an increase in conductivity.
- This shows that semiconductor materials have a negative temperature coefficient, meaning that resistance decreases as the temperature rises.
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10. On heating, the resistance of a semiconductor :
(a) increases
(b) decreases
(c) remains same
(d) first increases and then decreases
(e) None of the above / More than one of the above
[66th B.P.S.C. (Pre) 2020]
Ans. (b) decreases
- When a semiconductor is heated, it has fewer gaps and more electrons, which means its resistance goes down.
- So when you heat a semiconductor, its resistance decreases.
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11. Electric conduction in a semiconductor takes place due to
(a) Electrons only
(b) Holes only
(c) Both electrons and holes
(d) Neither electrons nor holes
[Uttarakhand P.C.S. (Pre) 2016]
Ans. (c) Both electrons and holes
- When a voltage is applied to a semiconductor, the current is conducted by free electrons and holes.
- These free electrons are caused by thermal energy splitting apart covalent bonds.
- Holes are left behind in the remaining covalent bonds.
- Both the free electrons and holes move under the influence of the electric field, allowing for the current to flow.
- The conductivity of a semiconductor can be improved by adding a small amount of certain elements.
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12. At absolute zero temperature, the electric resistance in a semiconductor is-
(a) Infinite
(b) Meager
(c) High
(d) Zero
[R.A.S./R.T.S. (Pre) 1992]
Ans. (a) Infinite
- At absolute zero temperature, a semiconductor does not allow electricity to pass through it and acts like an insulator.
- Its electric resistance is so high that it is considered to be infinite.
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13. Which one of the following metals is used as a semiconductor in transistors?
(a) Copper
(b) Germanium
(c) Graphite
(d) Silver
[44th B.P.S.C. (Pre) 2000]
Ans. (b) Germanium
- Germanium and silicon are the main materials used in transistors as semiconductors.
- They are not as good conductors as metals, but not as bad as insulators when it comes to electricity at normal temperatures.
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14. The most commonly used material for making transistors is –
(a) Aluminium
(b) Silicon
(c) Copper
(d) Silver
[U.P.P.C.S. (Pre) 2015]
Ans. (b) Silicon
- Transistors are usually made with very clean silicon or germanium, but sometimes they are made with other semiconductor materials.
- Depending on the type of transistor, they may either use one type of charge carrier or two different types.
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15. Which one of the following is an important component of a transistor?
(a) Arsenic
(b) Germanium
(c) Osmium
(d) Radium
[U.P. R.O./A.R.O. (Pre) 2016]
Ans. (b) Germanium
- Germanium is a very important part of transistors, which are devices that were used in the early days of technology.
- It is a type of metal that is silver-white in color and quite fragile.
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16. Which of the following are semiconductors?
1. Silicon
2. Quartz
3. Ceramics
4. Germanium
Choose the correct answer from the following alternatives:
(a) 1 and 2
(b) 1 and 3
(c) 1 and 4
(d) 3 and 4
[U.P.P.C.S. (Mains) 2005]
Ans. (c) 1 and 4
- Silicon or germanium can both be used as semiconductors in solid state electronics.
- They both have four electrons, but germanium has more free electrons and better conductivity.
- However, silicon is used much more often because it can handle higher temperatures than germanium.
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17. Which of the following is a semiconductor?
(a) Plastic
(b) Aluminium
(c) Wood
(d) Germanium
[U.P.P.C.S. (Pre) (Re. Exam) 2015]
Ans. (d) Germanium
- Semiconductors are substances that are in between being able to conduct electricity and not being able to conduct electricity.
- They can be either natural elements like silicon or germanium, or they can be made from a combination of elements like gallium arsenide or cadmium selenide.
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18. Which of the following elements is a semiconductor?
(a) Aluminium
(b) Silicon
(c) Silver
(d) Lead
[U.P. P.C.S. (Pre) 2018]
Ans. (b) Silicon
- Elements that are used to make semiconductors are made up of one type of atom, like Silicon (Si), Germanium (Ge), and Tin (Sn) from the column IV, and Selenium (Se) and Tellurium (Te) from the column VI on the periodic table.
- The right answer is Silicon.
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19. The Government of India has totally banned the export of burnt coal (Jhama) because it contains recoverable amounts of metal/element that is used in transistors. Which one of the following is that element?
(a) Phosphorus
(b) Germanium
(c) Silicon
(d) Tungsten
[I.A.S. (Pre) 1993]
Ans. (b) Germanium
- Germanium is a type of metal that is brittle, white, and shiny.
- It is usually obtained by melting zinc or from the ashes of burned coal.
- This metal is used in making transistors.
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