Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define energy transfer and give real-world examples.
• Describe conduction, convection, and radiation.
• Explain the law of conservation of energy.
• Identify energy transfer in South African contexts.
• Explain how insulation reduces energy transfer.

Lesson summary

This week, we delve deeper into the fascinating world of energy! Last week, we introduced what energy is. This week, we're focusing on how energy moves and changes form. Understanding energy transfer is crucial because it explains how everything around us works, from the food we eat giving us the power to play soccer, to the electricity that powers our homes and schools. In South Africa, with its unique energy challenges and abundant natural resources, understanding energy transfer is especially important for future innovators and problem-solvers. We can use this knowledge to build more sustainable and efficient energy systems for our country.

Lesson notes

Energy transfer is the movement of energy from one object or system to another. Energy doesn't just disappear; it transforms from one form to another or moves from one place to another. The Law of Conservation of Energy states that energy cannot be created or destroyed, only transformed or transferred. There are primarily three ways energy is transferred as heat: Conduction: This is the transfer of heat through a material by direct contact. Heat moves from a hotter object to a colder object when they are touching. Conduction works best in solids because the particles are close together.

Why it works:* When you heat one part of a solid, the particles in that area vibrate faster. These faster-vibrating particles bump into their neighbours, causing them to vibrate faster as well. This process continues, transferring the energy (in the form of heat) throughout the material.

Example:* Imagine you're stirring hot pap (maize porridge) with a metal spoon. The end of the spoon in the pap gets hot quickly because heat is conducted through the metal. A wooden spoon would heat up much slower because wood is a poor conductor of heat (it's an insulator). Another

Example:* Walking barefoot on hot tar (asphalt) in South Africa. The tar conducts the heat from the sun into your feet. Ouch!

Convection: This is the transfer of heat by the movement of fluids (liquids and gases). When a fluid is heated, it becomes less dense and rises. Cooler, denser fluid then sinks to take its place, creating a convection current.

Why it works:* Heated fluids expand, making them less dense. This less dense fluid is buoyed upwards by the surrounding denser fluid, creating currents. These currents carry the heat throughout the fluid.

Example:* Think of heating water in a pot on a stove. The water at the bottom gets heated by conduction from the pot. This hot water becomes less dense and rises. Cooler water from the top sinks to take its place, creating a convection current. Eventually, all the water in the pot will be hot. Another

Example:* The sea breeze in coastal South African towns. During the day, the land heats up faster than the sea. The hot air over the land rises, creating a low-pressure area. Cooler air from over the sea rushes in to replace it, creating a sea breeze.

Radiation: This is the transfer of heat by electromagnetic waves. Unlike conduction and convection, radiation does not require a medium (like a solid, liquid, or gas) to travel. This is how the Sun's energy reaches Earth.

Why it works:* All objects emit electromagnetic radiation. The hotter the object, the more radiation it emits, and the shorter the wavelength of that radiation. When this radiation is absorbed by an object, the object's particles gain energy, and it heats up.

Example:* Feeling the warmth of a braai (barbecue) even if you're not directly touching the coals. The hot coals are radiating heat in all directions. Another

Example:* Warming yourself in the sun on a cold winter's day in the Karoo. The sun's radiant energy is warming your skin directly.

Energy Transformations: Often, energy isn't just transferred; it's also transformed from one form to another.

Example:* In a solar geyser, radiant energy from the sun is transformed into thermal energy (heat), which is then transferred to the water. The heated water can then be used for showering or washing.