Thermal stress Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. The rate at which the physical dimensions of an object change according to changes in temperature is known as

1. Thermal expansion
2. Object contraction
3. Temperature expansion
4. None of the above

Ans. The correct answer is a. Thermal expansion

Explanation: Thermal expansion is the tendency of matter to change its shape, area, volume, and density in accordance with temperature changes.

Ques. Cracks on the object are formed due to

1. Thermal degree
2. Thermal increase
3. Thermal shock
4. Thermal constant

Ans. The correct answer is c. Thermal shock

Explanation: Thermal shock is the process through which a component suffers large-scale changes in thermal stresses and strains when the heat flux and component temperature gradient change quickly. Thermal shock causes cracks as a result of rapid temperature changes in components.

Ques. ______ is the instrument used to maintain a constant temperature.

1. Thermistor
2. Resistor
3. Rheostat
4. Thermostat

Ans. The correct answer is d. Thermostat

Explanation: A thermostat is a device that detects temperature changes in order to keep the temperature of an enclosed area approximately constant.

Ques. which one of the following are true for the thermal expansion coefficient?

1. α_copper > α_steel > α_aluminium > α_brass
2. α_steel > α_aluminium > α_brass > α_copper
3. α_brass > α_aluminium > α_copper > α_steel
4. α_aluminium > α_brass> α_copper > α_steel

Ans. The correct answer is d. α_aluminium > α_brass> α_copper > α_steel

Explanation: Aluminium has the highest thermal expansion coefficient, followed by brass and copper. Steel has the lowest thermal expansion coefficient of any of them.

Ques. What will be the effects of stress due to temperature change?

1. The body elongates if temperature decreases
2. The body elongates if the temperature rises
3. Body shortens of temperature rises
4. No change

Ans. The correct answer is b. The body elongates if the temperature rises

Explanation: Thermal stress is developed in a body When there is a change in temperature of the body, but if the body is free to expand or contract no thermal stress develops. Hence if the temperature of a body rises, the body elongates.

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Short Answers Questions [2 Marks Questions]

Ques. What is thermal stress?

Ans. Thermal stress is mechanical stress caused by a change in a material's temperature. Depending on the other heating factors, such as material types and constraints, these stresses might cause fracture or plastic deformation. Thermal stress can be caused by temperature gradients, thermal expansion or contraction, and thermal shocks.

Ques. What is the formula to calculate thermal stress?

Ans. Thermal stress can be calculated by the formula

Thermal stress = – YαΔT = – Yα(T₂ – T₁)

Where

• Y is the Young’s modulus of the material
• α is the coefficient of linear expansion of the material
• ΔT is the change in temperature

Ques. What is thermal shock?

Ans. Thermal shock is defined as an abrupt change in temperature that causes a transient mechanical load on an object. The load is created by the temperature change causing differential expansion of different parts of the object.

Ques. A thick glass tumbler often cracks when boiling water is poured into it. Why?

Ans. When you pour hot water into a thick tumbler, the inner surface expands. However, due to the low heat conductivity of glass, the expansion of the tumbler's exterior surface is relatively small. The tumbler breaks due to unequal expansion of the outer and inner surfaces.

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Long Answers Questions [3 Marks Questions]

Ques. What are the applications of thermal stress?

Ans. The practical applications of thermal stress are

• Thermostat: The most well-known use of thermal stress is a thermostat. A thermostat is a device used to regulate/maintain a steady temperature. "Thermo" means temperature and "stat" means static. They are commonly utilized in the manufacture of a wide range of electronic equipment, from air conditioners to iron boxes.
• Thermometers: Mercury thermometers measure temperature using the property of thermal expansion. The volume expansion or contraction with the change in temperature of the mercury is used to measure the temperature.

Ques. What is a bimetallic strip? Mention any two applications of it.

Ans. A bimetallic strip is used to convert temperature variation into mechanical displacement. It is made up of two strips of different metals that expand at different rates when heated. A bimetallic strip is often made of steel and copper, or in cases of steel and brass.

