Class 11 Physics Notes Chapter 14 (Chapter 14) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 14 from your Physics Lab Manual. This chapter often groups together several important activities that demonstrate fundamental physics principles. Understanding these activities, their underlying theory, and the expected observations is crucial, not just for your practical exams, but also for competitive government exams where conceptual clarity is tested.

Here are detailed notes covering some key activities typically found in this section:

Chapter 14: Activities

This chapter consolidates various experimental activities designed to reinforce theoretical concepts learned in Class 11 Physics. We will focus on a few representative activities often included here.

Activity 1: Cooling Curve and Change of State

• Aim: To observe the change of state (specifically, solidification) of a substance (like paraffin wax or naphthalene) by plotting its cooling curve (Temperature vs. Time).

• Apparatus: Boiling tube/Calorimeter, thermometer, stirrer, substance (wax/naphthalene), water bath, heater, clamp stand, stopwatch.

• Principle:

  • When a hot substance cools, it loses heat to the surroundings. According to Newton's Law of Cooling (approximately), the rate of cooling (rate of heat loss) is proportional to the temperature difference between the substance and the surroundings, assuming this difference is not too large.
  • During a change of state (e.g., liquid to solid), the substance releases latent heat at a constant temperature (the freezing point or melting point). This release of heat compensates for the heat loss to the surroundings, causing the temperature to remain constant for some time.

• Procedure Outline:

  1. Melt the substance completely in a boiling tube using a water bath. Ensure the thermometer bulb is fully immersed in the molten substance.
  2. Remove the tube from the water bath, place it in a suitable insulating enclosure (like a beaker with cotton wool or a double-walled enclosure) to ensure slow cooling.
  3. Start the stopwatch and record the temperature of the substance at regular intervals (e.g., every 30 seconds or 1 minute). Keep stirring gently for uniform cooling.
  4. Continue recording until the substance has completely solidified and its temperature has fallen significantly below the freezing point.
  5. Plot a graph with Time (t) on the X-axis and Temperature (T) on the Y-axis.

• Observations and Graph:

  • The graph will show temperature decreasing with time.
  • Initially (liquid phase), the temperature drops relatively quickly.
  • A plateau (horizontal or nearly horizontal section) appears on the graph. This corresponds to the freezing point (or melting point) of the substance. During this phase, the substance is solidifying, releasing latent heat of fusion, which balances the heat loss to the surroundings, keeping the temperature constant.
  • After the entire substance has solidified, the temperature starts dropping again (solid phase), usually at a slower rate than the initial liquid cooling.
  • The temperature corresponding to the plateau gives the freezing point (T_f) of the substance.

  (Typical Cooling Curve Shape)

     T |       /
       |      /  <-- Liquid Cooling
       |     /
       |----/-------  <-- Plateau (Solidification at Tf)
       |           \
       |            \ <-- Solid Cooling
       |             \
       +------------------> t
  

• Key Concepts: Newton's Law of Cooling, Change of State, Latent Heat of Fusion, Freezing Point/Melting Point.

Activity 2: Effect of Detergent on Surface Tension

• Aim: To observe the effect of adding detergent (a surfactant) on the surface tension of water.
• Apparatus: Beaker/Petri dish, water, sewing needle/pin, detergent powder/solution, dropper.
• Principle:
  • Surface Tension: It is the property of a liquid surface that allows it to resist an external force, due to the cohesive nature of its molecules. The surface behaves like a stretched elastic membrane. It arises from the net inward pull experienced by molecules at the surface compared to those in the bulk.
  • Surfactants (Surface Active Agents): Substances like detergents and soaps, when dissolved in a liquid (like water), reduce its surface tension. They accumulate at the interface and interfere with the cohesive forces between liquid molecules.
• Procedure Outline & Observations:
  1. Method 1 (Needle Float): Fill a beaker with clean water. Carefully place a dry needle or pin horizontally onto the water surface. Due to surface tension, it can float (if placed gently). Now, add a tiny amount of detergent powder or a drop of detergent solution near the needle. Observe that the needle immediately sinks. This happens because the detergent drastically reduces the surface tension, which can no longer support the weight of the needle.
  2. Method 2 (Drop Shape): Place a drop of water on a clean, non-absorbent surface (like plastic or waxed paper). Observe its tendency to form a spherical shape (minimizing surface area). Now, mix detergent with water and place a drop of this solution. Observe that the drop spreads out more and is flatter, indicating reduced surface tension.
• Conclusion: Detergents significantly reduce the surface tension of water. This property is crucial for their cleaning action, as lower surface tension allows water to wet surfaces more effectively and penetrate fabrics.
• Key Concepts: Surface Tension, Cohesive Forces, Adhesion, Surfactants, Wetting.

