Heat Transfer | Jamb(UTME)

Jamb(utme) key points on conductivities of common materials; the thermos flask and vacuum flask; land and sea breeze

Conductivities of Common Materials

1. Conductivity refers to a material's ability to conduct heat or electricity.
2. Materials are classified as conductors (good at conducting) or insulators (poor at conducting).
3. Metals like copper, silver, and aluminum are excellent thermal and electrical conductors.
4. Copper is commonly used in electrical wiring due to its high electrical conductivity.
5. Silver has the highest electrical conductivity but is expensive, so it’s used selectively.
6. Aluminum is lightweight and a good conductor, often used in overhead power lines.
7. Non-metals like wood, rubber, and plastic are poor conductors, making them good insulators.
8. Air is a poor conductor but a good insulator, often used in double-pane windows.
9. Water is a poor conductor of heat but can conduct electricity if it contains dissolved salts.
10. Materials like wool and fiberglass trap air, making them excellent thermal insulators.
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The Thermos Flask and Vacuum Flask

11. A thermos flask (or vacuum flask) is designed to minimize heat transfer and keep liquids hot or cold.
12. It consists of a double-walled container with a vacuum between the walls.
13. The vacuum reduces heat transfer by conduction and convection.
14. The inner walls are usually coated with a reflective material, like silver, to reduce heat loss by radiation.
15. The stopper or lid is made of an insulating material to prevent heat transfer through the opening.
16. The flask's design ensures minimal heat exchange with the environment.
17. A hot liquid in a thermos stays hot because heat cannot escape easily.
18. Similarly, a cold drink remains cold because external heat cannot enter.
19. Thermos flasks are widely used for carrying beverages, storing liquid nitrogen, and in laboratory settings.
20. Their effectiveness depends on maintaining the vacuum seal and the insulating materials.
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Land and Sea Breeze

21. Land and sea breezes are caused by differences in temperature between land and sea.
22. During the day, the land heats up faster than the sea because land has a lower specific heat capacity.
23. The warmer air over land rises, creating a low-pressure area.
24. Cooler air from the sea flows in to replace the rising warm air, creating a sea breeze.
25. Sea breezes are common during sunny days and provide a cooling effect.
26. At night, the sea retains heat longer than the land because water has a higher specific heat capacity.
27. The cooler air over the land flows toward the sea to replace the rising warm air, creating a land breeze.
28. Land and sea breezes are examples of natural convection currents in the atmosphere.
29. These breezes help regulate coastal temperatures, keeping them moderate.
30. Fishermen and sailors often rely on these breezes for navigation.
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Combustion Engine

31. A combustion engine is a machine that converts chemical energy from fuel into mechanical energy.
32. It operates by burning fuel (e.g., gasoline or diesel) in a combustion chamber.
33. There are two main types of combustion engines: internal combustion engines (ICE) and external combustion engines.
34. Internal combustion engines burn fuel inside the engine, as in cars, motorcycles, and airplanes.
35. External combustion engines burn fuel outside the engine, as in steam engines.
36. The most common internal combustion engines are four-stroke engines.
37. The four strokes are intake, compression, power, and exhaust.
38. In the intake stroke, the air-fuel mixture enters the combustion chamber.
39. During the compression stroke, the piston compresses the mixture, increasing pressure.
40. In the power stroke, the mixture is ignited, causing an explosion that drives the piston down.
41. During the exhaust stroke, waste gases are expelled from the chamber.
42. Combustion engines require fuel, air, and a spark to operate efficiently.
43. They are used in vehicles, generators, ships, and aircraft for their ability to produce power on demand.
44. Proper maintenance, like regular oil changes and spark plug replacements, ensures engine efficiency.
45. Combustion engines contribute to air pollution, emitting carbon dioxide and nitrogen oxides.
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Applications and Practical Observations

46. High-conductivity metals are essential in electrical grids and electronic devices.
47. Insulating materials like fiberglass and rubber are used in homes and appliances for safety and energy efficiency.
48. Thermos flasks are used in daily life for beverages and in industries for storing temperature-sensitive materials.
49. Land and sea breezes are crucial for coastal weather patterns, fishing activities, and tourism.
50. Combustion engines power most modern transportation but are gradually being replaced by electric motors to reduce environmental impact.
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Jamb(utme) key points on conduction, convection and radiation as modes of heat transfer; temperature gradient, thermal conductivity and heat flux

Conduction as a Mode of Heat Transfer

1. Conduction is the transfer of heat through a material without the material itself moving.
2. It occurs when faster-moving particles transfer energy to slower-moving particles by collision.
3. Solids are the best conductors because their particles are closely packed.
4. Metals like copper and aluminum are excellent conductors of heat due to free-moving electrons.
5. Poor conductors, such as wood and plastic, are called insulators.
6. Conduction requires direct contact between the heat source and the material.
7. An example of conduction is a metal spoon heating up when placed in a hot liquid.
8. Heat transfer by conduction is slow in insulating materials like foam or glass.
9. The rate of conduction depends on the material’s thermal conductivity.
10. Conduction only happens efficiently in solids; it’s slower in liquids and negligible in gases.
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Convection as a Mode of Heat Transfer

