Thermal transfer

Question 1

What happens to most materials when they are heated?

Answer 1

They expand in all directions

Question 2

What occurs when a material cools?

Answer 2

It contracts and gets smaller in all directions

Question 3

What factors influence thermal expansion?

Answer 3

• Amount of temperature change
• The material

Question 4

What are the three types of thermal expansion?

Answer 4

• Linear expansion
• Area expansion
• Volume expansion

Question 5

Why do railway lines need gaps between rails?

Answer 5

To prevent buckling as they expand in hot weather

Question 6

What happens to overhead electrical cables in summer?

Answer 6

They hang low and could break in winter if not allowed to contract

Question 7

How are bridges designed to accommodate thermal expansion?

Answer 7

One end is free and able to move

Question 8

What is the coefficient of linear expansion?

Answer 8

The increase in length of a material when its temperature is raised by one degree

Question 9

What symbol represents the coefficient of linear expansion?

Answer 9

The Greek letter α

Question 10

What are the units for the coefficient of linear expansion?

Answer 10

K−1

Question 11

What is the relationship between Kelvin and Celsius temperature changes?

Answer 11

A change of 25 K is the same as a change of 25 °C

Question 12

What is the coefficient of linear expansion of brass?

Answer 12

18×10−6 K−1

Question 13

If a 1-metre-long brass bar is heated by 1 K, how much does it expand?

Answer 13

0.000018 m

Question 14

How much will a 5 m length of brass expand if its temperature increases by 30 K?

Answer 14

0.0027 m or 2.7 mm

Question 15

What is the coefficient of linear expansion for steel?

Answer 15

15×10−6 K−1

Question 16

What is the coefficient of linear expansion for nylon?

Answer 16

100×10−6 K−1

Question 17

What is the formula for linear thermal expansion?

Answer 17

Δl = l1 α (T2 − T1)

Question 18

What is the formula to calculate the new length after thermal expansion?

Answer 18

l2 = l1 [1 + α (T2 − T1)]

Question 19

Calculate the length of a 30 m steel pipe at 300 °C, given α = 16×10−6 K−1.

Answer 19

30.133 m

Question 20

What is the formula for area expansion?

Answer 20

ΔA = αA ΔT

Question 21

What is the formula for volume expansion?

Answer 21

ΔV = αV ΔT

Question 22

What does the heat transfer coefficient (h) represent?

Answer 22

kJ h−1 m−2 °C−1

Question 23

What is the heat transfer equation?

Answer 23

Q = h × A × ΔT

Question 24

Find the heat transfer rate for ΔT = 30 °C, A = 1.2 m², h = 45 kJ h−1 m−2 °C−1.

Answer 24

1620 kJ h−1 or 1.62 MJ h−1