9th Physics Federal Board Notes: Unit 7 - Density and Temperature
📋 Table of Contents
- 1. Multiple Choice Questions
- 2. Short Response Questions
- 2.1 Liquid Density and Buoyancy
- 2.2 Measuring Human Body Density
- 2.3 Plasma as Fourth State of Matter
- 2.4 Water in Thermometers
- 2.5 Internal Energy and Temperature
- 2.6 Fixed Point Scales
- 2.7 Mercury vs Alcohol Thermometers
- 2.8 Absolute Zero
- 2.9 Thermocouple vs Liquid-in-Glass Thermometers
- 2.10 Increasing Thermometer Sensitivity
- 2.11 Verifying Thermometer Sensitivity
- 3. Long Response Questions
🌡️ Introduction to Unit 7
Unit 7: Density and Temperature explores fundamental concepts of matter and thermal physics. This unit helps students understand how matter exists in different states, how we measure density, and how temperature affects materials. You'll learn about various thermometers, the concept of absolute zero, and the relationship between internal energy and temperature.
Multiple Choice Questions
There are four fundamental states of matter: solid, liquid, gas, and plasma. Each state has distinct properties based on particle arrangement and energy.
In solids, particles are tightly packed in fixed positions and can only vibrate around these fixed points. They cannot move freely like in liquids or gases.
Density is an intensive property, meaning it doesn't depend on the amount of substance. All samples of pure water at the same temperature have the same density regardless of quantity.
Using the density formula:
Both gases and liquids are classified as fluids because they can flow and take the shape of their containers. This distinguishes them from solids which maintain a fixed shape.
Gas particles have high kinetic energy and move randomly in all directions. They are far apart with weak intermolecular forces, allowing them to fill any container.
Gases are highly compressible because their particles are far apart with empty space between them. They expand to fill any container completely.
When a solid melts into a liquid, particles gain energy and move farther apart. They break free from their fixed positions but remain close enough to maintain a definite volume.
Internal energy includes kinetic energy from particle motion, potential energy from intermolecular forces, and chemical energy from bonds. Light energy is external, not part of a substance's internal energy.
For an ideal gas, internal energy depends only on temperature. If temperature remains constant during expansion, the internal energy stays the same regardless of volume changes.
Mercury thermometers work on the principle of thermal expansion - mercury expands when heated and contracts when cooled. The change in volume is used to measure temperature.
A larger bulb contains more liquid, which expands more for the same temperature change. This greater expansion in a narrow capillary tube makes the thermometer more sensitive to small temperature variations.
Thermocouples have very fast response times because they have low thermal mass and directly convert temperature to electrical voltage, making them ideal for rapidly changing temperatures.
The temperature range is 100°C (from 0°C to 100°C). The mercury rises 4 cm (from 2 cm to 6 cm) over this range.
Thermocouples generate a voltage proportional to the temperature difference between two junctions of different metals. This voltage is measured to determine temperature.
Short Response Questions
1. Two liquids A and B, have densities 1 g/mL and 1.2 g/ml, respectively. When both liquids are poured into a container, one liquid floats on top of the other. Which liquid is on top, and why?
💧 Liquid Density and Buoyancy
When two immiscible liquids with different densities are poured into the same container, the liquid with the lower density will float on top of the liquid with the higher density.
In this case:
- Liquid A has a density of 1 g/mL
- Liquid B has a density of 1.2 g/mL
Since liquid A (1 g/mL) is less dense than liquid B (1.2 g/mL), liquid A will float on top of liquid B. This occurs due to buoyancy principles - the less dense liquid is "lighter" and rises above the denser liquid.
2. Write a method to find the volume and density of a human body?
📏 Measuring Human Body Density
To find the volume and density of a human body, we use the water displacement method:
- Measure Mass: Use a weighing scale to find the mass of the person in kilograms (kg).
- Water Displacement Setup:
- Fill a large container (like a tank) with water
- Note the initial water level (V₁)
- Submerge the Person:
- Fully submerge the person in the water without touching the sides or bottom
- Measure the new water level (V₂)
- Calculate Volume:
Volume = V₂ - V₁
- Calculate Density:
Density = Mass / VolumeExpress the density in kg/m³
This method provides accurate body volume and density measurements using Archimedes' principle of displacement.
