Punjab 9th Physics Ch 7 Thermal Properties of Matter Short Questions

Solids have fixed volume and shape because their particles are closely packed and have a fixed position in space, with strong forces holding them together. The particles can only vibrate about their mean positions, which prevents them from moving freely and changing the shape or volume of the solid.

Gases have neither fixed volume nor a fixed shape because:
• Molecules are widely spaced with large empty spaces between them
• Molecules are free to move in any direction with high kinetic energy
• Weak intermolecular forces allow particles to move independently
• Gas particles expand to fill any container they occupy

• In solids: The particles are closely spaced due to strong interatomic forces. They vibrate in fixed positions with minimal movement.

• In liquids: The spacing is greater than solids because of moderate intermolecular forces. Particles can slide past each other but remain close.

• In gases: The spacing is much greater than solids and liquids due to weak forces. Particles move freely and independently with large distances between them, making gases highly compressible.

Raising the temperature of a liquid increases the kinetic energy of its particles, causing them to:
• Move faster and more vigorously
• Spread out slightly, causing thermal expansion
• Eventually overcome intermolecular forces and turn into vapour (evaporation/boiling)
• Increase in pressure if contained in a closed vessel

Temperature is the degree of hotness or coldness of a body. It is a physical quantity that determines the direction of flow of thermal energy. Heat always flows from a body at higher temperature to a body at lower temperature until thermal equilibrium is reached.

Heat can be defined as “The energy that is transferred from one body to another due to a temperature difference between the two bodies.” Heat is energy in transit – it only exists during the process of transfer. Once transferred, it becomes part of the internal energy of the receiving body.

A thermometric property is a physical property that changes measurably with temperature. Some common thermometric properties include:

• Volume expansion: e.g., mercury or alcohol in a thermometer expands with temperature
• Colour change: e.g., liquid crystal thermometers change colour with temperature
• Electrical resistance: e.g., thermistors change resistance with temperature
• Pressure change: e.g., gas thermometers measure pressure changes at constant volume
• Electromotive force (EMF): e.g., thermocouples generate voltage with temperature difference

The three main temperature scales are:

1. Celsius Scale (°C):
• Ice point: 0°C
• Steam point: 100°C
• 100 equal divisions between fixed points

2. Fahrenheit Scale (°F):
• Ice point: 32°F
• Steam point: 212°F
• 180 equal divisions between fixed points

3. Kelvin Scale (K):
• Absolute zero: 0 K
• Ice point: 273 K
• Steam point: 373 K

Conversion formulas:
• °F = (9/5 × °C) + 32
• °C = (5/9) × (°F – 32)
• K = °C + 273
• °C = K – 273

Sensitivity of a thermometer refers to how small a change in temperature it can detect and measure accurately. A more sensitive thermometer shows a larger change in its thermometric property (like mercury level) for a small change in temperature. Sensitivity can be increased by:
• Using a larger bulb with more thermometric liquid
• Using a narrower capillary tube
• Using a liquid with higher coefficient of expansion

Linearity of a thermometer refers to how directly its readings correspond to the actual temperature – whether the temperature scale is evenly spaced and consistent throughout its range. A linear thermometer shows equal changes in its thermometric property for equal changes in temperature. Mercury has good linearity over a wide range, making it suitable for accurate thermometers.

The scale reading of a thermometer is accurate due to:
• Calibration: The thermometer is calibrated against known temperature standards (ice point and steam point)
• Linear expansion: The thermometric property (e.g., mercury expansion) changes linearly with temperature
• Accurate markings: The temperature scale is accurately marked and evenly spaced
• Proper construction: The thermometer is constructed to minimize errors (thin-walled bulb, uniform bore, proper sealing)
• Quality of materials: Using pure mercury or alcohol with consistent expansion properties

Temperature difference determines the direction of heat flow. Heat always flows spontaneously from a body at a higher temperature to a body at a lower temperature. This continues until both bodies reach the same temperature (thermal equilibrium). This principle is based on the Second Law of Thermodynamics.

