Master Newton's Law of Cooling: Practice Quizzes for Heat Transfer

Question 1

A metal sphere with a surface area of 0.1 m² is heated to 100°C and then placed in a room at 20°C. The convection coefficient for the sphere is 10 W/m²K. What is the rate of heat transfer by convection from the sphere to the air?

Accepted Answer

80 W

Answer

100 W

Answer

160 W

Question 2

A hot object is placed in a room with a constant temperature. According to Newton's Law of Cooling, the rate of cooling is:

Accepted Answer

Proportional to the temperature difference between the object and the room

Answer

Proportional to the temperature of the object

Answer

Constant

Question 3

A spherical object with a diameter of 0.2 m is placed in a wind tunnel with an air velocity of 10 m/s. Which of the following would increase the convection coefficient?

Accepted Answer

Increasing the air velocity

Answer

Decreasing the temperature difference between the object and the air

Answer

Increasing the diameter of the object

Answer

Decreasing the air velocity

Question 4

A cylindrical object with a surface area of 0.5 m² is placed in a water bath at 15°C. The object is initially at 80°C. The convection coefficient for the object in water is 500 W/m²K. How long will it take for the object to cool down to 30°C?

Accepted Answer

This cannot be determined without knowing the object's thermal properties

Answer

20 minutes

Answer

10 minutes

Question 5

Which of the following is NOT an application of Newton's Law of Cooling?

Accepted Answer

Calculating the efficiency of a heat engine

Answer

Designing cooling systems for electronic devices

Answer

Predicting the cooling time of a hot object

Answer

Estimating the temperature of a body after a certain time

Question 6

A hot object is placed in a vacuum. Which of the following is true about its cooling rate?

Accepted Answer

It will cool down very slowly due to the absence of convection

Answer

It will cool down at the same rate as in air

Answer

It will cool down faster than in air

Question 7

A hot cup of coffee cools down from 90°C to 80°C in 5 minutes. If the surrounding temperature is 20°C, how long will it take to cool down further from 80°C to 70°C?

Accepted Answer

Less than 5 minutes

Answer

Exactly 5 minutes

Answer

It's impossible to determine without more information

Answer

More than 5 minutes

Question 8

A metal sphere is heated to 100°C and then placed in a room at 20°C. The temperature of the sphere decreases exponentially with time. Which of the following correctly describes the relationship between the temperature difference (T - T_∞) and time (t) according to Newton's Law of Cooling?

Accepted Answer

(T - T_∞) is proportional to exp(-kt)

Answer

(T - T_∞) is proportional to 1/t

Answer

(T - T_∞) is proportional to t^2

Answer

(T - T_∞) is proportional to t

Question 9

A convection coefficient (h) is dependent on several factors. Which of the following factors DOES NOT directly influence the convection coefficient?

Accepted Answer

Material properties of the object being cooled

Answer

Fluid properties like viscosity and thermal conductivity

Answer

Geometry of the object being cooled

Answer

Flow conditions like velocity and turbulence

Question 10

Two identical metal blocks are heated to the same temperature and then placed in separate rooms. Room A has a temperature of 10°C, while Room B has a temperature of 20°C. Which block will cool down faster?

Accepted Answer

The block in Room A

Answer

They will cool down at the same rate

Answer

It depends on the material of the blocks

Answer

The block in Room B

Question 11

A large, flat plate is being cooled by air flowing over it. If the air velocity is increased, what will happen to the convection coefficient?

Accepted Answer

It will increase

Answer

It depends on the temperature difference

Answer

It will decrease

Answer

It will remain unchanged

Question 12

A cup of hot chocolate is placed in a refrigerator at 4°C. The hot chocolate cools down from 70°C to 50°C in 10 minutes. What is the approximate time constant for this cooling process?

Accepted Answer

Around 17 minutes

Answer

Around 5 minutes

Answer

It's impossible to determine without more information

Answer

Around 10 minutes

Question 13

Newton's Law of Cooling is primarily used to describe heat transfer through:

Accepted Answer

Convection

Answer

Conduction

Answer

Radiation

Question 14

A metal rod is heated to 50°C and then placed in a room at 25°C. The temperature of the rod is measured at different times. Which of the following graphs best represents the relationship between the rod's temperature and time based on Newton's Law of Cooling?

Accepted Answer

An exponential decay curve

Answer

A parabolic curve

Answer

A sinusoidal wave

Answer

A linear decreasing line

Question 15

Consider two identical objects, one made of steel and the other of wood, both heated to the same temperature and then placed in the same room. Which object will cool down faster?