3. How is plasma the fourth state of matter? Give a reason
⚡ Plasma - The Fourth State of Matter
Plasma is considered the fourth state of matter because it has distinct properties that differentiate it from solids, liquids, and gases.
Formation: Plasma forms when a gas is heated to extremely high temperatures, causing atoms to ionize into positive ions and free electrons.
Distinctive Properties:
- Conducts electricity efficiently
- Responds strongly to magnetic fields
- Contains charged particles rather than neutral atoms/molecules
Examples: Found in stars (including our Sun), neon lights, fluorescent tubes, and lightning.
These unique electrical and magnetic properties justify classifying plasma as a separate, fourth state of matter beyond the traditional three states.
4. Why water is not used in liquid in glass thermometers?
🚫 Limitations of Water in Thermometers
Water is generally not used in liquid-in-glass thermometers due to several limitations:
- Limited Temperature Range: Water freezes at 0°C and boils at 100°C, restricting its use to a narrow temperature range
- Non-linear Expansion: Water doesn't expand uniformly with temperature changes, especially near 4°C where it has maximum density
- Opacity Issues: Pure water is transparent, making it difficult to read the temperature level
- Wetting Properties: Water sticks to glass surfaces, causing inaccurate readings due to meniscus effects
- Freezing Expansion: Unlike most liquids, water expands when freezing, which could break the thermometer
Mercury and alcohol are preferred because they expand more uniformly, have wider liquid ranges, and are more visible in capillary tubes.
5. Can we increase internal energy of a substance without increasing its temperature?
🔥 Internal Energy vs Temperature
Yes, we can increase the internal energy of a substance without increasing its temperature during phase changes.
Explanation: During phase transitions (like melting or vaporization), the energy added is used to overcome intermolecular forces rather than increase kinetic energy.
Examples:
- When ice melts at 0°C, heat energy breaks hydrogen bonds but temperature remains constant until all ice becomes water
- When water boils at 100°C, energy separates water molecules into vapor without temperature increase
This "hidden" energy is called latent heat and increases the substance's internal energy (potential energy component) while temperature (related to kinetic energy) stays constant.
6. What are fixed points in temperature scales?
🎯 Fixed Points in Temperature Scales
Fixed points are specific, reproducible temperatures used to calibrate thermometers and define temperature scales.
Common Fixed Points:
- Ice Point (Lower Fixed Point): The temperature of pure melting ice at standard atmospheric pressure (0°C or 32°F)
- Steam Point (Upper Fixed Point): The temperature of steam from pure boiling water at standard atmospheric pressure (100°C or 212°F)
Purpose: These reference points allow accurate calibration of thermometers by marking specific positions on the scale that correspond to known, stable temperatures.
Other fixed points include the triple point of water (0.01°C) and the freezing point of various metals, used for more precise scientific measurements.
7. Why is mercury preferred over alcohol as a thermometric substance?
| Property | Mercury | Alcohol |
|---|---|---|
| Temperature Range | -39°C to 357°C (wider range) | -115°C to 78°C (limited range) |
| Expansion | Uniform expansion with temperature | Non-uniform expansion |
| Visibility | Shiny, easily visible | Usually dyed for visibility |
| Conductivity | Good heat conductor (quick response) | Poor heat conductor (slower response) |
| Adhesion | Doesn't stick to glass | Sticks to glass, causes errors |
| Boiling Point | High (357°C) | Low (78°C) |
8. What is absolute zero? Can it be achieved practically?
❄️ Absolute Zero
Definition: Absolute zero is the theoretical temperature at which all molecular motion ceases. It is the lowest possible temperature in the universe.
Value: -273.15°C or 0 Kelvin
Practical Achievement: No, absolute zero cannot be achieved practically. Scientists have come extremely close (within billionths of a degree), but the Third Law of Thermodynamics states that it's impossible to reach absolute zero through any finite process.
Reasons it cannot be reached:
- As temperature approaches absolute zero, the energy required to remove the remaining heat increases exponentially
- Quantum mechanical effects prevent complete cessation of motion (zero-point energy)
- Perfect isolation from all external energy sources is impossible
Despite this limitation, research at temperatures near absolute zero has led to important discoveries like superconductivity and Bose-Einstein condensates.
