Physics Lab Quiz - Mechanics & Heat

Experiment M1: Instrumental Errors

(Not in books)

Q1: What is the Least Count of a Screw Gauge? Ans: The least count of the screw gauge is defined as the distance moved by the screw when the cap rotates through one division on the circular scale. It is calculated by dividing the pitch of the screw by the total number of divisions on the circular scale.

Q2: What is the Pitch of the Screw Gauge? Ans: The pitch is the distance between two consecutive screw threads, measured parallel to the axis, or the distance the screw moves forward or backward during one complete rotation of the cap. The pitch of a screw gauge is usually 0.5 mm or 1 millimeter.

Q3: Explain the two types of Instrumental Error for the Slide Calipers. Ans: There are two types of zero errors:

1. Positive Zero Error: The zero of the Vernier scale is in advance of the zero line of the main scale by an amount x mm.

2. Negative Zero Error: The zero of the Vernier scale is behind that of the main scale by an amount x mm.

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4. Experiment E6 (H1): Specific Heat of Liquid by the Method of Cooling

Q1: State Newton’s law of cooling. Ans: The original source states: See the text. (Self-Generated Answer based on theory): Newton’s law of cooling states that the rate of loss of heat ($\frac{dQ}{dt}$) from a body is directly proportional to the temperature difference ($\theta -{\theta }_0$) between the body and its surroundings, provided this difference is small. Mathematically: $\frac{dQ}{dt}\propto (\theta -{\theta }_0)$.

Q2: Does Newton’s law of cooling hold well when the temperature difference between the body and its surroundings is very large? Ans: No, it is strictly true only for a small difference of temperature. This radiation law holds true when the temperature difference between the body and its surroundings is not more than $30^{\circ }\text{C}$. It is better when the temperature difference is only $10^{\circ }\text{C}$.

Q3: Is it an accurate method? Ans: No, it is a quick method. The law of cooling does not hold strictly for a large temperature difference.

Q4: What is the principal process by which heat is lost from the calorimeter in the arrangement you have made? Ans: Radiation is the principal process by which heat is lost from the calorimeter.

Q5: Will the experiment succeed if we keep the calorimeter open and why? Ans: In this case, radiation loss will increase due to the difference of temperature between the liquid and the surroundings.

Q6: Why is it necessary to use the same volume of the two liquids? Ans: In this way, the surface area from which radiation occurs is kept equal in both cases.

Q7: Can we heat the liquids to any temperature we like? Ans: No, the temperature must not rise less than the boiling point of the liquids.

Q8: Should the liquids have the same volume or the same masses? Ans: The volume of the liquids taken in the calorimeter must be same.

Q9: Would you prefer a calorimeter well-polished or painted black? Ans: We prefer a calorimeter should have thin walls and their outer surfaces should be painted black.

Q10: Does Newton’s law of cooling hold for solids also? Ans: Newton’s law of cooling holds only for liquids, not for solids.

Q11: What are the factors on which the radiation of heat from a hot body depends? Ans: The original source states: See the text. (Self-Generated Answer based on theory): The radiation of heat depends on (i) the temperature of the radiating body, (ii) the nature of the surface (area and color/texture), and (iii) the temperature of the surroundings.

Q12: What is the chief source of error and how to avoid it? Ans: The chief source of error is due to evaporation of the liquid. To avoid this, the calorimeter should be provided with a lid having a hole for the thermometer and a stirrer. When necessary, a rotary stirrer should be employed for up and down stirring to promote evaporation.

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8. Experiment E7 (H2): Co-efficient of Thermal Conductivity by Lees and Charlton’s Method

Q1: Define the co-efficient of thermal conductivity. Ans: The co-efficient of thermal conductivity of a substance is the quantity of heat that flows in one second through the opposite faces of a block of material having an area of each face $1{\text{ cm}}^2$ and a distance between the faces of $1\text{ cm}$, when the opposite faces are maintained at a temperature difference of $1^{\circ }\text{C}$.

Q2: Is Ice’s method applied in the case of (i) a conductor and (ii) a liquid? Ans: i) No, it can’t be applied in the case of a conductor. ii) Yes, provided the disc $A$ is replaced by a thin walled flat-end receptacle containing the liquid.

Q3: Distinguish between steady and variable states. Ans:

• Steady state: This state occurs when there is no absorption of heat by the bar and no temperature increase of any part of the bar is called the steady state.
• Variable state: The state during which the temperature of each portion of the bar is changing is known as the variable state.

Q4: Does the thermal conductivity depend on temperature? Ans: Yes, it decreases with a rise of temperature.

Q5: Why should the experimental disc be thin in Lee’s and Charlton’s method? Ans: In measuring the conductivity of such a poor conductor, one must remember that a thin disc of the material must be used. If the disc is thin, then the surrounding of the disc should be thin and heat loss due to radiation would be minimal.

Q6: What is the temperature gradient and what is its mathematical expression? Ans: The change of temperature per unit distance is called the temperature gradient. The mathematical expression of temperature gradient is $\frac{d\theta }{dx}$.

Q7: Does the value of thermal conductivity depend on the dimension of the substance? Ans: No, it depends only on the material of the substance.

Q8: Why do the two thermometers should be placed close to the face of the disc of the specimen? Ans: The steady state temperatures should be recorded only when the two thermometers remain constant.

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