Question

Give an example for each of the following situations: (a) adding heat to a system raises its temperature, (b) adding heat to a system does not change its temperature, and (c) a system's temperature changes despite no heat being added to it or removed from it.

Short answer

(a) Heating water in a kettle, (b) ice melting, (c) gas expanding in an insulated container.

Step-by-step solution

Understanding Situation (a)

In situation (a), when heat is added to a system, the kinetic energy of its particles increases, which raises the system's temperature. A common example is heating water in a kettle. As heat is added, the water's temperature increases until it starts to boil.

Understanding Situation (b)

Situation (b) describes a phase change where adding heat to a system does not change its temperature. An example is ice melting into water. The temperature remains at 0°C while the ice absorbs heat until it completely melts.

Understanding Situation (c)

In situation (c), the temperature can change without adding or removing heat, usually due to a change in pressure or volume. An example is gas expanding in an insulated container. As the gas expands, it does work on its surroundings, leading to a decrease in temperature even though no heat is added or removed from the system.

Key concepts

Temperature Change

Temperature change occurs when the thermal energy within a system is altered, usually by adding or removing heat. The temperature of an object is a measure of the average kinetic energy of its particles. So, when heat is added, the particles move faster, and the temperature rises. Conversely, when heat is removed, the particles slow down, and the temperature decreases.

For example, consider heating a pot of water. As the heat energy from the stove is transferred to the water, the average kinetic energy of water molecules increases. This rise in kinetic energy results in an increase in temperature until the water reaches its boiling point and undergoes a phase change.

Heat Transfer

Heat transfer defines the movement of thermal energy from one object or substance to another. It occurs through conduction, convection, or radiation.

- **Conduction** involves direct contact, like touching a hot pan. - **Convection** is the movement of heat by fluid motion, seen in boiling water. - **Radiation** transfers heat through electromagnetic waves, like sunlight warming your face.

In any heat transfer process, the energy flows from a warmer object to a cooler one until thermal equilibrium is reached. An example is leaving a metal spoon in a hot cup of tea; the heat conducts from the tea to the spoon, making it warm to the touch.

Phase Change

A phase change happens when a substance transitions from one state of matter to another, such as solid to liquid, or liquid to gas. These changes occur at specific temperatures and require energy addition or removal without changing the temperature of the substance.

For instance, the melting of ice into water is a phase change that takes place at 0°C. Even though ice absorbs heat, its temperature remains constant until all the ice has melted. This energy absorbed or released during a phase change is called latent heat. It is crucial in applications like cooling systems and weather phenomena.

Kinetic Energy

Kinetic energy is the energy of motion. In the context of thermodynamics, it refers to the energy possessed by particles as they move. The faster the particles move, the higher the kinetic energy and, typically, the temperature.

When heat is added to a system, the kinetic energy of the particles increases, which generally results in an increase in temperature. This link between kinetic energy and temperature is foundational for understanding why heating substances often result in temperature change, except during a phase change as previously discussed. In a gas, this energy is visible as increased pressure, explained by the ideal gas laws.

Pressure and Volume Effects

Pressure and volume changes can also influence the temperature of a system without adding or removing heat. This phenomenon is observable when gases expand or compress adiabatically, meaning there is no heat exchange with the environment.

For example, when a gas in a piston expands, it does work on its environment. The energy for this work comes from the internal energy of the gas, so its temperature decreases even though no heat is transferred out of the system. This principle is a cornerstone of processes like refrigeration and explains why aerosol cans cool as they are used.