Metals are good conductors of electricity because
Answer: B
Because metals contain free electrons.
The valence electrons in a metal are delocalised over the whole lattice rather than bound to individual atoms, and an applied field sets them drifting. The same electron sea carries heat, which is why good electrical conductors are also good thermal ones.
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Railway tracks are banked on curves
Answer: C
So that the necessary centripetal force is obtained from the horizontal component of the normal reaction.
Banking tilts the reaction inward: \[\tan\theta = \frac{v^2}{rg}\] The rails then do not have to supply the force by pressing sideways on the flanges, which would cause wear and risk derailment.
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The fact that two large ships travelling on close parallel courses in the same direction tend to move towards each other can be explained by
Answer: A
This is explained by Bernoulli's principle.
Water forced through the narrow gap between the hulls speeds up, and where the speed rises the pressure falls: \[P + \tfrac{1}{2}\rho v^2 + \rho gh = \text{constant}\] The pressure on the outer sides then exceeds that between them, pushing the ships together. The same effect makes a passing express train tug at a person on the platform.
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If a red-green flag is seen in green light it appears to be of which of the following colour?
Answer: D
It appears black and green.
A coloured surface shows the colour it reflects. In green light the green portion of the flag reflects green and looks green; the red portion has no red light to reflect and absorbs the green, so it looks black.
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During thunderstorm you are travelling in a car. To protect yourself from the lightning you will have to
Answer: A
You should remain in the car.
The metal body acts as a Faraday cage: a lightning strike travels over the outer shell to the ground, and the field inside a closed conductor is zero, so the occupants are unharmed. Standing under a tree is the most dangerous choice, since a tall isolated object attracts the strike.
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Rainbow is due to
Answer: D
A rainbow is due to refraction and reflection of sunlight by water droplets.
Light entering a raindrop is refracted and dispersed into colours, reflected once off the inside of the far surface, then refracted again on leaving. It emerges at about 42° to the incoming ray, which fixes the radius of the bow.
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Choke is used to_______.
Answer: C
A choke is used to reduce the current in an AC circuit.
It is an inductor, whose reactance \(X_L = 2\pi f L\) opposes alternating current. Because the reactance is 90° out of phase with the current, it limits the current while dissipating almost no power - unlike a resistor, which would waste energy as heat.
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One thousand microns is equal to
Answer: A
One thousand microns is \(10^{-3}\) m.
\[1000 \times 10^{-6}\ \mathrm{m} = 10^{-3}\ \mathrm{m}\] which is one millimetre. The micron, now called the micrometre, is \(10^{-6}\) m.
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The SI unit of luminous intensity is
Answer: C
The SI unit of luminous intensity is the candela.
It is one of the seven base units. The lumen measures luminous flux and the lux illuminance - one lux is one lumen per square metre - while the watt measures radiant power without regard to the eye's sensitivity.
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The minimum number of Non zero non collinear vectors required to produce a zero vector is
Answer: A
Three non-zero, non-collinear vectors are needed.
Two non-collinear vectors can never cancel, since their sum has a component along each. Three can, if they close a triangle head to tail - which requires them to be coplanar.
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