Technical Science - Term 2 - Week 3

Thermodynamics is the branch of physics that deals with heat, work, and energy. It is governed by four fundamental laws that describe how energy is transferred and transformed. The first law, known as the law of energy conservation, states that energy cannot be created or destroyed, only transformed from one form to another. The second law introduces the concept of entropy, indicating that energy transformations are not 100% efficient and that systems tend to move towards a state of disorder.

Heat can be transferred in three primary ways: conduction, convection, and radiation. Conduction occurs through direct contact between materials, where heat flows from the hotter object to the cooler one. Convection involves the movement of fluids (liquids or gases) where warmer areas of a liquid or gas rise and cooler areas sink, creating a cycle. Radiation is the transfer of heat through electromagnetic waves, which can occur in a vacuum, such as the heat from the sun reaching the Earth.

Consider a closed system where 500 J of heat is added to a gas, and the gas does 200 J of work on its surroundings. According to the first law of thermodynamics, the change in internal energy (ΔU) of the gas can be calculated using the formula ΔU = Q - W, where Q is the heat added and W is the work done. Therefore, ΔU = 500 J - 200 J = 300 J. This means the internal energy of the gas increases by 300 J.

If a metal rod is heated at one end, the heat will travel to the cooler end through conduction. For instance, if the hot end of the rod is at 100°C and the cool end is at 20°C, the temperature gradient causes heat to flow from the hot end to the cool end. The rate of heat transfer can be calculated using Fourier's law of heat conduction, which states that the heat transfer rate is proportional to the temperature difference and the area through which heat is conducted.

In a thermodynamic process, a gas expands against a piston, doing work. If the gas expands from a volume of 1 m³ to 3 m³ under a constant pressure of 100 kPa, calculate the work done by the gas. The work done (W) can be calculated using the formula W = P × ΔV, where ΔV is the change in volume. Here, ΔV = 3 m³ - 1 m³ = 2 m³. Thus, W = 100 kPa × 2 m³ = 200 kJ. This exercise helps students understand how to apply the concepts of pressure and volume in thermodynamics.

Students will solve a set of problems related to thermodynamics, including calculating changes in internal energy, work done in various processes, and identifying heat transfer methods in different scenarios. For example, if a system absorbs 400 J of heat and does 150 J of work, what is the change in internal energy? Students will work individually and then discuss their solutions in small groups to reinforce their understanding.

• Energy can be created or destroyed.
• Energy cannot be created or destroyed.
• Energy is always lost as heat.
• Energy can only be transformed into work.

Answer: Energy cannot be created or destroyed.

The first law of thermodynamics, also known as the law of energy conservation, states that energy can only change forms but cannot be created or destroyed.

• Conduction
• Convection
• Radiation
• Insulation

Answer: Radiation

Radiation is the transfer of heat through electromagnetic waves and can occur in a vacuum, unlike conduction and convection which require a medium.

Answer: Entropy is a measure of the disorder or randomness in a system.

In thermodynamics, entropy quantifies the amount of energy in a physical system that is not available to do work, reflecting the level of disorder.

• W = P × V
• W = P × ΔV
• W = ΔU + Q
• W = Q - ΔU

Answer: W = P × ΔV

The work done by a gas during expansion or compression is calculated using the formula W = P × ΔV, where P is pressure and ΔV is the change in volume.

Answer: Conduction is the transfer of heat through direct contact between materials.

In conduction, heat energy is transferred from the hotter part of a material to the cooler part through molecular collisions.

• Heating a metal rod in a flame
• Boiling water in a pot
• Feeling warmth from a heater
• Sunlight warming the ground

Answer: Boiling water in a pot

Convection occurs in fluids where warmer areas rise and cooler areas sink, as seen when boiling water.