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Q:

The base of an electric iron is brightly polished mainly

A) to make it smooth and frictionless B) to make it rust-proof
C) to reduce heat loss by radiation D) to make it more durable
 
Answer & Explanation Answer: C) to reduce heat loss by radiation

Explanation:

Polished surfaces are poor radiators of heat. So loss of heat on that account is reduced and the electric iron remains hotter than otherwise.

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Q:

Radio waves sent out by a radio stations are reflected by

A) troposphere B) stratosphere
C) mesosphere D) ionosphere
 
Answer & Explanation Answer: D) ionosphere

Explanation:

This is a layer of atmosphere about 60 to 70 km above the earth and extends to about 400km. It contains ionised particles which can reflect radio waves making radio communication round the world possible.

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Q:

Skating on ice is possible because 

A) the surface of ice is smooth B) ice is cold
C) the pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water D) the pressure on the ice due to skate raises its melting point
 
Answer & Explanation Answer: C) the pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water

Explanation:

Here is a case of  lowering of melting point of ice with increase of pressure

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Q:

Tap water is placed in an open pad and allowed to evaporate. After some time the temperature of water 

A) increases to a great extent B) increases slightly
C) decreases slightly D) decreases rapidly
 
Answer & Explanation Answer: C) decreases slightly

Explanation:

Evaporation causing cooling.Here the evaporation is less. Therefore cooling is less.

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Q:

In a freezer ice cubes will be formed more quickly in trays made of 

A) rubber B) plastic
C) aluminium D) wood
 
Answer & Explanation Answer: C) aluminium

Explanation:

Aluminium being a good conductor of heat conducts away the heat from the substance quite easily and causes quick freezing.

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Q:

The cover of a solar coker is made of glass because 

A) it absorbs sun's rays B) it reflects sun's rays
C) it retains the heat inside by the green house effect D) it rediates heat better than metals
 
Answer & Explanation Answer: C) it retains the heat inside by the green house effect

Explanation:

As the inside of the cooker is painted black, heat absorption is more producing heat waves of longer wavelength which cannot escape through glass and hence are trapped inside. Thus the food is cooked.

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Q:

A stone floor feels cold to the bare feet but a carpet on the same floor feels warm. This is because 

A) the floor and carpet are at different temperatures B) the stone conveys the heat away from the feet more rapidly than the carpet
C) the carpet is a better conductor of heat than the stone D) the human body maintains a constant temperature
 
Answer & Explanation Answer: B) the stone conveys the heat away from the feet more rapidly than the carpet

Explanation:

Both the floor and the carpet have the same temperature as both are exposed to the same atmosphere. But the former conducts away the heat from the feet better than the carpet as the latter is a non-conductor. So the feet losing more heat, the floor appears cooler to the feet.

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Q:

A cup of hot coffee is placed on a metal table in a room. By which of the following methods does it lose heat?

A) conduction and convection only B) conduction and radiation only
C) convection and radiation only D) conduction, convection and radiation
 
Answer & Explanation Answer: D) conduction, convection and radiation

Explanation:

Part of the heat is conducted away by the metal table. Convection currents in air near the surface of the coffee are set up which accounts for some heat loss. Heat is also lost by radiation from the hot coffee to the surroundings. Most of the heat is lost by the last method.

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