9. Differentiate between thermocouple thermometer and liquid in glass thermometer.
| Aspect | Thermocouple Thermometer | Liquid-in-Glass Thermometer |
|---|---|---|
| Principle | Thermoelectric effect (voltage generation) | Thermal expansion of liquid |
| Temperature Range | Very wide (-200°C to 2000°C) | Limited (depends on liquid used) |
| Response Time | Very fast (seconds) | Slow (minutes) |
| Accuracy | High accuracy possible | Moderate accuracy |
| Remote Measurement | Possible with long wires | Not possible |
| Fragility | Robust | Fragile (glass can break) |
| Cost | Generally more expensive | Inexpensive |
| Applications | Industrial processes, furnaces | Medical, laboratory, household |
10. How can we increase the sensitivity of a liquid in glass thermometer?
📈 Increasing Thermometer Sensitivity
Sensitivity refers to how much the liquid expands for a small temperature change. Several design modifications can increase sensitivity:
- Use a Larger Bulb: More liquid expands more for the same temperature change
- Use a Narrower Capillary Tube: The same volume expansion causes greater length change
- Select Appropriate Liquid: Choose liquids with high coefficient of thermal expansion
- Reduce Wall Thickness: Thinner glass allows faster heat transfer
- Optimize Bulb Shape: Spherical bulbs provide maximum surface area for heat exchange
Trade-off: Increased sensitivity often comes with reduced range, as the same capillary tube can accommodate less total expansion.
11. How can we verify that a liquid in glass thermometer is sensitive?
✅ Verifying Thermometer Sensitivity
To verify the sensitivity of a liquid-in-glass thermometer:
- Small Temperature Change Test:
- Place the thermometer in water at a known temperature (e.g., 20°C)
- Add a small amount of hot water to increase temperature by 1°C
- Observe if the liquid level shows a noticeable, clear movement
- Compare with Standard:
- Compare readings with a certified, high-sensitivity thermometer
- Check if both show the same response to small temperature changes
- Resolution Check:
- Verify that the scale divisions are fine enough to detect small changes (e.g., 0.1°C divisions)
- Ensure the liquid movement is smooth and not "sticky"
- Repeatability Test:
- Measure the same temperature multiple times
- A sensitive thermometer should give consistent readings
A sensitive thermometer will show clear, repeatable movement for temperature changes as small as 0.1°C.
Long Response Questions
1. Define density. Write different methods to measure the density of a liquid and irregular solid.
⚖️ Density Definition and Measurement
Definition: Density is defined as mass per unit volume of a substance. It's a fundamental physical property that indicates how much matter is packed into a given space.
Units: kg/m³ (SI units) or g/cm³
📊 Measuring Liquid Density
Method 1: Using Density Bottle
- Weigh empty, dry density bottle (m₁)
- Fill with liquid and weigh (m₂)
- Calculate liquid mass: m = m₂ - m₁
- Volume is known from bottle capacity
- Density = m / V
Method 2: Using Hydrometer
- Float hydrometer in liquid
- Read density directly from scale
- Based on buoyancy principle
🧱 Measuring Irregular Solid Density
Method 1: Water Displacement
- Weigh solid in air (m)
- Partially fill measuring cylinder with water (V₁)
- Submerge solid, note new volume (V₂)
- Volume = V₂ - V₁
- Density = m / (V₂ - V₁)
Method 2: Eureka Can
- Fill eureka can until water flows from spout
- Place measuring cylinder under spout
- Gently lower solid into can
- Collect displaced water in cylinder
- Volume = volume of displaced water
2. Compare the properties of three states of matter i.e., solid, liquid and gas.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite shape | Takes container shape | Takes container shape |
| Volume | Definite volume | Definite volume | Fills entire container |
| Density | High | Moderate | Low |
| Compressibility | Negligible | Very low | High |
| Particle Arrangement | Ordered, fixed positions | Random, close together | Random, far apart |
| Particle Motion | Vibrational only | Vibrational + rotational | Vibrational + rotational + translational |
