Physics Questions and Answers

Answer: A

Both Statement I and Statement II are correct.

Magnetite (Fe₃O₄) is the iron ore that occurs naturally magnetised as lodestone. It was the first natural magnet known and was used in early compasses.

A freely suspended bar magnet turns to lie along the Earth's magnetic field, with its north pole pointing roughly towards geographic north. That is the basis of the magnetic compass. Strictly it points to magnetic north, which differs slightly from true north, but the textbook statement is accepted.

Answer: A

Both Statement I and Statement II are correct.

Inertia is a body's resistance to a change in its state of rest or motion, and mass is the measure of that resistance: the larger the mass, the larger the force needed for a given acceleration (F = ma).

Friction acts along the surfaces in contact and always opposes the relative motion, or the tendency to relative motion, between them. That is why a sliding block slows down. In the school-level form of the statement both are true, which rules out the options marking either one wrong.

Answer: A

The audible range is 20 Hz to 20 kHz.

A healthy young human ear hears sound frequencies from about 20 hertz to 20,000 hertz. The upper limit falls with age, often to around 15 kHz or lower in older adults.

Frequencies below 20 Hz are infrasound, which elephants and whales use. Frequencies above 20 kHz are ultrasound, heard by dogs and bats and used in medical imaging. So the other ranges lie outside human hearing.

Answer: C

The correct statements are (3) and (4) only.

A concave lens is diverging, so whatever the position of the object it forms a virtual, erect and diminished image. White light is a mixture of seven colours, the VIBGYOR spectrum seen when it passes through a prism. Both statements are correct.

Statement 1 is wrong because a concave mirror forms a virtual, magnified image when the object is between the pole and the focus, as in a shaving mirror. Statement 2 is wrong because a real image is formed by rays that actually meet, so it can be caught on a screen.

Answer: B

Rear-view mirrors in vehicles are convex.

A convex mirror always forms an upright, smaller (diminished) virtual image, so it shows a much wider field of view than a plane mirror of the same size. That lets the driver see more of the traffic behind, which is why such mirrors carry the warning that objects are closer than they appear.

Concave mirrors magnify nearby objects and are used for shaving mirrors and headlight reflectors. Plane mirrors give a true-size view but a narrow field.

Answer: B

The weight of a body would decrease.

On the rotating Earth, part of gravity is used to supply the centripetal force that keeps a body moving in its circle, so the apparent weight is

\[ g' = g - \omega^{2} R \cos^{2}\lambda \]

where \(\omega\) is the angular speed and \(\lambda\) the latitude. A faster rotation means a larger \(\omega\), so \(g'\) and the weight fall everywhere except at the poles, where \(\cos\lambda = 0\). At the equator, rotating about 17 times faster would make objects weightless. So weight neither increases nor stays the same in general.

Answer: A

The Higgs boson is popularly called the God Particle.

The nickname comes from the title of Leon Lederman's 1993 popular book The God Particle. The particle itself, predicted by Peter Higgs and others in 1964, is the quantum of the Higgs field that gives elementary particles their mass. Its discovery at CERN's Large Hadron Collider was announced in 2012.

The electron is a lepton, and the proton and neutron are composite particles made of quarks. None of them is the Higgs boson. Physicists generally dislike the nickname, but it is the name the question is after.

Answer: A

An LED works on electroluminescence.

In a light emitting diode, a forward voltage drives electrons across a p-n junction. When they recombine with holes they fall to a lower energy level and give out the difference as light. Light produced directly by an electric current in a material in this way is electroluminescence.

A laser relies on stimulated emission and an optical cavity, which an ordinary LED does not have. Thermionic emission is the release of electrons from a hot filament, as in a vacuum tube. The photoelectric effect is the reverse process, light releasing electrons, and is the principle of a photodiode or solar cell.

Answer: A

The one that is not an electromagnetic wave is Supersonic waves.

'Supersonic' describes motion faster than sound, and the waves meant are sound waves, which are mechanical waves. They need a material medium to travel through and cannot cross a vacuum.

Gamma rays, X-rays and ultraviolet rays are all parts of the electromagnetic spectrum. They are oscillating electric and magnetic fields that travel through a vacuum at the speed of light, differing only in wavelength: gamma rays shortest, then X-rays, then ultraviolet.

Answer: C

The outer rail is raised to obtain the necessary centripetal force, as the key's option puts it, from the horizontal component of the force on the train.

A train going round a curve needs a centripetal force directed towards the centre of the curve. Raising the outer rail (superelevation or banking) tilts the track, so the reaction force of the rails on the wheels leans inward, and its horizontal component supplies that centripetal force. Strictly it is the normal reaction, not the weight itself, that is tilted, but the purpose is the one the option states.

This keeps the flanges of the wheels from pressing hard on the outer rail and reduces wear. Centrifugal force is not something the track needs to supply, friction is not avoided by banking, and banking is designed to allow a speed, not to reduce it.