Convection Examples: 44 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Convection.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

Heat transfer through the bulk movement of a fluid (liquid or gas) that carries thermal energy from one place to another.

Hot air rises and cool air sinks — this circulation carries heat through the room.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Convection starts by identifying what is warmer, what is cooler, and what energy or state variable changes.

Common stuck point: Students often know a formula related to convection but skip the recognition step: Am I tracking thermal energy transfer, particle motion, temperature change, or pressure-volume-temperature relationships? That leads to a correct-looking substitution attached to the wrong physical model.

Sense of Study hint: Ask: Am I tracking thermal energy transfer, particle motion, temperature change, or pressure-volume-temperature relationships?

Worked Examples

Example 1

medium

Switching on a fan raises

h

from

5

25 \text{ W/(m}^2\cdot {}^\circ \text{C)}

over a

0.5 \text{ m}^2

surface at

\Delta T = 40°C

. By how much does the convective heat loss increase?

Answer

Δ Q = 400 W

First step

Q_{\text{before}} = 5 \times 0.5 \times 40 = 100 \text{ W}

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Example 2

medium

An attic vent uses convection to push hot air out. Hot attic air at

40°C

rises while cooler outside air at

25°C

enters at the soffits. What property of the hot air drives this motion?

Example 3

hard

Why do desert nights feel colder than expected even when air temperature is moderate?

Example 4

challenge

An object of heat capacity

C = 200 \text{ J/}{}^\circ \text{C}

cools by convection with

hA = 0.5 \text{ W/}{}^\circ \text{C}

20°C

air. Starting at

80°C

, estimate the time to cool to

50°C

using Newton's law of cooling.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

Warm air near a radiator rises and cooler air sinks, circulating around the room. What heat-transfer mode is this?

Example 2

easy

Can convection occur in a solid steel block? Why or why not?

Example 3

easy

Why does warm air rise during convection?

Example 4

easy

In Newton's law of cooling

Q = hA\Delta T

, if the temperature difference

\Delta T

triples, what happens to the convective heat flow?

Example 5

easy

Which two states of matter can transfer heat by convection?

Example 6

easy

A pot of water is heated from the bottom. How does heat reach the top of the water?

Example 7

easy

Is the convective heat transfer coefficient

h

a fixed material property like thermal conductivity?

Example 8

easy

Why does blowing on hot soup cool it faster?

Example 9

medium

A surface loses heat by convection with

h = 25 \text{ W/(m}^2\cdot {}^\circ \text{C)}

, area

2 \text{ m}^2

, and

\Delta T = 40°C

. Find the heat flow.

Example 10

medium

A radiator surface (

A = 1.5 \text{ m}^2

h = 10

) is at 60°C in a 20°C room. Find the convective heat output.

Example 11

medium

Convective heat flow is 800 W. If the airflow doubles

h

and the temperature difference stays the same, find the new heat flow.

Example 12

medium

A cup of coffee loses 50 W by convection at

\Delta T = 50°C

. Find its convective coefficient times area (

hA

Example 13

medium

Why do convection ovens cook food faster than conventional ovens at the same temperature?

Example 14

medium

Sea breezes occur because land heats faster than water during the day. How does convection create the breeze?

Example 15

medium

A heated plate transfers 1200 W by convection over

3 \text{ m}^2

with

h = 20

. Find the temperature difference.

Example 16

challenge

A surface loses heat by convection (

h = 15

A = 2 \text{ m}^2

, surface 80°C) into 20°C air. Each second this energy is removed from 0.5 kg of water (

c = 4200

) behind it. Find the water's cooling rate in °C per second.

Example 17

challenge

Two identical surfaces lose heat by convection. Surface A has still air (

h = 10

); surface B has a fan (

h = 60

). Both have

\Delta T = 30°C

and area

1 \text{ m}^2

. How much more power does B lose?

Example 18

challenge

A house wall loses heat by conduction (1000 W) and the outer surface then loses it by convection (

h = 20

A = 10 \text{ m}^2

). In steady state, find the temperature difference between the outer surface and the outside air.