11. Convection is the transfer of heat by the movement of fluids (liquids or gases).
12. Warm fluid rises because it becomes less dense, while cooler, denser fluid sinks, creating a current.
13. This circulation is called a convection current.
14. Convection occurs naturally in boiling water and atmospheric wind patterns.
15. Forced convection happens when an external force, like a fan or pump, moves the fluid.
16. Examples include air conditioning, cooling fans, and heating systems.
17. Convection explains why water in a pot heats evenly when boiling.
18. It also plays a key role in weather systems, ocean currents, and mantle convection beneath Earth’s crust.
19. The speed of convection increases with larger temperature differences.
20. Convection is impossible in solids because their particles cannot move freely.
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Radiation as a Mode of Heat Transfer

21. Radiation is the transfer of heat through electromagnetic waves without needing a medium.
22. Heat from the Sun reaches Earth via radiation through the vacuum of space.
23. Infrared radiation is the primary form of heat transfer in radiation.
24. All objects emit radiation depending on their temperature.
25. Dark, rough surfaces are better at absorbing and emitting radiation than light, smooth surfaces.
26. Radiation doesn’t require contact; it can happen even across empty space.
27. Examples include feeling the warmth of a campfire or heat from a radiator.
28. The amount of energy radiated increases with the temperature of the object.
29. Stefan-Boltzmann’s law quantifies radiation as proportional to the fourth power of the object’s temperature.
30. Radiation is the dominant mode of heat transfer in space, where conduction and convection are absent.
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Temperature Gradient

31. A temperature gradient is the rate of temperature change over a distance within a material.
32. It is represented mathematically as:
    
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    $Temperature Gradient = \frac{\Delta T}{\Delta x}$
33. A higher temperature gradient results in faster heat transfer.
34. The direction of heat transfer is always from higher temperature to lower temperature.
35. Temperature gradients are steeper near intense heat sources like flames.
36. In solids, a steep gradient leads to rapid conduction.
37. Temperature gradients are used to design heat-efficient systems in industries and buildings.
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Thermal Conductivity

38. Thermal conductivity measures a material’s ability to conduct heat.
39. High thermal conductivity materials, like metals, transfer heat quickly.
40. Low thermal conductivity materials, like wood or foam, act as insulators.
41. The formula for conduction involving thermal conductivity is:
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    $Q = kA \frac{\Delta T}{\Delta x}$ where:
    • $Q$ = Heat transferred,
    • $k$ = Thermal conductivity,
    • $A$ = Area,
    • $\frac{\Delta T}{\Delta x}$ = Temperature gradient.
42. Thermal conductivity is measured in $W/m·K$ (watts per meter per Kelvin).
43. Materials with low thermal conductivity are used in clothing, building insulation, and refrigerators.
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Heat Flux

44. Heat flux is the amount of heat transferred per unit area over a period of time.
45. It is given by:
    
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    $q = \frac{Q}{A}$ where $q$ = Heat flux, $Q$ = Heat energy, $A$ = Area.
46. Heat flux is measured in $W/m^2$ (watts per square meter).
47. It indicates the efficiency of heat transfer in a given system.
48. Heat flux depends on the temperature gradient, material properties, and the surface area.
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Effect of Surface Nature on Energy Radiated and Absorbed

49. The ability of a surface to radiate or absorb heat depends on its color, texture, and material.
50. Dark, rough surfaces are excellent absorbers and emitters of heat radiation.
51. Light, shiny surfaces reflect most radiation and are poor absorbers and emitters.
52. Emissivity is a measure of how effectively a surface emits radiation, ranging from 0 (perfect reflector) to 1 (perfect emitter).
53. A surface with high emissivity radiates more heat energy at a given temperature.
54. Solar panels are designed with dark surfaces to maximize heat absorption.
55. Aluminum foil is used for thermal insulation because it reflects heat well.
56. Surface properties are crucial in designing efficient radiators, heaters, and cooling systems.
57. Stefan-Boltzmann’s law explains the radiative power of surfaces:
    
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    $P = \sigma A T^4$ where $P$ = Power radiated, $\sigma$ = Stefan-Boltzmann constant, $A$ = Area, $T$ = Temperature.
58. The color of cars affects how much heat they absorb—dark-colored cars heat up faster than light-colored ones.
59. Surface coatings and finishes are chosen to enhance or minimize heat transfer in industrial applications.
60. Understanding surface radiation is critical for energy efficiency in buildings, solar panels, and thermal management systems.

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