Temperature, Thermal Expansion and Heat Energy for SS 1

Heat Energy

Performance Objectives

By the end of the lesson, students should be able to:

1. define temperature and explain thermal expansion, change of state, and vaporization using the kinetic molecular theory;
2. explain conduction, convection, and radiation in terms of the kinetic molecular theory;
3. illustrate convection current in water;
4. measure temperature and thermal expansion of metallic materials.

Introduction to Heat Energy

Heat energy is a form of energy that flows from a region of higher temperature to a region of lower temperature. It is associated with the random motion of atoms and molecules within a substance.

All matter is made up of tiny particles (atoms and molecules) that are constantly in motion. This constant motion means that these particles possess kinetic energy. The total kinetic energy of these particles within a substance is related to its internal energy, and heat is the transfer of this internal energy.

Concept of Temperature

Temperature is a measure of the average kinetic energy of the particles within a substance.

When a substance gains heat energy, its particles move faster and vibrate more vigorously, increasing their average kinetic energy. This increase in average kinetic energy is observed as a rise in temperature. Conversely, when a substance loses heat energy, its particles slow down, their average kinetic energy decreases, and the temperature falls.

Temperature is measured using a thermometer, and common units include degrees Celsius (°C), Kelvin (K), and degrees Fahrenheit (°F).

Effects of Heat on Matter

When substances absorb heat energy, they can undergo several changes. These changes include thermal expansion, changes in state, and vaporization.

Thermal Expansion

Thermal expansion is the increase in the size (length, area, or volume) of a substance when its temperature rises.

According to the kinetic molecular theory, when a substance is heated, its particles gain kinetic energy and vibrate more vigorously. This increased vibration causes the particles to move further apart from each other, leading to an increase in the overall dimensions of the substance. This effect is noticeable in solids, liquids, and gases.

Examples of thermal expansion are observed in everyday life. For instance, railway lines have small gaps between sections to allow for expansion on hot days, preventing the tracks from buckling. Similarly, bridges are built with expansion joints to accommodate changes in length due to temperature variations.

Expansivity

Expansivity is a measure of how much a substance expands per unit change in temperature.

Different materials expand by different amounts for the same temperature change. For solids, there are three types of expansivity:

1. Linear Expansivity: This refers to the increase in length per unit original length per degree Celsius rise in temperature. It is most relevant for long, thin objects like metal rods.
2. Area (Superficial) Expansivity: This refers to the increase in surface area per unit original area per degree Celsius rise in temperature.
3. Volume (Cubical) Expansivity: This refers to the increase in volume per unit original volume per degree Celsius rise in temperature. This applies to solids, liquids, and gases.

For a small change in temperature, the change in length (ΔL) of a material is directly proportional to its original length (L₀) and the change in temperature (ΔT). This relationship can be expressed as:

ΔL = L₀αΔT

Where α (alpha) is the coefficient of linear expansivity, measured in per degree Celsius (°C⁻¹) or per Kelvin (K⁻¹).

Applications of Thermal Expansion

Thermal expansion has several practical applications, including:

1. Riveting: Hot rivets are inserted into holes and allowed to cool. As they cool, they contract, pulling the metal plates tightly together.
2. Fitting wheels on axles: Metal tyres are heated to expand them, then fitted onto wheels. Upon cooling, they contract and grip the wheel tightly.
3. Bimetallic strips: These are used in thermostats and fire alarms. They consist of two different metals joined together. When heated, the metals expand at different rates, causing the strip to bend and complete or break an electrical circuit.
4. Thermometers: The expansion of a liquid (like mercury or alcohol) in a narrow tube is used to measure temperature.

Change of State

A change of state is a physical process where matter transitions from one state (solid, liquid, or gas) to another due to the absorption or release of heat energy.

According to the kinetic molecular theory, when a solid is heated, its particles gain enough kinetic energy to overcome the strong forces holding them in fixed positions. They begin to move more freely, and the solid melts into a liquid. If the liquid continues to absorb heat, its particles gain even more kinetic energy, moving far apart and escaping the liquid surface to become a gas. The reverse processes occur when heat is removed.

Melting and Freezing

Melting is the process by which a solid changes into a liquid upon absorbing heat energy.

Freezing is the process by which a liquid changes into a solid upon releasing heat energy.

At the melting point, the absorbed heat energy is used to break the bonds between particles, not to increase their kinetic energy, so the temperature remains constant during the phase change.

Boiling and Condensation

Boiling is the rapid vaporization of a liquid, occurring throughout the liquid when it reaches its boiling point and forms bubbles of vapour.

Condensation is the process by which a gas or vapour changes into a liquid upon releasing heat energy.

Similar to melting, during boiling, the temperature of the liquid remains constant as the absorbed heat energy is used to overcome intermolecular forces and change the liquid into a gas.

Vaporization

Vaporization is the process by which a liquid changes into a gas or vapour.

Vaporization can occur in two main ways: evaporation and boiling. Evaporation happens slowly at the surface of a liquid at any temperature below its boiling point. Boiling occurs rapidly throughout the liquid at its specific boiling point. In both cases, particles at the surface or within the liquid gain enough kinetic energy to escape into the gaseous state.

Heat Transfer

Heat energy can be transferred from one place to another by three main methods: conduction, convection, and radiation.

Conduction

Conduction is the transfer of heat energy through direct contact between particles, without any overall movement of the material itself.

This method is most common in solids, especially metals. According to the kinetic molecular theory, when one end of a metal rod is heated, the particles at that end gain kinetic energy and vibrate more vigorously. These energetic particles collide with their less energetic neighbours, transferring some of their energy. This process continues along the rod, transferring heat from the hotter end to the colder end. Materials that conduct heat well are called conductors (e.g., metals), while those that conduct heat poorly are called insulators (e.g., wood, plastic, air).