| Intermolecular Forces | Very strong | Moderate | Very weak |
| Examples | Ice, wood, metal | Water, oil, milk | Air, oxygen, steam |
3. Write down the properties of a substance used for temperature measurement.
🌡️ Properties of Thermometric Substances
An ideal substance for temperature measurement should have these properties:
📈 Thermal Expansion
The substance should expand and contract uniformly with temperature changes. The expansion should be:
- Linear and predictable
- Reversible (same contraction when cooled)
- Significant enough to measure accurately
🌊 Wide Temperature Range
The substance should remain in the same state (liquid, gas) over a wide temperature range:
- Low freezing point
- High boiling point
- No decomposition at high temperatures
👁️ Good Visibility
The substance should be easily visible in the thermometer:
- Opaque or brightly colored
- Doesn't stick to glass walls
- Forms clear meniscus
⚡ Good Thermal Conductivity
The substance should quickly respond to temperature changes:
- Rapid heat transfer
- Low specific heat capacity
- Quick thermal equilibrium
🛡️ Chemical Stability
The substance should be chemically inert:
- Doesn't react with container
- Doesn't oxidize or decompose
- Non-toxic and safe to handle
💰 Cost and Availability
Practical considerations include:
- Reasonably priced
- Easily available
- Easy to purify and handle
4. What is a gas thermometer? On which principle does it work? Describe its construction and working.
💨 Gas Thermometers
Definition: A gas thermometer is a precision instrument that uses the thermal properties of gases to measure temperature. It's one of the most accurate thermometers and serves as a standard for calibrating other thermometers.
Working Principle: Gas thermometers work on the principle that the pressure of a gas at constant volume, or the volume of a gas at constant pressure, changes predictably with temperature.
There are two main types:
- Constant Volume Gas Thermometer: Measures pressure changes at fixed volume
- Constant Pressure Gas Thermometer: Measures volume changes at fixed pressure
🏗️ Construction
Main Components:
- Bulb: Contains the gas (usually nitrogen, helium, or hydrogen)
- Capillary Tube: Connects bulb to pressure measuring device
- Manometer: Measures gas pressure
- Reservoir: For adjusting mercury levels (in constant volume type)
- Scale: Calibrated temperature scale
⚙️ Working (Constant Volume Type)
Operation Steps:
- The bulb is placed in the environment whose temperature is to be measured
- Gas in the bulb expands or contracts with temperature changes
- Mercury reservoir is adjusted to keep gas volume constant
- Pressure change is measured using the manometer
- Temperature is calculated using the gas law equation:
Where P is pressure and T is absolute temperature in Kelvin.
📊 Advantages and Applications
Advantages:
- Extremely accurate and precise
- Wide temperature range (-200°C to 1600°C)
- Serves as primary standard for temperature measurement
- Can measure very high and very low temperatures
Applications:
- Calibration of other thermometers
- Scientific research and laboratories
- Measurement of thermodynamic temperatures
- Establishing international temperature scales
5. How can we improve the liquid in glass thermometer?
🔧 Improving Liquid-in-Glass Thermometers
Several enhancements can improve the performance and accuracy of liquid-in-glass thermometers:
🎯 Increased Accuracy
Methods:
- Use finer capillary tubes for better resolution
- Implement laser-etched scales for precise markings
- Add magnifying lens for easier reading
- Use high-purity thermometric liquids
- Include calibration certificates
📏 Extended Range
Methods:
- Use multiple liquids with overlapping ranges
- Implement gas-filled bulbs for high-temperature use
- Use special glass that withstands higher temperatures
- Design with expansion chambers for over-range protection
⚡ Faster Response
Methods:
- Reduce bulb wall thickness
- Use liquids with higher thermal conductivity
- Design smaller, more compact bulbs
- Use thin-walled, responsive glass
🛡️ Enhanced Durability
Methods:
- Use borosilicate glass (heat-resistant)
- Add protective metal or plastic casing
- Implement shock-absorbing mounting
- Use reinforced glass for the capillary
👁️ Better Readability
Methods:
- Use colored liquids (red alcohol, silver mercury)
- Implement backlighting for low-light conditions
- Add digital displays for precise reading
- Use contrasting background colors
🔍 Specialized Designs
Methods:
- Clinical thermometers with constrictions
- Maximum-minimum recording thermometers
- Bimetallic strips for automatic recording
- Remote reading capabilities
📚 Master 9th Physics
Understanding density and temperature is fundamental to grasping more advanced physics concepts. Continue your journey through the fascinating world of physics with our comprehensive notes for all units.
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Comprehensive study guide based on Federal Board curriculum with additional insights from educational resources
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