Activity 3: Factors Affecting Rate of Loss of Heat

• Aim: To study factors affecting the rate of cooling of a body, such as the nature of its surface and the surface area exposed. (Qualitative verification of aspects of Newton's Law of Cooling).
• Apparatus: Two identical calorimeters (one polished, one painted black), two thermometers, heater, water, stopwatch, insulating enclosure.
• Principle:
  • Newton's Law of Cooling: The rate of loss of heat (-dQ/dt) of a body is directly proportional to the temperature difference (ΔT = T - T₀) between the body (T) and its surroundings (T₀), provided ΔT is small. Rate of cooling (-dT/dt) is also proportional to ΔT.
  • Factors: The rate of heat loss also depends on:
    • Surface Area (A): Larger the surface area exposed, faster the rate of cooling.
    • Nature of Surface (Emissivity, e): Black and rough surfaces are good emitters (and absorbers) of radiation, hence they cool faster than polished, shiny surfaces (poor emitters).
• Procedure Outline:
  1. Effect of Surface Nature: Take two identical calorimeters, one polished (A) and one painted black (B). Fill both with the same amount of hot water at the same initial temperature (e.g., 80°C). Place them in the same environment (same T₀) and record their temperatures simultaneously at regular intervals using separate thermometers and a stopwatch. Plot T vs. t for both.
  2. Effect of Surface Area: (Can be done similarly) Use two calorimeters of different sizes (hence different surface areas) but made of the same material and surface finish, filled with the same initial temperature water. Or use identical calorimeters but fill them with different volumes of water (affecting the exposed surface area if open, or total heat capacity).
• Observations:
  • Surface Nature: The cooling curve for the blackened calorimeter (B) will be steeper than that for the polished calorimeter (A). This shows that the black surface loses heat faster than the polished surface.
  • Surface Area: The calorimeter with the larger exposed surface area will cool faster.
• Conclusion: Rate of cooling increases with increase in surface area and depends on the nature of the surface (black surfaces cool faster than polished ones).
• Key Concepts: Heat Transfer (Conduction, Convection, Radiation), Newton's Law of Cooling, Emissivity, Absorptivity, Surface Area.

Activity 4: Measuring Force of Limiting Friction

• Aim: To measure the force of limiting static friction and kinetic friction between a block and a horizontal surface.
• Apparatus: Wooden block (with hook), horizontal surface (tabletop), pulley, weight box, pan, string, spring balance (optional).
• Principle:
  • Friction: A force that opposes relative motion (or tendency of relative motion) between surfaces in contact.
  • Static Friction (f_s): Friction acting when there is no relative motion. It is self-adjusting, increasing with the applied force up to a maximum value.
  • Limiting Static Friction (f_s(max)): The maximum value of static friction that comes into play when the body is just about to move over the surface. It is proportional to the normal reaction (N). f_s(max) = μ_s N, where μ_s is the coefficient of static friction.
  • Kinetic Friction (f_k): Friction acting when there is relative motion between the surfaces. It is nearly independent of the relative velocity (at low speeds) and is usually less than limiting static friction. It is also proportional to the normal reaction. f_k = μ_k N, where μ_k is the coefficient of kinetic friction. Generally, μ_k < μ_s.
• Procedure Outline:
  1. Place the wooden block on the horizontal surface. Attach a string to its hook, pass it over a frictionless pulley fixed at the edge of the table, and connect a pan to the other end.
  2. Limiting Friction: Gradually add small weights to the pan. Note the total weight (Applied Force, P = Mg) in the pan just when the block starts to slide. This weight equals the force of limiting static friction. f_s(max) = P_start.
  3. Kinetic Friction: Add slightly more weight to the pan so the block starts moving. Then, adjust the weights slightly (usually reduce them a bit) so that the block moves with a constant, slow velocity when given a gentle push. The weight (P = Mg) in the pan now equals the force of kinetic friction. f_k = P_move.
  4. Measure the mass of the block (m). The normal reaction N = mg (where g is acceleration due to gravity).
  5. Calculate μ_s = f_s(max) / N and μ_k = f_k / N.
  6. Repeat for different surfaces or by adding weight on top of the block (to change N) to study the dependence on normal reaction.
• Observations: Record the weight required to just start motion (P_start) and the weight required to maintain uniform motion (P_move). Observe that P_start > P_move.
• Key Concepts: Friction, Static Friction, Limiting Friction, Kinetic Friction, Coefficient of Static Friction (μ_s), Coefficient of Kinetic Friction (μ_k), Normal Reaction.

