Which part of the flame is the hottest?
Answer: A
The blue zone is the hottest part of the flame.
There the air supply is plentiful and combustion is complete, so the temperature reaches about 1,500 °C. The luminous yellow region is cooler, its light coming from glowing unburnt carbon, and the innermost dark zone is unburnt gas and coldest of all.
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The wavelength of a wave is the distance between
Answer: D
The wavelength is the distance between crest and crest, or trough and trough.
It is the length of one complete cycle, so any two corresponding points in phase are a wavelength apart. Crest to the adjacent trough is only half a wavelength.
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What is the minimum velocity with which a body of mass m must enter a vertical loop of radius R at the bottom so that it can complete the loop?
Answer: D
The minimum entry speed is \(\sqrt{5gR}\).
At the top of the loop, gravity must at least supply the centripetal force: \[mg = \frac{mv_{top}^2}{R} \Rightarrow v_{top}^2 = gR\]
Applying conservation of energy between bottom and top, a height \(2R\) apart: \[\tfrac{1}{2}mv^2 = \tfrac{1}{2}mv_{top}^2 + mg(2R)\]
\[v^2 = gR + 4gR = 5gR \Rightarrow v = \sqrt{5gR}\]
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The amount of water vapour in the air is called
Answer: B
The amount of water vapour in the air is called humidity.
Absolute humidity is the mass of vapour per unit volume; relative humidity, the figure quoted in weather reports, is the ratio of the actual vapour pressure to the saturation value at that temperature. Dew forms when air is cooled until its relative humidity reaches 100 per cent.
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The maximum velocity upto which fluid motion is streamline is called
Answer: B
This is the critical velocity.
Above it the flow turns turbulent. It follows from the Reynolds number: \[v_c = \frac{R_e\,\eta}{\rho D}\] Flow is laminar below about \(R_e = 2000\) and turbulent above about 3000.
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Materials for rain-proof coats and tents owe their water-proof properties to
Answer: A
Waterproofing depends on surface tension.
The fabric is treated so that water does not wet it: the contact angle becomes obtuse, cohesion within the drop exceeds its adhesion to the fibre, and surface tension pulls the drop into a bead that rolls off instead of soaking through the pores.
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Electronegativity is a measure of
Answer: B
Electronegativity is a measure of non-metallic character.
It is the tendency of an atom to attract a shared pair of electrons towards itself, and it rises across a period and falls down a group - exactly the trend of non-metallic character. Metals are electropositive, giving electrons up rather than attracting them.
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The velocity required to place a vehicle in orbit round the moon is _______ that required to place it in orbit round the earth
Answer: A
The velocity needed is less than that for an earth orbit.
Orbital velocity is \[v = \sqrt{\frac{GM}{R}}\] and the moon's mass is about 1/81 of the earth's while its radius is about 1/3.7, so \(v\) works out at roughly \(1.7\ \mathrm{km\,s^{-1}}\) against \(7.9\) for the earth - a little over a fifth.
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When one enters a dark room after being in bright light one is not able to see anything clearly for a while because
Answer: C
The iris cannot dilate the pupil quickly enough.
In bright light the pupil is constricted to a small aperture; on entering darkness the iris muscles need time to open it, and the retinal pigment rhodopsin, bleached by the bright light, takes several minutes to regenerate. Full dark adaptation takes about twenty to thirty minutes.
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In the electrical circuit of a house, the fuse acts as
Answer: C
The fuse is a safety device.
It is a short length of wire of low melting point in series with the circuit. If the current exceeds the rated value, the heat \(I^2Rt\) melts the wire and breaks the circuit before the wiring can overheat and start a fire.
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