What does thermal efficiency in heat transfer refer to?
A Rate of heat transfer
B Material conductivity
C Amount of heat lost
D Percentage of heat used effectively
Thermal efficiency measures the effectiveness of a system in converting heat energy into useful work. It is defined as the ratio of the useful heat energy transferred to the total heat energy supplied to the system.
What does the Stefan-Boltzmann law describe?
A Thermal expansion
B Rate of radiation from a surface
C Conduction of heat
D Rate of convection
The Stefan-Boltzmann law describes the total energy radiated by a blackbody. It is proportional to the fourth power of its absolute temperature. This law is essential in calculating radiant heat transfer from surfaces.
How does temperature affect the Stefan-Boltzmann law’s radiation rate?
A No effect
B Proportional to the fourth power
C Proportional to square of temperature
D Inversely proportional
The Stefan-Boltzmann law states that the power radiated by a blackbody is proportional to the fourth power of its temperature. A small increase in temperature leads to a large increase in radiative heat transfer.
What does thermal expansion refer to?
A Increase in temperature
B Decrease in density
C Increase in volume due to temperature rise
D Increase in thermal conductivity
Thermal expansion occurs when a material’s volume increases as its temperature rises. This is due to the increased movement of molecules, which push them apart as the material heats up, causing expansion.
How does increasing the temperature of a material affect its thermal conductivity?
A Increases thermal conductivity
B Decreases thermal conductivity
C No effect
D Depends on material
For most materials, thermal conductivity increases with temperature, but in some cases (especially in non metals), it may decrease. The relationship between temperature and thermal conductivity is material dependent.
What happens to the rate of heat transfer as the thermal resistance of a material increases?
A Increases heat transfer
B Decreases heat transfer
C No change
D Reduces temperature difference
Thermal resistance opposes heat flow. As the thermal resistance of a material increases, the rate of heat transfer decreases because it becomes harder for heat to pass through the material.
In which of the following scenarios is heat transfer most likely governed by the Stefan-Boltzmann law?
A Cooling of a liquid
B Heat transfer in metals
C Radiation from a hot object
D Heat conduction in solids
The Stefan-Boltzmann law governs the heat transfer by radiation, particularly from hot objects. It is crucial in calculating the amount of thermal radiation emitted by an object based on its temperature.
What role does thermal efficiency play in the performance of a heat engine?
A Determines energy loss
B Measures the proportion of energy used effectively
C Determines the power output
D Affects heat capacity
Thermal efficiency in heat engines measures how effectively the engine converts heat energy into useful work. A higher thermal efficiency means more of the supplied heat is converted into work, reducing energy waste.
How does an increase in temperature affect the coefficient of thermal expansion?
A Increases expansion
B Affects density
C No effect
D Reduces expansion
As the temperature increases, most materials expand due to the increased kinetic energy of their molecules. The coefficient of thermal expansion quantifies the degree to which a material expands with a temperature increase.
What type of material would be best for minimizing thermal expansion in an application?
A Metal
B Insulator
C Material with low coefficient of expansion
D High conductivity material
Materials with a low coefficient of thermal expansion expand less when heated. These materials are ideal for applications where dimensional stability is crucial, such as in precision instruments and high temperature environments.
What does the law of thermal radiation state?
A All bodies radiate heat
B Only metals radiate heat
C Heat can’t be radiated
D Radiation occurs in the absence of matter
The law of thermal radiation states that all bodies emit radiation based on their temperature, even if they are not hot enough to glow visibly. The amount of radiation emitted is dependent on the temperature of the body.
What happens to the radiated energy from a body as its temperature increases?
A Decreases
B Remains constant
C Increases significantly
D Changes in irregular pattern
According to the Stefan-Boltzmann law, the radiated energy from a body increases significantly as its temperature increases. The rate of radiation is proportional to the fourth power of the temperature, so even small temperature increases result in large increases in energy emission.
How does insulation affect the thermal efficiency of a system?
A Increases heat transfer
B Reduces heat transfer
C No effect
D Affects material conductivity
Insulation reduces the rate of heat transfer by increasing the material’s thermal resistance. This leads to better thermal efficiency by reducing unwanted heat loss or gain, thereby conserving energy within the system.
What does the thermal expansion of a material depend on?
A Color
B Density
C Temperature change
D Volume
Thermal expansion is directly proportional to the temperature change. As a material is heated, its molecules move faster and tend to push apart, causing the material to expand. The amount of expansion depends on the temperature difference.
How does a higher thermal conductivity affect heat transfer in a material?
A Reduces heat transfer
B No effect
C Increases heat transfer
D Decreases temperature gradient
A higher thermal conductivity allows heat to transfer more efficiently through a material. Materials with high thermal conductivity, like metals, allow heat to flow faster, which is crucial in applications where rapid heat transfer is needed.