| Heat | Internal Energy |
|——|—————-|
| Energy transferred between systems due to temperature difference | Total energy of all particles in a system |
| Can be gained or lost by a body | Includes kinetic energy of particles + potential energy of bonds |
| Measured in joules (J) | Also measured in joules (J) |
| Exists only during transfer | Exists within the body at all times |
| Depends on temperature difference | Depends on mass, temperature, and nature of substance |

Energy (heat) travels from your hand to the cold surface, not cold traveling to your hand. This is because heat always flows from higher temperature to lower temperature. Your hand is warmer than the cold surface, so thermal energy transfers from your hand to the surface. The sensation of “cold” is actually your hand losing heat energy.

No, you cannot reliably feel your own fever by touching your forehead. This is because:
• Your hand and forehead are at approximately the same temperature when you have a fever
• Temperature perception requires a temperature difference between the sensing surface and the object
• Without a reference point or temperature difference, your nerves cannot detect the elevated temperature
• A thermometer or another person’s hand (at normal temperature) is needed to accurately detect fever

(i) The walls of the thermometer bulb are made thin to:
• Increase heat transfer rate between the bulb and the substance being measured
• Reduce thermal mass for quicker response to temperature changes
• Allow the thermometric liquid to expand/contract more rapidly

(ii) The inner bore (capillary tube) must be narrow to:
• Increase sensitivity – a small volume change produces a large visible movement
• Improve precision in reading temperature changes
• Reduce the amount of thermometric liquid needed
• Minimize thermal lag for faster readings

Pressure and resistance are thermometric properties because they change predictably with temperature:

• Pressure: The pressure of a given mass of gas at constant volume increases with temperature. This principle is used in gas thermometers where pressure changes indicate temperature changes.

• Resistance: The electrical resistance of a given length of wire (like platinum) increases with temperature. This property is used in platinum resistance thermometers where resistance measurements are converted to temperature readings.

Both properties show consistent, measurable changes with temperature, making them reliable for temperature measurement.

Thermometric properties are physical properties of a substance that change appreciably and predictably with temperature changes. These properties are used to construct thermometers. Examples include:

• Volume expansion of liquids (mercury, alcohol)
• Pressure of gases at constant volume
• Electrical resistance of metals
• Electromotive force in thermocouples
• Colour change in liquid crystals
• Length expansion of solids

A good thermometric property should be: easily measurable, change significantly with temperature, and be reproducible.

A thermistor is a type of resistor whose electrical resistance changes significantly with temperature. The word “thermistor” comes from “thermal resistor.”

Types:
• NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases
• PTC (Positive Temperature Coefficient): Resistance increases as temperature increases

Applications:
• Digital thermometers
• Temperature sensors in electronic devices
• Overheat protection circuits
• Automotive temperature monitoring

Thermistors are highly sensitive and can detect very small temperature changes, making them useful for precise temperature measurements.

Absolute zero is the theoretical temperature at which all molecular motion stops and particles have minimum possible energy. At this temperature, a substance has zero thermal energy.

Value:
• 0 Kelvin (0 K)
• -273.15°C on Celsius scale
• -459.67°F on Fahrenheit scale

Absolute zero cannot be reached in practice, but scientists have achieved temperatures very close to it. The Kelvin scale starts at absolute zero, which is why it has no negative values.

Molecular Theory of Matter states that:

1. All matter is composed of very small particles called molecules/atoms
2. These particles are always in motion (vibrational, rotational, or linear)
3. There are spaces between particles (intermolecular spaces)
4. Particles exert attractive forces on each other (intermolecular forces)
5. The strength of intermolecular forces depends on the distance between particles – forces decrease as distance increases
6. Temperature affects the kinetic energy of particles – higher temperature means faster motion

This theory explains the properties of solids, liquids, and gases, as well as phenomena like diffusion, expansion, and change of state.

Plasma is a state of matter in which most atoms are ionized, containing positively charged ions and free electrons moving independently within the gas volume.

