Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define potential and kinetic energy with South African examples.
• Explain energy transfer: conduction, convection, and radiation.
• Describe energy transformations in everyday appliances.
• Explain the Law of Conservation of Energy.
• Describe how energy is used in sectors like electricity generation.

Lesson summary

Energy is the driving force behind everything that happens around us. From the sun warming our faces to a car moving down the N1, energy is constantly being transferred and transformed. Understanding energy and its transfer is crucial because it helps us understand how things work, solve problems related to energy usage (like saving electricity at home), and even develop new technologies for a more sustainable future. In South Africa, with ongoing challenges related to energy supply (load shedding) and environmental concerns, grasping these concepts becomes even more important.

Lesson notes

2.1 What is Energy? Energy is the ability to do work. Work, in a scientific sense, is done when a force causes an object to move a certain distance. Energy is measured in Joules (J). 2.2 Forms of Energy: There are many different forms of energy, including: Kinetic Energy: The energy of motion. Anything that is moving has kinetic energy. The faster it moves, the more kinetic energy it has.

Example: A speeding taxi, water flowing in the Orange River, a child running to catch a kite on Clifton Beach.

Potential Energy: Stored energy that has the potential to do work. There are several types of potential energy: Gravitational Potential Energy: Energy stored due to an object's height above the ground. The higher the object, the more gravitational potential energy it has.

Example: Water stored in a dam high in the Drakensberg mountains, a rock poised at the top of a hill in the Karoo.

Elastic Potential Energy: Energy stored in a stretched or compressed object.

Example: A stretched elastic band, a compressed spring in a car's suspension.

Chemical Potential Energy: Energy stored in the bonds of molecules.

Example: The energy stored in firewood, coal, food, or a battery.

Heat (Thermal)

Energy: Energy associated with the temperature of an object. The hotter the object, the more heat energy it has.

Example: Boiling water in a kettle, the hot air rising from a fire.

Light (Radiant)

Energy: Energy that travels in electromagnetic waves.

Example: Sunlight, energy from a light bulb, energy from a fire.

Sound Energy: Energy that travels in waves through a medium (like air or water).

Example: The sound of music from a radio, the noise of traffic.

Electrical Energy: Energy associated with the flow of electric charge.

Example: Electricity powering a light bulb, electricity used in a cell phone. 2.3 Energy Transfer: Energy can be transferred from one object to another or from one form to another. There are three main ways heat energy is transferred: Conduction: The transfer of heat energy through a material by direct contact. Heat moves from the hotter part of the object to the colder part. This works best in solids. Think of a metal spoon in a hot cup of tea; the spoon heats up because of conduction.

Example: Heating up a pot on a stove. The heat from the stove conducts through the pot to cook the food inside.

Convection: The transfer of heat energy by the movement of fluids (liquids and gases). Hotter fluids are less dense and rise, while cooler fluids are denser and sink, creating a current.

Example: A hot air balloon rises because the hot air inside is less dense than the cool air outside, the ocean currents along the South African coast.

Radiation: The transfer of heat energy by electromagnetic waves. This is the only way heat can travel through a vacuum (like space).

Example: The sun warming the Earth, the heat you feel from a fire. 2.4 Energy Transformation: Energy can change from one form to another. This is called energy transformation.

Examples: A car engine converts chemical potential energy (from petrol/diesel) into heat energy and then into kinetic energy. A solar panel converts light energy from the sun into electrical energy. A kettle converts electrical energy into heat energy to boil water. A coal-burning power station converts chemical potential energy (from coal) into heat energy, then into kinetic energy (steam turning turbines), and finally into electrical energy. 2.5 The Law of Conservation of Energy: The Law of Conservation of Energy states that energy cannot be created or destroyed, but it can be transferred from one object to another or transformed from one form to another. The total amount of energy in a closed system remains constant.

Example: When a light bulb is switched on, electrical energy is converted into light energy and heat energy. The total amount of energy before and after the transformation remains the same. 2.6 South African

Examples: Solar Geyser: Converts light energy (sunlight) into heat energy to warm water.

Coal-burning Power Station: Converts chemical potential energy (coal) into heat energy, then kinetic energy (steam turning turbines), and finally into electrical energy.

Eskom Power Lines: Transfers electrical energy from power stations to homes and industries.

Burning Wood for Cooking (Braai): Converts chemical potential energy (wood) into heat and light energy.

Example Calculation: A 2 kg book is held 1.5 meters above the floor. Calculate its gravitational potential energy (GPE). Use g = 9.8 m/s².

Solution: GPE = mgh Where: m = mass (2 kg) g = acceleration due to gravity (9.8 m/s²) h = height (1.5 m) GPE = (2 kg) (9.8 m/s²) (1.5 m) GPE = 29.4 J Therefore, the book has 29.4 Joules of gravitational potential energy. Guided Practice (With Solutions)

Question 1: Identify the main type of energy transformation that occurs in a burning candle. Explain your answer.

Solution: Chemical potential energy (stored in the wax) is transformed into light energy and heat energy.