Physics 8 Lesson 23: Convection- Heat radiation
1. Theoretical summary
b) Answer the question:
– C1: Does the purple water move in a stream from the bottom up and then from the top or does it move chaotically in all directions?
The purple water moves in a stream from the bottom up and then from the top.
– C2: Why does the bottom layer of hot water go to the top while the upper layer of cold water goes down?
The bottom layer of water heats up first, expands, its specific gravity is less than the specific gravity of the cold water layer above, so the hot water layer rises and the cold water layer sinks to form a convection current.
– C3: How do you know that the water in the cup is hot?
Thanks to the thermometer.
The transfer of heat by the formation of currents as in the above experiment is called convection.
The convection also occurs in a gas.
– Observe the experiment and describe the phenomenon that occurs when we light candles and incense.
– Smoke helps us to observe the phenomenon of air convection more clearly.
Phenomenon occurs: see incense smoke also moves into a stream.
Explanation: The lower layer of air is heated lighter than the upward movement, the cold air layer above is heavier than the downward movement. Just like that, a convection current is formed.
– C5: Why do you want to heat liquids and gases from below?
To heat liquids and gases, they must be heated from the bottom so that the lower part heats up first (d decreases) up, the unheated upper part goes down to form a convection current.
– C6: Does convection occur in vacuum and in solids?
In a vacuum and in solids, convection does not occur because in a vacuum, in solids, convection currents cannot be created.
Conclusion: Convection is the transfer of heat by streams of liquids or gases.
1.2. Heat radiation
b) Answer the question:
– What does the drop of colored water moving towards the end of B show?
– Predict what happens to colored water drops when we take a piece of wood between the heat source and the sphere?
– Result: The colored water drop tends to move back to the A end.
– C8: The drop of colored water moving back to end A shows what? What is the use of a piece of wood?
The air in the cooler has cooled, causing the colored water droplet to move to end A, the piece of wood has prevented heat from transferring from the heat source to the flask. Show that heat is transferred from the heat source to the vessel in a straight line.
Thermal radiation is the transfer of heat by direct rays of heat. Thermal radiation can occur even in a vacuum.
The rougher the surface and the darker the color, the more heat rays are absorbed.
The smoother the surface and the brighter the color, the less heat rays are absorbed.
2. Illustrated exercise
Question 1: Why do we often wear white clothes in the summer and not black ones?
Because light-colored objects absorb less heat, wearing white clothes in the summer will reduce the ability to absorb heat rays, making us feel cooler.
Verse 2: Is heat transfer from the heat source to the vessel conduction and convection? Why?
- Heat transfer from the heat source to the vessel is not conduction because air is a poor conductor of heat. Nor is it convection because heat is transferred in a straight line.
- In the experiment on heat was transferred by direct heat rays, This form of heat transfer is called thermal radiation.
- Thermal radiation can occur even in a vacuum.
3.1. Essay exercises
Question 1: Why does convection not occur in solids?
Verse 2: What is thermal radiation?
Question 3: How does solar energy reach the Earth?
Question 4: Standing near a fire, we feel hot. How is heat transferred from a flame to a person?
3.2. Multiple choice exercises
Question 1: Choose the wrong answer:
A. When an object absorbs incoming heat radiation, the object’s temperature will increase.
B. Thermal radiation is the transfer of heat by emitting direct rays of heat.
C. Objects that are too cold cannot radiate heat.
D. Thermal radiation can occur in a vacuum.
Verse 2: A test tube is filled with water, at what position of the tube should the tube be heated to make all the water in the tube boil faster?
A. Burn in the middle of the tube.
B. Burning at the mouth of the tube.
C. Burn at the bottom of the tube.
D. Can burn in any position
Question 3: Which of the following objects absorbs heat well?
A. The object has a smooth, dark surface.
B. The object has a rough, light-colored surface.
C. The object has a smooth, light-colored surface.
D. The object has a rough, dark surface.
Question 4: Select the wrong comment:
A. In the phenomenon of convection, there is a mechanical phenomenon: the hot water layer rises, the cold water layer sinks.
B. In convection, heat is transferred from an object with a higher temperature to an object with a lower temperature.
C. In convection, there is thermal expansion.
D. Convection occurs when two solids of different temperatures come into contact.
Through this lecture, students will be acquainted with the knowledge related to Convection – Heat radiation along with related exercises in many levels from easy to difficult…, they need to understand:
Identify convection currents in liquids and gases.
Find real-life examples of thermal radiation.
Name the main forms of heat transfer in solids, liquids, gases, and vacuum.