Characteristics:
• Highly energetic and electrically conductive
• Responds strongly to electromagnetic fields
• Found in stars, lightning, neon signs, and fluorescent lamps
• Makes up about 99% of the visible universe

Due to the presence of charged particles (ions and electrons), plasma is a conducting state of matter that allows electric current to pass through it. It is often called the fourth state of matter after solid, liquid, and gas.

Yes, the kinetic molecular theory applies to plasma, but with additional considerations:

Basic applicability:
• Plasma particles (ions and electrons) are in constant random motion
• Particle motion increases with temperature
• Collisions between particles transfer energy

Additional complexities for plasma:
• Coulomb interactions: Charged particles exert electromagnetic forces on each other over long distances
• Collective behavior: Plasma exhibits wave phenomena and oscillations not seen in neutral gases
• Magnetic field effects: Plasma motion is influenced by magnetic fields due to charged particles
• Ionization/recombination: Particles can gain or lose electrons, changing their charge state

While the basic kinetic theory principles apply, plasma requires additional physics (electromagnetism, quantum mechanics) for complete description.

Mercury is preferred to alcohol as a thermometric liquid because:

Advantages of Mercury:
• High boiling point (357°C): Can measure high temperatures without vaporizing
• Low freezing point (-39°C): Remains liquid at moderately low temperatures
• Uniform linear expansion: Provides accurate, consistent readings across its range
• High density: Allows for compact thermometer design with visible column
• Does not wet glass: Forms a clear meniscus for easy reading
• Good thermal conductivity: Responds quickly to temperature changes
• Opaque and shiny: Easily visible in glass tube
• Chemically stable: Does not react with glass or decompose easily

Disadvantages of Alcohol:
• Lower boiling point (78°C): Cannot measure high temperatures
• Wets glass: Can stick to tube walls, causing reading errors
• Colourless: Requires dye for visibility
• Lower density: Requires larger bulb for same sensitivity

Water is not suitable for use in thermometers because:

• Freezing point: Water freezes at 0°C, making it unusable for measuring temperatures below freezing
• Boiling point: Water boils at 100°C, limiting its range for higher temperature measurements
• Anomalous expansion: Water expands when freezing (0°C to 4°C), which can burst the glass tube
• Non-linear expansion: Water’s expansion is not uniform across temperature ranges, reducing accuracy
• Low sensitivity: Small coefficient of expansion makes small temperature changes hard to detect
• Wetting property: Water adheres to glass, causing reading errors

Equivalent temperature of 373 K:
• Celsius: 373 K – 273 = 100°C
• Fahrenheit: (9/5 × 100) + 32 = 212°F

Two ways to increase sensitivity of a liquid-in-glass thermometer:

1. Narrower capillary bore:
• Reducing the diameter of the glass tube (bore) makes the thermometer more sensitive
• A smaller bore requires less liquid expansion to produce a visible rise in the column
• This allows detection of smaller temperature changes with greater precision

2. Larger bulb volume:
• Increasing the size of the bulb contains more thermometric liquid
• More liquid expands more for the same temperature change
• This produces a larger movement in the capillary tube for better sensitivity

Additional methods:
• Using a liquid with higher coefficient of expansion (like alcohol instead of mercury)
• Using thinner bulb walls for faster heat transfer
• Ensuring uniform bore diameter throughout the capillary

When the same amount of heat energy is supplied to different masses of the same substance, the temperature rise is inversely proportional to the mass.

Given:
• 1 litre of water rises by 2°C with a certain heat input
• 2 litres of water (twice the mass) receives the same heat input

Since Q = mcΔT (where Q = heat, m = mass, c = specific heat, ΔT = temperature change):
• For constant Q and c: ΔT ∝ 1/m
• Doubling the mass halves the temperature rise

Therefore, the temperature rise for 2 litres of water will be approximately 1°C (half of the original 2°C rise).

Note: This assumes no heat loss to surroundings and that the stove supplies the same energy in the same time.

