Pieces of camphor placed on water move about rapidly. This is because of
Answer: C
Because of surface tension.
Camphor dissolves unevenly around the fragment, lowering the surface tension of the water locally. The stronger pull from the untouched water on the other side drags the piece that way, and as the dissolution shifts, so does the direction - producing the darting motion.
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When a bottle of perfume is opened in one corner of a room the smell spreads soon throughout the room. This is an example of
Answer: D
This is an example of diffusion.
The perfume molecules move randomly and spread from the region of high concentration to regions of low concentration until they are evenly distributed. In a gas the molecules travel at hundreds of metres a second, though collisions make the net spread far slower.
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Formation of shadows can be explained by
Answer: A
Shadows are explained by the rectilinear propagation of light.
Light travels in straight lines in a uniform medium, so an opaque body blocks the rays behind it and leaves a dark region of the same outline. The sharpness depends on the source: a point source gives a crisp umbra, an extended source a fuzzy penumbra as well.
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The core of an electromagnet is made of soft iron because soft iron has
Answer: B
Because soft iron has large susceptibility and small retentivity.
High susceptibility means it magnetises strongly and concentrates the flux; low retentivity means it loses that magnetism almost entirely the moment the current is switched off - exactly what an electromagnet must do. Steel, with high retentivity, is used for permanent magnets.
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Ball bearings are used in cycles, scooters etc to
Answer: B
To reduce the friction between the wheel and the axle.
The balls turn the sliding contact of the axle in its housing into a rolling one, and rolling friction is smaller by a factor of a hundred or more. The point of contact is momentarily at rest, so the surfaces do not scrape.
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If a band is played on the moon the sound will
Answer: D
The sound would not be heard at all.
Sound is a mechanical wave and needs a material medium to carry the compressions and rarefactions. The moon has effectively no atmosphere, so there is nothing to transmit the vibration.
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One astronomical unit is the average distance between
Answer: A
One astronomical unit is the average distance between the earth and the sun.
It is about \(1.496\times10^{11}\) m, or 150 million km, and light covers it in some eight minutes. It is the natural yardstick for distances inside the solar system: Jupiter is 5.2 AU from the sun and Neptune 30.
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Temperature can be expressed as derived quantity in terms of any of the following
Answer: D
Temperature cannot be expressed in terms of any of them.
It is one of the seven base quantities of the SI system, alongside length, mass, time, current, amount of substance and luminous intensity. A base quantity is by definition independent of the others and cannot be derived from them.
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A boat loaded with rocks floats in the middle of a swimming pool. A man in the boat throws the rocks overboard. The level of water in the pool will
Answer: B
The water level will decrease.
In the boat, each rock displaces its own weight of water; sunk on the bottom, it displaces only its own volume. Since the rocks are denser than water, the volume is the smaller of the two, so the total displacement falls and the level drops.
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In a freezer ice cubes will be formed more quickly in trays made of
Answer: C
Ice forms fastest in trays made of aluminium.
Metals conduct heat far better than rubber, plastic or wood, so heat leaves the water through the aluminium tray much more rapidly. Aluminium's thermal conductivity is around \(205\ \mathrm{W\,m^{-1}\,K^{-1}}\) against about 0.2 for plastic.
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