Which one of the following scientists mentioned that an electron in an atom could revolve in certain stable orbits without the emission of radiant energy ?
- A.Ernest Rutherford
- B.J.J. Thomson
- C.Niels Bohr
- D.Albert Einstein
Correct answer
Explanation
The correct answer is C, Niels Bohr. In his 1913 model of the atom, Bohr proposed that electrons move in certain fixed orbits, called stationary states, in which they do not radiate energy.
Rutherford's nuclear model had a serious flaw: by classical physics, an electron circling the nucleus should keep losing energy and spiral into it. Bohr solved this by saying that only orbits with particular energies are allowed, the angular momentum of the electron being a whole multiple of h/2π. An electron gives out or absorbs energy only when it jumps from one orbit to another, and the energy of the light equals the difference between the two levels. This explained the line spectrum of hydrogen, and Bohr received the Nobel Prize in Physics in 1922.
A is wrong because Rutherford discovered the nucleus through his gold foil experiment but could not explain the atom's stability. B is wrong because J. J. Thomson discovered the electron and gave the 'plum pudding' model. D is wrong because Einstein explained the photoelectric effect, not electron orbits.
Exam tip: Thomson, electron and plum pudding model; Rutherford, nucleus; Bohr, stationary orbits; Chadwick, neutron.
Practice Questions
View allWhich of the following statements about electromagnetic waves, sound waves and water waves is/are correct ? 1. They exhibit reflection 2. They carry energy 3. They exert pressure 4. They can travel in vacuum Select the correct answer using the code given below :
- A.1,2 and 3
- B.2 and 4
- C.1 and 3 only
- D.1 only
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All three kinds of wave reflect, carry energy and exert pressure, but only electromagnetic waves can travel through a vacuum.
Read the full explanation →Which one of the following is NOT the unit of energy?
- A.Joule
- B.Watt-hr
- C.Newton-metre
- D.kg-metre/sec²
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A kilogram-metre per second squared is mass times acceleration, which is force; it is the newton, not a unit of energy.
Read the full explanation →To protect steel and iron from rusting, a thin layer of which one of the following metals is applied?
- A.Magnesium
- B.Zinc
- C.Aluminium
- D.Lead
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Coating iron or steel with a thin layer of zinc is called galvanisation, and it is the most common way to protect them from rusting.
Read the full explanation →In an electric circuit, a wire of resistance 10 Ω is used. If this wire is stretched to a length double of its original value, the current in the circuit would become :
- A.half of its original value.
- B.double of its original value.
- C.one-fourth of its original value.
- D.four times of its original value.
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Stretching the wire to twice its length makes its resistance four times larger, so at the same voltage the current falls to a quarter.
Read the full explanation →In a solenoid, the current flowing through the wire is I and number of turns per unit length is n. This gives a magnetic field B inside the solenoid. If number of turns per unit length is increased to 2n, what will be the value of magnetic field in the solenoid ?
- A.B
- B.2B
- C.B/2
- D.B/4
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The field inside a long solenoid is directly proportional to the number of turns per unit length, so doubling n doubles the field.
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