Test: Physics - 1 - UPSC MCQ

Because the specific gravity of the needle is greater than that of water displaced by it. Specific gravity (or relative density) = Density of the substance / Density of the water at 4°C The average density of iron needle is greater than water, therefore, it sinks.

The cloudy nights are warmer because clouds prevent escape of radiation of heat from the ground and the air.

At that pressure, water boils at 121°C (250°F). That means food can cook at a much higher temperature than it ever could at atmospheric pressure—and since cooking reactions speed up at higher temperatures, your food cooks faster.

The white clothes reflect heat falling on them in summer and they absorb the least from the sun.

The dimmer bulb has larger resistance than the brighter one. Power of the bulb ∝ 1 / Resistance Resistance is the obstruction offer to the flow of current.

In a fluorescent tube, electrical energy is almost converted into light. The inner wall of the fluorescent tube is coated with phosphorus which immediately transformed into bright light.

In respect of a jet engine and a rocket, both the given statements are correct.

Because the vertical line through the centre of gravity of the tower falls within the base.

Correct option is A)

Electronic motors draw more current which is inversely proportional to the voltage, therefore, more heat
H=I²Rt is generated

If the doors of a refrigerator are left open for a few hours, the room temperature will increases. It is because system releases heat to the surroundings.

If a rock is brought from the surface of the moon to the earth, its weight will change but not its mass. Mass is a invariant physical quantity whereas weight of a body (w = mg) is variable as the value of acceleration due to gravity (g) changes.

Wavelength is the distance between two consecutive crests or troughs and 1 angstrom = 10–¹⁰m, joule is the unit of energy. Intensity of sound is measured in decibel. Frequency is measured in hertz (Hz).

The unit of temperature is kelvin, power is measured in watt, pressure in pascal and force in newton.

Cosmic rays have very high frequency >10²³ Hz and wavelength <10^–24 m.

Pendulum clocks become slow in summer because the length of the pendulum increases. Increase in length ∆l = l_o α ∆T where l_o = original length, α = coefficient of linear expansion and ∆T = change in temperature.

A piece of ice added to the drink cool it because ice takes latent heat from the drink for melting resulting in the cooling of the drink.

Revolver was discovered by Colt. Dynamite was discovered by Alfred Nobel. Law of cooling was discovered by Newton and law of pressure by Pascal.

We don’t feel the atmospheric pressure because the pressure inside our bodies is approximately equal to the atmospheric pressure outside. This balance of forces prevents us from feeling the immense pressure exerted by the atmosphere.

Hypermetropia is corrected by using a convex lens.

Myopia is corrected by concave lens.

Assertion (A): If ice accumulates on the freezer coils, the cooling efficiency of a refrigerator decreases.
True. Ice acts as an insulating layer, reducing heat transfer from the refrigerator compartment to the cooling coils, so cooling is affected.

Reason (R): Ice is a poor conductor of heat.
True. Ice has low thermal conductivity compared to metal, which is why it slows down the transfer of heat.

Relation between A and R:The poor thermal conductivity of ice directly explains why ice buildup reduces cooling efficiency.

Therefore, Correct answer is Option (a)  Both A and R are true, and R is the correct explanation of A.