All Free Physics MCQs with Answers

221. A current carrying coil placed in a uniform magnetic field experiences

• A. a net force that accelerates it across the field
• B. a torque that tends to align its plane perpendicular to the field
• C. neither force nor torque
• D. a force along the field lines

Explanation: The forces on opposite sides of the coil are equal and opposite, so they cancel and there is no net translational force, but because they act along different lines they form a couple that rotates the coil. The rotation continues until the plane of the coil is perpendicular to the field, at which point the torque is zero. A motor uses a commutator to reverse the current at that instant so rotation continues.

222. The Earth's magnetic field is approximately

• A. 5 tesla
• B. 50 microtesla
• C. 50 millitesla
• D. zero at the equator

Explanation: At around 50 microtesla the geomagnetic field is weak enough that a compass needle needs to be delicately mounted to respond to it, and strong enough to deflect charged particles from the Sun. The field is horizontal near the magnetic equator and dips steeply near the poles. A laboratory electromagnet is thousands of times stronger.

223. The emission spectrum of hydrogen consists of

• A. a continuous band of all wavelengths
• B. a set of sharp, discrete lines at fixed wavelengths
• C. a single wavelength only
• D. no radiation at all

Explanation: A line spectrum arises because the electron can occupy only certain energy levels, so only certain energy differences, and therefore only certain photon frequencies, are possible. A hot solid gives a continuous spectrum instead, because its atoms interact strongly. Every element has its own line pattern, which is why spectra identify elements in stars.

224. According to Bohr's model, an electron in a hydrogen atom radiates energy when it

• A. moves in a stable orbit
• B. jumps from a higher energy level to a lower one
• C. jumps from a lower energy level to a higher one
• D. is at rest in the nucleus

Explanation: Bohr postulated that an electron in an allowed orbit does not radiate at all, contradicting classical electromagnetism, and that a photon is emitted only when it drops to a lower level, carrying exactly the energy difference. Absorbing a photon does the reverse and lifts the electron to a higher level. This is why emission and absorption spectra of an element show lines at the same wavelengths.

225. The energy of the electron in the ground state of a hydrogen atom is

• A. minus 13.6 eV
• B. plus 13.6 eV
• C. zero
• D. minus 3.4 eV

Explanation: The negative sign means the electron is bound, and 13.6 eV is therefore the energy needed to remove it completely, which is the ionisation energy of hydrogen. The n equals 2 level lies at minus 3.4 eV, which is minus 13.6 divided by 4, since the energy varies as one over n squared. Energy is defined as zero for a free electron at rest.

226. The Balmer series of hydrogen lines lies in the visible region because all its transitions end at

• A. n equals 1
• B. n equals 2
• C. n equals 3
• D. n equals 4

Explanation: Transitions down to the second level involve energy differences that correspond to visible photons, which is why Balmer lines are the ones seen with a spectroscope in the laboratory. The Lyman series ends at n equals 1 and lies in the ultraviolet, since those energy gaps are larger. Paschen and the higher series end at n equals 3 or above and fall in the infrared.

227. The Lyman series of hydrogen appears in which region of the spectrum?

• A. Infrared
• B. Visible
• C. Ultraviolet
• D. Radio

Explanation: Every Lyman transition ends on the ground state, so the energy released is large and the photons are ultraviolet. Larger energy gaps always mean higher frequency and shorter wavelength. Working out which series a line belongs to is simply a matter of identifying the level it ends on.

228. Bohr postulated that the angular momentum of an electron in an allowed orbit is

• A. any value at all
• B. an integral multiple of h divided by 2 pi
• C. always zero
• D. equal to hf

Explanation: Quantising angular momentum in units of h over 2 pi is what restricts the electron to particular orbits and therefore particular energies, and de Broglie later showed this is the condition for a whole number of electron wavelengths to fit round the orbit. Without the postulate, classical physics predicts the electron should spiral into the nucleus. It was an assumption Bohr made because it reproduced the observed spectrum.

229. An absorption spectrum is produced when

• A. a hot gas emits light
• B. white light passes through a cooler gas, which removes photons of its own characteristic energies
• C. light passes through a vacuum
• D. a solid is heated until it glows

Explanation: Atoms in the cooler gas absorb exactly the photon energies that match their own energy level differences, leaving dark lines in the continuous background at those wavelengths. The dark lines therefore fall at the same positions as the bright lines the same gas would emit when hot. Fraunhofer lines in sunlight are produced this way by the Sun's own atmosphere.

230. The ionisation energy of hydrogen is the energy needed to move the electron from

• A. n equals 1 to n equals 2
• B. n equals 2 to n equals 1
• C. n equals 1 to infinity
• D. infinity to n equals 1

Explanation: Ionisation means removing the electron completely, so it must be raised from the ground state to the level where its energy is zero, which is n at infinity, and that requires 13.6 eV. Moving between two bound levels needs less energy and simply excites the atom. The energy released in the reverse process is emitted as a photon.