There are no negative numbers on the Kelvin scale because:

• Absolute zero foundation: The Kelvin scale starts at absolute zero (0 K), which is the lowest theoretically possible temperature where all molecular motion ceases

• Energy basis: Temperature measures the average kinetic energy of particles. Since kinetic energy cannot be negative (particles cannot have less than zero motion), temperature cannot be negative on an absolute scale

• Thermodynamic definition: Kelvin is defined based on fundamental thermodynamic principles, not arbitrary reference points like Celsius or Fahrenheit

• Physical meaning: Negative Kelvin would imply particles having negative kinetic energy, which is physically impossible

The Kelvin scale is an absolute temperature scale, making it essential for scientific calculations involving gas laws, thermodynamics, and energy equations.

The statement “A thermometer measures its own temperature” is partially true but misleading:

What’s correct:
• A thermometer actually measures the temperature of its thermometric substance (mercury, alcohol, etc.)
• The reading reflects the thermometer’s own thermal state after reaching equilibrium

What’s incomplete:
• The purpose is to measure an external object’s temperature, not the thermometer itself
• Through thermal equilibrium, when the thermometer and object reach the same temperature, the thermometer’s reading represents the object’s temperature

Proper understanding:
• A thermometer measures its own temperature as an indicator of the temperature of whatever it is in thermal contact with
• Accurate measurement requires: good thermal contact, sufficient time for equilibrium, and minimal heat exchange with surroundings

The statement emphasizes that thermometers work by coming to thermal equilibrium with the measured object.

When touching different materials on a winter night:

Metals feel colder than air temperature because:
• Metals are good conductors of heat
• They quickly transfer heat away from your skin
• This rapid heat loss creates a sensation of coldness, even if the metal is at air temperature

Wooden objects feel closer to air temperature because:
• Wood is a poor conductor (insulator)
• Heat transfer from your hand is slow
• The surface warms slightly at the contact point

Cotton and plastic objects feel slightly warmer because:
• These are excellent insulators with very low thermal conductivity
• Minimal heat is drawn from your hand
• They may retain some body heat from previous contact

Important note: All objects are actually at the same temperature (air temperature) after sufficient time. The different sensations are due to different rates of heat transfer, not different actual temperatures.

An increase in temperature of 1°C is greater than 1°F.

Reason:
• The Celsius scale has 100 divisions between ice point and steam point
• The Fahrenheit scale has 180 divisions between the same points
• Therefore: 1°C = 1.8°F or 1°F = 5/9°C ≈ 0.56°C

This means:
• A 1°C change represents a larger temperature difference than a 1°F change
• For the same physical temperature change, the numerical value in Fahrenheit will be 1.8 times larger than in Celsius

Example: A rise from 20°C to 21°C equals a rise from 68°F to 69.8°F (a 1.8°F increase).

Gas molecules do not all have the same speed due to:

• Random motion: Gas molecules move randomly in all directions with constant collisions

• Collisions: When molecules collide, they exchange energy and momentum, resulting in a distribution of speeds

• Maxwell-Boltzmann distribution: At any temperature, gas molecules follow a statistical distribution where:
– Some molecules move very slowly
– Most molecules have speeds near the average
– Some molecules move very fast

• Temperature dependence: Higher temperature increases the average speed but doesn’t make all speeds equal

• Energy distribution: Kinetic energy is distributed among molecules according to probability, not equally

This speed distribution explains phenomena like diffusion, evaporation, and why some molecules can escape a liquid surface even below boiling point.

In an ideal vacuum, where no particles exist, temperature does not have conventional meaning because:

• Temperature definition: Temperature measures the average kinetic energy of particles. No particles = no kinetic energy to measure

• No thermal equilibrium: Temperature requires matter to establish thermal equilibrium

However, in practical contexts, we can discuss effective temperature of a vacuum:

• Radiation temperature: A vacuum can contain electromagnetic radiation (photons) which has an associated temperature (e.g., cosmic microwave background at 2.7 K)

• Container temperature: The walls of a vacuum chamber have a temperature that affects objects inside through radiation

• Quantum vacuum: Even “empty” space has quantum fluctuations with energy, though this doesn’t correspond to conventional temperature

Conclusion: While ideal vacuum has no temperature in the traditional sense, we can meaningfully discuss radiation temperature or the temperature of objects within the vacuum.