Accepted Answer

The steel object

Answer

The wood object

Answer

They will cool down at the same rate

Answer

It depends on the size of the objects

Question 16

A hot object is placed in a vacuum chamber. What will happen to its cooling rate compared to cooling in air?

Accepted Answer

It will cool down slower

Answer

It depends on the material of the object

Answer

The cooling rate will remain the same

Answer

It will cool down faster

Question 17

A hot cup of coffee is left to cool in a room. Which of the following statements best describes the cooling process according to Newton's Law of Cooling?

Accepted Answer

The rate of cooling is proportional to the temperature difference between the coffee and the room.

Answer

The rate of cooling is proportional to the temperature of the coffee.

Answer

The coffee cools at a constant rate until it reaches room temperature.

Question 18

What does the convection coefficient (h) represent in Newton's Law of Cooling?

Accepted Answer

A measure of the heat transfer rate between a surface and a fluid.

Answer

The specific heat capacity of the cooling object.

Answer

The thermal conductivity of the cooling fluid.

Question 19

Which of the following factors does NOT significantly affect the convection coefficient (h) in natural convection?

Accepted Answer

The thermal conductivity of the solid object.

Answer

The properties of the cooling fluid (e.g., viscosity, density).

Answer

The geometric configuration of the cooling surface.

Answer

The temperature difference between the surface and the fluid.

Question 20

Two identical objects, one made of aluminum and the other of wood, are heated to the same temperature and allowed to cool in a room. Which object will cool faster, assuming the same convective conditions?

Accepted Answer

The aluminum object will cool faster.

Answer

Both objects will cool at the same rate.

Answer

The cooling rate cannot be determined without knowing the exact temperatures.

Answer

The wooden object will cool faster.

Question 21

A metal sphere is heated and then allowed to cool in still air. Which of the following changes would result in the SLOWEST cooling rate?

Accepted Answer

Increasing the diameter of the sphere and decreasing the air temperature.

Answer

Increasing the diameter of the sphere and increasing the air temperature.

Answer

Decreasing the diameter of the sphere and increasing the air temperature.

Question 22

In forced convection, how does increasing the velocity of the cooling fluid generally affect the convection coefficient (h)?

Accepted Answer

The convection coefficient (h) increases.

Answer

The convection coefficient (h) remains constant.

Answer

The convection coefficient (h) decreases.

Answer

The effect on the convection coefficient (h) is unpredictable.

Question 23

Which of the following units is the SI unit for the convection coefficient (h)?

Accepted Answer

W/(m²·K)

Answer

W/(m·K)

Answer

W/m²

Answer

J/(kg·K)

Question 24

A hot object is placed in a cold environment. According to Newton's Law of Cooling, what happens to the temperature difference between the object and the environment over time?

Accepted Answer

The temperature difference decreases exponentially.

Answer

The temperature difference remains constant.

Answer

The temperature difference decreases linearly.

Question 25

In the context of heat transfer, what is the primary difference between natural convection and forced convection?

Accepted Answer

Natural convection is driven by buoyancy forces, while forced convection is driven by external means like a fan or pump.

Answer

Natural convection occurs in liquids, while forced convection occurs in gases.

Question 26

How can Newton's Law of Cooling be used in practical applications?

Accepted Answer

All of the above

Answer

Estimating the cooling time of electronic components.

Answer

Designing heat exchangers for industrial processes.

Answer

Predicting the temperature change of food during storage.

Question 27

A hot cup of coffee cools down from 90°C to 70°C in 10 minutes in a room at 20°C. What is the approximate time it takes for the coffee to cool down from 70°C to 50°C in the same room?

Accepted Answer

More than 10 minutes

Answer

Less than 10 minutes

Answer

Cannot be determined from the given information

Answer

Exactly 10 minutes

Question 28

Which of the following factors does NOT affect the convection coefficient (h) in Newton's Law of Cooling?

Accepted Answer

Color of the object

Answer

Fluid properties (e.g., viscosity, thermal conductivity)

Answer

Fluid velocity

Question 29

What is the physical meaning of the time constant (τ) in Newton's Law of Cooling equation?

Accepted Answer

Time taken for the temperature difference between the object and the surrounding to decrease by 63.2%

Answer

Time taken for the object to reach the surrounding temperature

Question 30

A metal sphere is heated to 100°C and then placed in a room at 20°C. The sphere cools down to 60°C in 5 minutes. Which of the following statements is TRUE about the cooling process?

