QUESTION 17
In order for the aluminum bar to have its rate of conduction increased, which of the following changes should be made?
A. Increase its length
B. Increase its thickness
C. Decrease its thermal conductivity
D. Decrease the temperature gradient
QUESTION 18
In an experiment, Tom records the resistance value of a 20cm strip of metal that is kept at four (4) different temperatures. His results are recorded below.
Temperature of metal strip Resistance of metal strip
0°C 0.100
25°C 0.400
50°C 0.700
75°C 1.00
What is the average change in resistance of the metal for each 10°C rise in temperature?
A. 0.0120
B. 0.0130
C. 0.0140
D. 0.0160
QUESTION 19
John has four (4) shirts of different colors. On a hot sunny day, he tries to decide which shirt he will wear. Each of the colors of the shirts is given below.
Which colored shirt will reflect most of the heat radiation?
A. Black
B. White
C. Yellow
D. Red
QUESTION 20
In which part of a refrigerator will the refrigerant change from a low temperature, low pressure gas to a high temperature, high pressure gas?
A. Evaporator
B. Compressor
C. Expansion valve
D. Condenser
Answer (1)
Question 17: Increasing the thickness of the aluminium bar increases its rate of conduction.
Question 18: Calculate the average change in resistance per 10°C: 75 0.900 × 10 = 0.12 .
Question 19: White colored shirt reflects most of the heat radiation.
Question 20: The refrigerant changes from a low temperature, low pressure gas to a high temperature, high pressure gas in the compressor.
Explanation
Problem Analysis Let's analyze each question step by step.
Question 17 Solution Question 17: The rate of heat conduction through a material is given by the formula:
Q / t = k A ( d T / d x ) ,
where:
Q / t is the rate of heat conduction,
k is the thermal conductivity,
A is the cross-sectional area (related to thickness),
d T / d x is the temperature gradient,
L is the length.
To increase the rate of conduction, we need to:
Increase the thermal conductivity ( k ),
Increase the cross-sectional area (thickness) ( A ),
Increase the temperature gradient ( d T / d x ),
Decrease the length ( L ).
Based on the options, increasing the thickness is the correct choice.
Question 18 Solution Question 18: To find the average change in resistance for each 10°C rise in temperature, we use the data provided.
The resistance values are:
0°C: 0.100
25°C: 0.400
50°C: 0.700
75°C: 1.00
Calculate the total change in resistance: R f ina l − R ini t ia l = 1.00 − 0.100 = 0.900 .
Calculate the total change in temperature: T f ina l − T ini t ia l = 75 − 0 = 75° C .
Calculate the average change in resistance per degree Celsius: T o t a l C han g e in T e m p er a t u re T o t a l C han g e in R es i s t an ce = 75 0.900 .
Calculate the average change in resistance for each 10°C rise in temperature: A v er a g e C han g e p er 10° C = 75 0.900 × 10 = 0.12 .
So, the average change in resistance for each 10°C rise in temperature is 0.12.
Question 19 Solution Question 19: On a hot sunny day, the shirt color that reflects the most heat radiation is white. White surfaces reflect most of the light and heat, while dark surfaces absorb more heat.
Question 20 Solution Question 20: In a refrigerator, the refrigerant changes from a low temperature, low pressure gas to a high temperature, high pressure gas in the compressor. The compressor increases both the temperature and pressure of the refrigerant.
Final Answers Final Answers:
Question 17: B. Increase its thickness
Question 18: A. 0.120
Question 19: B. White
Question 20: B. Compressor
Examples
Understanding heat transfer and thermal properties is crucial in many real-world applications. For example, engineers use these principles to design efficient cooling systems for electronics, develop insulation materials for buildings, and optimize heat exchangers in power plants. Knowing how different materials respond to temperature changes and how they conduct or reflect heat helps in creating more energy-efficient and comfortable environments.