Convection

Convection is the transfer of heat energy in fluids (liquids and gases) through the actual movement of the heated particles themselves.

When a fluid is heated, the particles at the bottom gain kinetic energy, move faster, and spread further apart. This makes the heated fluid less dense than the surrounding cooler fluid. The less dense, hotter fluid rises, while the denser, cooler fluid sinks to take its place. This cooler fluid then gets heated, rises, and the cycle continues, forming a convection current. This process effectively transfers heat throughout the fluid.

Convection Current in Water

A convection current can be easily observed in water. When water in a beaker is heated from below, the water at the bottom becomes hot, expands, and becomes less dense. This hot water rises, and cooler, denser water from the top sinks to take its place. This cooler water then gets heated and rises, creating a continuous circulation of water known as a convection current.

The procedure to illustrate a convection current in water involves the following steps:

1. Pour water into a beaker, filling it about three-quarters full.
2. Place a few crystals of potassium permanganate at the bottom of the beaker, preferably to one side.
3. Gently heat the bottom of the beaker directly below where the potassium permanganate crystals are placed using a burner.
4. Observe the movement of the coloured water.

As the water at the bottom heats up, it rises, carrying the dissolved potassium permanganate with it. The cooler water at the top then sinks, gets heated, and rises, creating a visible current of coloured water moving in a circular path. This demonstrates the formation of a convection current.

Land and Sea Breezes

Convection currents also occur in the atmosphere, leading to phenomena like land and sea breezes. During the day, land heats up faster than the sea. The air above the land becomes warmer, expands, and rises. Cooler, denser air from over the sea then moves inland to replace the rising warm air, creating a sea breeze. At night, the land cools faster than the sea. The air above the sea remains warmer and rises, while cooler air from the land moves out to sea, creating a land breeze.

Radiation

Radiation is the transfer of heat energy in the form of electromagnetic waves, which do not require a material medium for transfer.

Unlike conduction and convection, radiation can transfer heat through a vacuum, such as the space between the Sun and the Earth. All objects emit and absorb thermal radiation. The hotter an object, the more thermal radiation it emits. Dark, dull surfaces are good absorbers and emitters of radiation, while light, shiny surfaces are poor absorbers and emitters (good reflectors).

According to the kinetic molecular theory, the particles of a hot object vibrate and oscillate, causing them to emit electromagnetic waves. When these waves strike another object, their energy is absorbed by the particles of that object, increasing their kinetic energy and thus raising its temperature.

Measuring Temperature Using a Simple Thermometer

A simple thermometer, such as a liquid-in-glass thermometer, measures temperature based on the thermal expansion of a liquid (mercury or alcohol).

The procedure for measuring temperature involves the following steps:

1. Select the correct thermometer: Ensure the thermometer is suitable for the temperature range to be measured.
2. Hold the thermometer correctly: Hold the thermometer by the top end, away from the bulb, to avoid transferring body heat to the bulb.
3. Place the bulb in contact: Immerse the bulb of the thermometer fully into the substance whose temperature is to be measured.
4. Wait for equilibrium: Allow sufficient time for the liquid in the thermometer to expand or contract and stabilise. The liquid column will stop moving when it reaches the same temperature as the substance.
5. Read the temperature: Read the temperature by looking at the level of the liquid column against the scale. Ensure your eye is level with the top of the liquid column to avoid parallax error.
6. Remove and clean: Carefully remove the thermometer and clean it if necessary.

Safety Tip: Handle glass thermometers with care to prevent breakage. Do not use a mercury thermometer to measure very high temperatures that exceed its range, as it may burst.

Measuring Thermal Expansion of Metallic Materials

Thermal expansion of metallic materials can be demonstrated and measured using apparatus like the ball and ring apparatus or a linear expansivity apparatus.

Using the Ball and Ring Apparatus

The ball and ring apparatus demonstrates linear expansion in a simple way.

The procedure involves the following steps:

1. Check initial state: At room temperature, ensure the metal ball can just pass through the metal ring.
2. Heat the ball: Heat the metal ball using a burner for a few minutes.
3. Attempt to pass the ball through the ring: Immediately after heating, try to pass the hot ball through the ring.
4. Observe: The hot ball will no longer pass through the ring because it has expanded.
5. Cool the ball: Allow the ball to cool down or cool it by immersing it in cold water.
6. Re-attempt to pass the ball: Once cooled, the ball will again pass through the ring, demonstrating that it has contracted back to its original size.

Using a Linear Expansivity Apparatus

A linear expansivity apparatus is used to measure the actual change in length of a metal rod for a given temperature change, allowing for the calculation of the coefficient of linear expansivity.

The procedure involves the following steps:

1. Set up the apparatus: Secure a metal rod (e.g., copper or brass) in the apparatus. One end of the rod is fixed, and the other end rests against a micrometer screw gauge or a dial gauge.
2. Measure initial length: Record the initial length (L₀) of the rod.
3. Measure initial temperature: Record the initial temperature (T₁) of the rod.
4. Heat the rod: Pass steam through a jacket surrounding the rod to heat it uniformly.
5. Measure final temperature: Once the temperature stabilises, record the final temperature (T₂) of the rod.
6. Measure change in length: Read the change in length (ΔL) directly from the micrometer screw gauge or dial gauge.
7. Calculate expansivity: Use the formula ΔL = L₀αΔT to calculate the coefficient of linear expansivity (α), where ΔT = T₂ – T₁.

Safety Tip: Be careful when handling hot apparatus and steam. Use heat-resistant gloves and ensure the steam generator is operated safely.