Accepted Answer

The rate of cooling is faster at the beginning than at the end.

Answer

The rate of cooling is slower at the beginning than at the end.

Answer

The rate of cooling is constant throughout the process.

Question 31

A hot object is placed in a cold environment. Which of the following scenarios will result in the fastest cooling rate?

Accepted Answer

High convection coefficient and large surface area

Answer

High convection coefficient and small surface area

Answer

Low convection coefficient and small surface area

Answer

Low convection coefficient and large surface area

Question 32

The rate of heat transfer by convection is directly proportional to:

Accepted Answer

The temperature difference between the object and the surrounding fluid

Answer

The volume of the object

Answer

The mass of the object

Question 33

Which of the following scenarios would lead to a higher convection coefficient?

Accepted Answer

A fan blowing air over a hot plate

Answer

A hot plate placed in a vacuum

Answer

A hot plate placed in a still room

Answer

A hot plate submerged in a stagnant fluid

Question 34

Which of the following statements accurately describes Newton's Law of Cooling?

Accepted Answer

The rate of heat loss from an object is proportional to the temperature difference between the object and its surroundings.

Answer

The rate of heat loss from an object is constant regardless of the temperature difference.

Question 35

A cup of hot tea is left on a table to cool down. Which of the following actions would NOT increase the cooling rate?

Accepted Answer

Covering the cup with a lid

Answer

Using a cup with a larger surface area

Answer

Placing the cup near an open window

Answer

Stirring the tea

Question 36

Two identical metal spheres, one at 80°C and the other at 60°C, are placed in a room at 20°C. Which sphere will cool down faster?

Accepted Answer

The sphere at 80°C

Answer

The cooling rate depends on the material of the sphere, not the initial temperature

Answer

The sphere at 60°C

Answer

Both spheres will cool down at the same rate

Question 37

A cup of hot coffee cools from 95°C to 85°C in 5 minutes when placed in a room at 25°C. What is the convection coefficient (h) between the coffee and the air?

Accepted Answer

10.6 W/m²K

Answer

15.9 W/m²K

Answer

5.3 W/m²K

Answer

21.2 W/m²K

Question 38

A cold beverage is taken out of a refrigerator at 5°C and placed in a room at 25°C. The beverage warms up to 15°C in 10 minutes. Determine the time constant (τ) of the beverage.

Accepted Answer

13.86 minutes

Answer

27.73 minutes

Answer

6.93 minutes

Answer

20.8 minutes

Question 39

A metal block with a high thermal conductivity is initially at a temperature of 200°C. It is suddenly immersed in a fluid at 50°C. Which of the following factors would increase the rate of cooling?

Accepted Answer

Increasing the fluid velocity

Answer

Increasing the thermal conductivity of the fluid

Answer

Using a fluid with a lower specific heat capacity

Answer

Decreasing the surface area of the block

Question 40

A thermometer is initially at a temperature of 0°C. It is placed in a water bath at 100°C. After 1 minute, the thermometer reads 63.2°C. What will be the temperature of the thermometer after 3 minutes?

Accepted Answer

86.4°C

Answer

77.6°C

Answer

95.2°C

Question 41

In Newton's Law of Cooling, the temperature difference (ΔT) between an object and its surroundings is:

Accepted Answer

Proportional to the rate of heat transfer.

Answer

Equal to the temperature of the surroundings.

Answer

Inversely proportional to the surface area of the object.

Answer

Independent of the fluid velocity.

Question 42

A hot object with an initial temperature of 100°C is placed in a room at 20°C. The temperature of the object after 10 minutes is 60°C. What is the temperature of the object after 20 minutes?

Accepted Answer

40°C

Answer

20°C

Answer

50°C

Answer

30°C

Question 43

Which of the following fluids has the highest convection coefficient?

Accepted Answer

Water

Answer

Air

Answer

Glycerin

Answer

Oil

Question 44

A heat exchanger is used to cool a fluid from 120°C to 60°C. The fluid passes through a tube with a surface area of 2 m² and a convection coefficient of 100 W/m²K. The heat transfer rate from the fluid to the surroundings is:

Accepted Answer

6 kW

Answer

9 kW

Answer

3 kW

Answer

12 kW

Question 45

In Newton's Law of Cooling, the time constant (τ) represents:

Accepted Answer

The time required for the temperature difference between the object and its surroundings to decrease by 63.2%

Answer

The time required for the object to reach its equilibrium temperature.