Example 19

medium

A heater surface (

A = 0.5 \text{ m}^2

h = 30

) is at 70°C in 25°C air. Find the convective heat output.

Example 20

medium

Convective heat loss is 500 W. If the area triples while

h

and

\Delta T

stay fixed, find the new heat loss.

Example 21

easy

A surface has

h=20 \text{ W/(m}^2\cdot {}^\circ \text{C)}

A=1.5 \text{ m}^2

\Delta T=30°C

. Find the convective heat loss.

Example 22

easy

A heater at

80°C

20°C

air has

h=15

A=0.4 \text{ m}^2

. Find

Q

Example 23

easy

Convective heat flow is

200 \text{ W}

. If

\Delta T

doubles, find the new

Q

Example 24

medium

Coffee at

80°C

sits in a

20°C

room with

hA = 0.8 \text{ W/}{}^\circ \text{C}

. Find the initial cooling rate.

Example 25

medium

A wall (

A=10 \text{ m}^2

, surface at

30°C

) loses heat to outside air at

5°C

with

h=12

. Find

Q

Example 26

medium

A bottle's surface at

40°C

loses

25 \text{ W}

20°C

air. The bottle has

A = 0.05 \text{ m}^2

. Find

h

Example 27

medium

0.2 \text{ m}^2

heater is at

60°C

20°C

air with

h=18

. Find

Q

Example 28

medium

Convective heat flow is

750 \text{ W}

h=15

A=2 \text{ m}^2

. Find

\Delta T

Example 29

medium

Wind chill: at

\Delta T = 10°C

, increasing wind raises

h

from

5

30

. Find the factor by which convective heat loss grows.

Example 30

medium

Convective heat loss is

400 \text{ W}

. If

A

doubles and

h

halves while

\Delta T

stays fixed, find the new

Q

Example 31

hard

A radiator at

70°C

20°C

air has

h=10

A=2 \text{ m}^2

. After 1 hour, how much energy did it deliver by convection? Assume steady

\Delta T

Example 32

hard

A surface at

50°C

loses heat by convection (

h=8

A=2 \text{ m}^2

) into

20°C

air. The heat removed must equal the energy lost by

4 \text{ kg}

of oil (

c=2000

) behind the surface. Find the oil's cooling rate.

Example 33

hard

A wall conducts

1500 \text{ W}

to its outer surface, which loses heat by convection (

h=15

A=10 \text{ m}^2

) into

5°C

air. In steady state, find the outer surface temperature.

Example 34

hard

Two surfaces lose heat: A has

h=10

\Delta T=50

A=1 \text{ m}^2

; B has

h=40

\Delta T=20

A=1.5 \text{ m}^2

. Which loses more heat, and by how much?

Example 35

hard

A radiator heats a

40 \text{ m}^3

room of air (

\rho = 1.2

c = 1000

). The radiator delivers

1200 \text{ W}

by convection. Find the maximum rate of room-air temperature rise. Ignore losses.

Example 36

hard

An object cools from

60°C

50°C

in still air at

20°C

with

h_1=5

. With a fan blowing (

h_2=20

), how does the initial cooling rate compare?

Example 37

hard

A heated pipe (

A = 0.3 \text{ m}^2

) at

80°C

sits in air at

20°C

. Free convection gives

h=8

; adding a fan raises it to

h=40

. Find the percentage increase in heat loss.

Example 38

hard

Why does water boil in a pot mostly at the bottom but heat the whole pot quickly?

Example 39

hard

A computer chip dissipates

50 \text{ W}

. Its case (

A = 0.005 \text{ m}^2

) sits in

25°C

air with

h=120

(forced fan). Find the steady case temperature.

Example 40

challenge

On Earth, free convection in a fluid requires gravity to produce buoyancy. In a microgravity space station, what happens to natural convection currents around a heater?

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Related Concepts

Heat Transfer Temperature Conduction Radiation (Heat Transfer)

Background Knowledge

These ideas may be useful before you work through the harder examples.

heat transfertemperature