Sp Unit 3.4
Practicals
Thermal Physics
Sp Unit 3.4PracticalsThermal PhysicsLearners should be able to demonstrate and apply their knowledge and understanding of: |
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|---|---|
| 1. | Estimation of Absolute zero by use of gas law |
| 2. | Measurement of the specific heat capacity of a solid |
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1. Estimation of Absolute Zero using the Gas Law
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⇒ Objective:
- To estimate the value of absolute zero (-273.15°) using the relationship between temperature and volume for a gas at constant pressure.
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⇒ Apparatus Required:
- – A glass capillary tube sealed at one end
- – A water bath
- – A thermometer
- – A ruler
- – A beaker
- – A heat source (e.g., Bunsen burner)
- ⇒ Theory:
- According to Charles’ Law, the volume (V) of a gas is directly proportional to its absolute temperature (T) at constant pressure:
- [math]V ∝ T[/math]
- or
- [math]V = kT[/math]
- Where:
- – V = volume of the gas
- – T = absolute temperature in Kelvin
- – k = proportionality constant
- By extrapolating a graph of volume vs. temperature in Celsius, the temperature at which the volume becomes zero gives an estimate of absolute zero.
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⇒ Procedure:
- 1. Setting Up the Experiment:
- A glass capillary tube with trapped air is placed in a beaker of water.
- The water is heated gradually using a heat source.
- A thermometer is placed in the beaker to measure the temperature.
- Figure 1 Estimation of absolute zero using the gas law
- 2. Measuring the Volume:
- As the temperature increases, the gas expands, pushing the liquid column down in the capillary tube.
- Measure the length (l) of the gas column at different temperatures.
- Assume the volume (V) of the gas is proportional to its length ( [math]V ∝ l[/math]).
- Data Collection:
- Record the length of the gas column at different temperatures.
- Repeat the experiment by allowing the water to cool down gradually.
- Plotting the Graph:
- Plot a graph of gas volume (V) vs. temperature (K) in Celsius.
- Extrapolate the straight-line graph to find the temperature at which V=0.
- This intercept gives an estimate of absolute zero.
- Figure 2 A graph of gas between volume (V) vs temperature (K) in Celsius
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⇒ Observations & Data Table:
| Temperature (°C) | Gas Column Length (cm) |
|---|---|
| 0 | 8.5 |
| 20 | 9.2 |
| 40 | 10.0 |
| 60 | 10.7 |
| 80 | 11.4 |
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⇒ Results:
- By extrapolating the graph, the intercept occurs around -273°C, which is an estimate of absolute zero.
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⇒ Precautions:
- – Ensure the gas does not leak from the capillary tube.
- – Stir the water to maintain uniform temperature.
- – Avoid rapid heating to prevent thermal expansion effects.
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⇒ Conclusion:
- The experiment successfully demonstrates Charles’ Law, and the estimated absolute zero is close to the accepted value of -273.15°C.
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2. Measurement of the Specific Heat Capacity of a Solid
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⇒ Objective:
- To determine the specific heat capacity (c) of a solid using the method of mixtures or electrical heating.
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⇒ Apparatus Required:
- – A solid metal block (e.g., copper, aluminum)
- – A heater with known power rating
- – A thermometer
- – A stopwatch
- – A calorimeter
- – A balance (to measure mass)
- – A beaker of water
- Figure 3 Measurement of the specific heat capacity of a solid
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⇒ Theory:
- The specific heat capacity (c) is the amount of heat energy required to raise the temperature of 1 kg of a substance by 1°C:
- [math]Q = mcΔT[/math]
- Where:
- – Q = heat energy supplied (J)
- – m = mass of the solid (kg)
- – c = specific heat capacity (J/kg°C )
- – ΔT = temperature change (°C)
- In the electrical method, we use:
- [math]Q = Pt[/math]
- Where:
- – P = power of the heater (W)
- – t = heating time (s)
- By equating:
- [math]Pt = mcΔT[/math]
- for c:
- [math]c = \frac{P t}{m \Delta T}[/math]
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⇒ Procedure:
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Method 1: Electrical Heating Method
- 1. Setting Up the Experiment:
- – Place the solid metal block in an insulating container to minimize heat loss.
- – Insert a heater and a thermometer into the block.
- – Measure the initial temperature of the block ([math]T_i[/math] ).
- 2. Heating the Metal Block:
- – Turn on the heater and start the stopwatch.
- – Record the power rating of the heater (P) and the heating time (t). Stir the system gently to distribute heat evenly.
- – Measure the final temperature ([math]T_f[/math] ) of the block.
- 3. Calculating the Specific Heat Capacity:
- – Use the formula
- [math]c = \frac{P t}{m \Delta T}[/math]
- to determine c.
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⇒ Observations & Data Table:
| Parameter | Value |
|---|---|
| Mass of block (m) | 0.5 kg |
| Power of heater (P) | 50 W |
| Heating time (t) | 120 s |
| Initial temperature ( [math]T_i[/math]) | 25°C |
| Final temperature ( [math]T_f[/math] ) | 45°C |
| Temperature change (ΔT) | 20°C |
- ⇒ Using:
- [math]c = \frac{50 \times 120}{0.5 \times 20}[/math]
- For a copper block, the accepted value is 385 J/kg°C, so there may be heat loss errors.
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Method 2: Mixture Method
- Heat a metal block in boiling water.
- Transfer it quickly into a known mass of cold water in a calorimeter.
- Measure initial and final temperatures.
- Use the heat lost by the solid = heat gained by water:
- [math]m_s c_s ∆T_s = m_w c_w ∆T_w[/math]
- Solve for [math]C_s[/math].
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⇒ Sources of Error & Precautions:
- – Heat Loss: Minimize by using an insulated container.
- – Temperature Measurement: Stir the liquid properly for uniform heat distribution.
- – Time Delay: Transfer the metal block quickly in the mixture method.
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⇒ Conclusion:
- The specific heat capacity of the metal was determined using electrical heating and mixture methods. The experimental values were close to theoretical values with minor errors due to heat loss.