The statement “A hot body does not contain heat” is scientifically accurate:

Correct understanding:
• Heat is energy in transit: Heat only exists during the process of energy transfer between bodies at different temperatures

• Internal energy vs. heat: A hot body contains internal energy (kinetic + potential energy of its particles), not “heat”

• After transfer: Once heat is transferred to a body, it becomes part of that body’s internal energy; it is no longer called “heat”

Analogy:
• Heat is like “work” – both describe energy transfer processes, not stored quantities
• Just as a moving object has kinetic energy but doesn’t “contain work,” a hot object has internal energy but doesn’t “contain heat”

Practical implication:
• We say “heat flows” not “heat is stored”
• A hot body can transfer energy as heat to a colder body, but the energy was internal energy before transfer

This distinction is fundamental to thermodynamics and helps avoid conceptual errors in understanding energy.

Yes, the Sun is matter, with important qualifications:

Why the Sun is matter:
• Composition: The Sun is composed of physical substances – primarily hydrogen (~74%) and helium (~24%), with trace amounts of heavier elements

• Mass: The Sun has enormous mass (~2 × 10³⁰ kg), which is a defining property of matter

• Occupies space: The Sun has volume and physical dimensions (radius ~696,000 km)

• Plasma state: The Sun’s matter exists primarily as plasma – ionized gas with free electrons and ions, which is still matter

Energy consideration:
• The Sun also emits energy (light, heat, radiation) through nuclear fusion
• This energy is not matter but a byproduct of matter interactions
• According to E=mc², matter and energy are interchangeable, but the Sun’s bulk remains matter

Conclusion: The Sun is matter in the plasma state that continuously converts some of its mass into energy through nuclear reactions. Both its material composition and energy emission are essential to understanding the Sun.

Particle Theory of Matter – Key Points Differentiating States:

SOLIDS:
• Particle arrangement: Closely packed in fixed, regular patterns
• Particle motion: Vibrate about fixed positions only
• Intermolecular forces: Very strong, holding particles in place
• Shape and volume: Fixed shape and fixed volume
• Compressibility: Very low (particles already close together)
• Examples: Ice, iron, wood

LIQUIDS:
• Particle arrangement: Close but not in fixed patterns; can slide past each other
• Particle motion: Move more freely than solids; can flow
• Intermolecular forces: Moderate – strong enough to keep particles together but allow movement
• Shape and volume: No fixed shape (takes container shape); fixed volume
• Compressibility: Low, but slightly more than solids
• Examples: Water, oil, mercury

GASES:
• Particle arrangement: Widely spaced with no fixed arrangement
• Particle motion: Move rapidly and randomly in all directions
• Intermolecular forces: Very weak; particles move independently
• Shape and volume: No fixed shape or volume; fills entire container
• Compressibility: High (large spaces between particles)
• Examples: Air, oxygen, carbon dioxide

These differences explain why each state has distinct physical properties and behaviors.

TEMPERATURE:
Temperature is the degree of hotness or coldness of a body. It is a physical quantity that determines the direction of heat flow – heat always flows from higher to lower temperature.

HOW IT IS MEASURED:
Temperature is measured using thermometers that exploit thermometric properties (properties that change with temperature):
• Liquid expansion (mercury/alcohol thermometers)
• Gas pressure (gas thermometers)
• Electrical resistance (resistance thermometers)
• Voltage generation (thermocouples)

CONSTRUCTION OF MERCURY-IN-GLASS THERMOMETER:

1. Bulb: Thin-walled glass bulb at the bottom contains mercury
2. Capillary tube: Very narrow, uniform bore glass tube connected to bulb
3. Mercury: Pure mercury as thermometric liquid (uniform expansion, visible)
4. Stem: Glass stem with calibrated scale marked on it
5. Reference points: Calibrated at ice point (0°C) and steam point (100°C)
6. Sealed top: Tube sealed to prevent mercury evaporation and contamination
7. Scale: Evenly spaced divisions between fixed points (linear expansion of mercury)

Working: When temperature rises, mercury expands and rises in the capillary. The height indicates temperature on the calibrated scale.