The following are the applications of the bimetallic strip

• Thermistor
• Heat engine
• Clocks
• Heaters
• Iron box

Ques. What are the different types of thermal stresses?

Ans. The different types of thermal stresses are

• Internal thermal stress: Internal thermal stress arises when the temperature within an object suddenly rises or falls.
• External thermal stress: External thermal stress occurs when there is a temperature difference between the surface of the material and its surroundings.

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Very Long Answers Questions [5 Marks Questions]

Ques. An aluminium cylinder 10 cm long, with a cross-sectional area of 20 cm² is to be used as a spacer between two walls. At 17.2 ℃ it just slips in between the walls. When it warms to 22.3 ℃, calculate the stress in the cylinder and the total force it exerts on each wall, assuming that the walls are perfectly rigid and a constant distance apart. (For aluminium Y = 7.0 x 10¹⁰ Pa and α = 2.4 x 10^-5 K^-1)

Ans. Given

• Initial temperature, T₁ = 17.2 ℃
• Final temperature, T₂ = 22.3 ℃
• Area of cross-section of the aluminium cylinder, A = 20 cm² = 20 x 10^-4 m²
• Young’s modulus for aluminium, Y = 7.0 x 10¹⁰ Pa
• Coefficient of linear expansion of aluminium, α = 2.4 x 10^-5 K^-1

Thermal stress developed in the aluminium cylinder due to change in temperature is given by

Thermal stress = – YαΔT = – Yα(T₂ – T₁)

On substituting the values, we get

Thermal stress = - 7.0 x 10¹⁰ x 2.4 x 10^-5 (22.3 – 17.2)

⇒ Thermal stress = – 8.6 x 10⁶ Pa

The total force exerted on the walls by the cylinder is given by

F = Thermal stress x Cross-sectional area of the aluminium cylinder (A)

⇒ F = – 8.6 x 10⁶ x 20 x 10^-4

⇒ F = – 1.7 x 10⁴ N

Ques. What will be the pressure that has to be applied to the ends of a steel wire of length 5 cm to keep its length constant when its temperature is raised by 50 K? (For steel Young’s modulus is 2 x 10¹¹ N m^-2 and coefficient of thermal expansion is 1.1 x 10^-5 K^-1)

Ans. Given

• Increase in temperature, ΔT = 50 K
• Length of the steel wire, l = 5 cm = 5 x 10^-2 m
• Young’s modulus for steel, Y = 2 x 10¹¹ N m^-2
• Coefficient of linear expansion of steel, α = 1.1 x 10^-5 K^-1

Thermal stress developed in the steel wire due to change in temperature is given by

Thermal stress = – YαΔT

On substituting the values, we get

Thermal stress = - 2 x 10¹¹ x 1.1 x 10^-5 x 50

⇒ Thermal stress = - 1.1 x 10⁸ N/m²

The pressure that has to be applied to the ends of a steel wire to keep its length is equal to the magnitude of thermal stress developed in the steel wire. i.e.

P = 1.1 x 10⁸ N/m²

Ques. What are the thermal properties of a material?

Ans. The following are the thermal properties of a material

• Specific Heat: The amount of heat required to increase the temperature of one gram of a material by one degree Celsius is referred to as specific heat.
• Heat capacity: The quantity of heat required to change the temperature of a substance by one degree is known as its heat capacity.
• Thermal conductivity: Thermal conductivity refers to a material's ability to conduct/transfer heat.
• Melting point: The melting point of a material is the temperature at which it changes from solid to liquid. The melting point of pure metal is constant, but the melting point of alloys varies.
• Thermal diffusivity: Thermal diffusivity is defined as the ratio of thermal conductivity to heat capacity.
• Thermal shock resistance: Thermal shock resistance is a solid's ability to withstand sudden temperature fluctuations, either during heating or cooling.

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