Heat and Energy

The One Thing

Energy is conserved while being transferred or transformed between forms. Heat is energy transferred because of a temperature difference, whereas work is energy transferred by a force causing displacement and power is the rate of that transfer.

Reactions, Processes and Experiments

Key Terms

• Energy: The capacity of a body or system to do work. Its SI unit is the joule (J).
• Heat: A form of energy transferred from a body at higher temperature to a body at lower temperature.
• Temperature: A measure of the degree of hotness or coldness of a body.
• Internal Energy: The total energy associated with the random motion and interactions of particles inside a substance.
• Conduction: Transfer of heat through a substance without the actual movement of its particles from one place to another; it is common in solids.
• Convection: Transfer of heat through the bulk movement of fluids such as liquids and gases.
• Radiation: Transfer of heat through electromagnetic waves without requiring a material medium.
• Thermal Conductor: A material that allows heat to pass through it easily, such as copper or aluminium.
• Thermal Insulator: A material that does not allow heat to pass through it easily, such as wood, plastic, or air.
• Work: Work is done when a force produces displacement in the direction of the force.
• Power: The rate at which work is done or energy is transferred.
• Kinetic Energy: The energy possessed by a body due to its motion.
• Potential Energy: The energy possessed by a body because of its position, shape, or condition.
• Mechanical Energy: The sum of the kinetic energy and potential energy of a body.
• Law of Conservation of Energy: Energy can neither be created nor destroyed; it can only be transformed from one form to another.
• Renewable Energy: Energy obtained from sources that are naturally replenished, such as sunlight, wind, and flowing water.
• Non-renewable Energy: Energy obtained from limited sources that take a very long time to form, such as coal, petroleum, and natural gas.
• Thermal Equilibrium: The condition in which bodies in thermal contact attain the same temperature.
• Latent Heat: Heat absorbed or released during a change of state without a change in temperature.
• Joule (J): The SI unit of heat, energy, and work.
• Kilowatt-hour (kWh): A common commercial unit of electrical energy; 1 kilowatt-hour = 3.6 × 10^6 joules.
• Watt (W): The SI unit of power; 1 watt = 1 joule per second.

Easily Confused

• Heat and temperature: Temperature describes the thermal condition of a body, whereas heat is energy transferred because of a temperature difference.
• Heat and internal energy: Heat is energy being transferred, whereas internal energy is the total energy associated with particles within a substance.
• Conduction and convection: Conduction transfers heat without bulk particle movement and is common in solids; convection transfers heat through the bulk movement of fluids.
• Convection and radiation: Convection requires a fluid medium, whereas radiation can travel through a vacuum.
• Work and energy: Energy is the capacity to do work, whereas work is energy transferred by a force causing displacement.
• Work and power: Work measures energy transferred; power measures how quickly the transfer occurs.
• Kinetic energy and potential energy: Kinetic energy results from motion, whereas potential energy results from position, shape, or condition.
• Renewable and non-renewable energy: Renewable sources are naturally replenished, whereas non-renewable sources are limited and take a very long time to form.
• Thermal conductors and thermal insulators: Thermal conductors allow heat to pass through easily, whereas thermal insulators restrict heat transfer.

What Gets Asked

• Define heat, temperature, and internal energy, and distinguish heat from temperature. The key mark is recognising that heat is transferred energy, not simply the degree of hotness.
• Identify the method of heat transfer in a situation involving a solid, a moving liquid or gas, or empty space. The common slip is assigning convection to solids or radiation to a process requiring particle movement.
• State and apply the equations W = F × s, W = F s cos θ, P = W/t = E/t, K.E. = 1/2 mv^2, and P.E. = mgh. Marks are lost by confusing work with power or omitting the angle factor in the general work equation.
• Convert between commercial and SI units of energy using 1 kilowatt-hour = 3.6 × 10^6 joules. The specific error is failing to distinguish kilowatt-hour from watt.
• Explain conservation of energy in machines, moving bodies, electrical devices, and heat processes. The required point is that energy may be transformed or dissipated as heat, but total energy is not created or destroyed.
• Describe radiation from different surfaces and changes of state. The relevant distinctions are that dark, dull surfaces are good absorbers and emitters, bright, polished surfaces are poor absorbers and emitters, and latent heat is transferred without a temperature change.