Comparison of Temperature Scales:

| Feature | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|———|————-|—————–|————|
| Ice point | 0°C | 32°F | 273 K |
| Steam point | 100°C | 212°F | 373 K |
| Number of divisions | 100 | 180 | 100 |
| Size of 1 degree | 1°C | 5/9°C | 1 K = 1°C |
| Absolute zero | -273°C | -459.67°F | 0 K |
| Negative values | Possible | Possible | Not possible |
| Usage | Scientific, most countries | USA, some applications | Scientific (SI unit) |
| Basis | Water’s phase changes | Brine solution & human body | Absolute thermodynamic scale |

CONVERSION FORMULAS:
• °F = (9/5 × °C) + 32
• °C = (5/9) × (°F – 32)
• K = °C + 273
• °C = K – 273
• °F = (9/5 × (K – 273)) + 32

KEY POINT:
• Kelvin is the SI base unit for temperature
• Celsius and Kelvin have same degree size, just different zero points
• Fahrenheit degree is smaller (5/9 of Celsius degree)

SENSITIVITY:
Definition: The ability of a thermometer to detect and show small changes in temperature.

Example: A clinical thermometer is highly sensitive – it can detect 0.1°C changes in body temperature. This is achieved by:
• Narrow capillary tube
• Large bulb containing more mercury
• Mercury’s good expansion properties

RANGE:
Definition: The span of temperatures a thermometer can measure accurately from minimum to maximum.

Examples:
• Clinical thermometer: Range 35°C to 42°C (limited to human body temperatures)
• Laboratory thermometer: Range -10°C to 110°C (wider for experiments)
• Mercury thermometer: Range -39°C to 357°C (limited by mercury’s freezing/boiling points)

LINEARITY:
Definition: The property where equal temperature changes produce equal changes in the thermometric property (e.g., mercury column height).

Example: Mercury has excellent linearity – a 10°C rise always produces the same mercury expansion whether going from 20°C to 30°C or 80°C to 90°C. Alcohol has poorer linearity at extreme temperatures.

IMPORTANCE:
• Sensitivity: Crucial for precise measurements (medical, scientific)
• Range: Determines application suitability (industrial, laboratory, domestic)
• Linearity: Ensures accurate, consistent readings across the entire scale

Improving Thermometer Parameters Through Design:

IMPROVING SENSITIVITY:
1. Thin-walled bulb: Reduces thermal mass for faster heat transfer and quicker response
2. Larger bulb volume: Contains more thermometric liquid, producing greater expansion for small temperature changes
3. Narrower capillary bore: Small volume changes produce larger visible movement in the tube
4. High-expansion liquid: Using mercury or alcohol with large coefficients of expansion

IMPROVING RANGE:
1. Longer capillary tube: Allows measurement over wider temperature span without overflow
2. Appropriate liquid selection: Mercury for high temperatures (-39°C to 357°C), alcohol for low temperatures (-112°C to 78°C)
3. Strong glass construction: Withstands thermal stress at temperature extremes
4. Proper sealing: Prevents liquid evaporation or contamination at high temperatures

IMPROVING LINEARITY:
1. Uniform bore diameter: Ensures consistent expansion-to-height ratio throughout the scale
2. Pure thermometric liquid: Impurities can cause non-linear expansion
3. Accurate calibration: Multiple reference points ensure even scale markings
4. Quality glass: Low thermal expansion glass minimizes container effects on readings

ADDITIONAL DESIGN FEATURES:
• Magnifying lens: Improves reading precision
• White backing: Enhances visibility of liquid column
• Protective casing: Prevents breakage and maintains calibration
• Immersion depth marking: Ensures correct usage for accurate readings

These structural improvements make thermometers reliable tools for accurate temperature measurement across various applications.