Multiple Choice Questions (MCQs)

1. In the cooling curve experiment for change of state (liquid to solid), the temperature remains constant during solidification because:
   a) Heat loss to surroundings stops.
   b) The thermometer is faulty.
   c) Latent heat released compensates for heat loss to surroundings.
   d) The specific heat capacity becomes infinite.

2. The part of the cooling curve representing the change of state from liquid to solid is typically:
   a) A steeply falling line.
   b) A gradually falling line.
   c) A horizontal or nearly horizontal line.
   d) A rising line.

3. Surface tension of a liquid is a result of:
   a) Gravitational force between molecules.
   b) Net adhesive force on surface molecules.
   c) Net cohesive force on surface molecules.
   d) Viscous force within the liquid.

4. Adding detergent to water causes the surface tension of water to:
   a) Increase
   b) Decrease
   c) Remain unchanged
   d) Become zero instantly

5. According to Newton's Law of Cooling, the rate of loss of heat of a body is directly proportional to the:
   a) Temperature of the body.
   b) Temperature of the surroundings.
   c) Specific heat capacity of the body.
   d) Temperature difference between the body and surroundings.

6. Two identical objects, one painted black and the other polished silver, are heated to the same high temperature and allowed to cool in the same surroundings. Which object will cool faster?
   a) The polished silver object.
   b) The black painted object.
   c) Both will cool at the same rate.
   d) Cooling rate depends on the material, not the surface.

7. The maximum value of static friction that acts when a body is just about to slide over another surface is called:
   a) Kinetic friction
   b) Rolling friction
   c) Limiting friction
   d) Fluid friction

8. Which statement is generally true regarding the coefficients of static (μ_s) and kinetic (μ_k) friction?
   a) μ_s = μ_k
   b) μ_s > μ_k
   c) μ_s < μ_k
   d) μ_s = 0

9. The force of limiting static friction between two surfaces in contact is:
   a) Independent of the normal reaction.
   b) Directly proportional to the normal reaction.
   c) Inversely proportional to the normal reaction.
   d) Directly proportional to the area of contact.

10. While plotting a cooling curve, temperatures are recorded at regular intervals. To ensure accuracy, the thermometer bulb should be:
    a) Touching the bottom of the container.
    b) Just above the surface of the liquid.
    c) Fully immersed in the substance being cooled.
    d) Held outside the container near it.

Answers to MCQs:

1. c) Latent heat released compensates for heat loss to surroundings.
2. c) A horizontal or nearly horizontal line.
3. c) Net cohesive force on surface molecules.
4. b) Decrease
5. d) Temperature difference between the body and surroundings.
6. b) The black painted object.
7. c) Limiting friction
8. b) μ_s > μ_k
9. b) Directly proportional to the normal reaction.
10. c) Fully immersed in the substance being cooled.

Study these activities carefully, focusing on the principles, procedures, expected results, and the interpretation of graphs. Good luck with your preparation!