Lesson Notes By Weeks and Term v5 - Grade 9
The national electricity supply system – Week 7 focus
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The national electricity supply system – Week 7 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Natural Sciences
Class: Grade 9
Term: 2nd Term
Week: 7
Theme: General lesson support
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In South Africa, access to electricity is not just a convenience; it's fundamental to our daily lives, impacting everything from studying after dark and cooking meals, to running businesses and accessing healthcare. Unfortunately, we also face significant challenges with electricity supply, including load shedding and rising costs. Understanding how our national electricity supply system works is crucial for all South Africans to be informed citizens and potentially contribute to future solutions. This week, we'll investigate where our electricity comes from, how it's distributed, and some of the challenges we face.
2. 1.
The National Electricity Supply System: An Overview The South African national electricity supply system is a complex network that involves generating electricity, transmitting it over long distances, and distributing it to homes, businesses, and industries. It can be broken down into three main stages: Generation: Electricity is produced in power stations.
Transmission: High-voltage electricity is transported over long distances through a network of transmission lines.
Distribution: Electricity is distributed to end-users (homes, businesses, etc.) through a network of distribution lines. 2.
2. Electricity Generation: Coal-Fired Power Stations South Africa relies heavily on coal-fired power stations for electricity generation.
Here's how it works: Coal Combustion: Coal is burned in a furnace to produce heat. This is a chemical reaction – the carbon in the coal combines with oxygen in the air, releasing a large amount of energy in the form of heat. Why Coal?* South Africa has abundant coal reserves, making it a relatively inexpensive fuel source.
Water Heating: The heat is used to boil water, producing high-pressure steam.
Steam Turbine: The high-pressure steam is directed at a turbine, causing it to spin. A turbine is essentially a large fan connected to a generator.
Generator: The spinning turbine turns a generator, which converts mechanical energy (the turbine's spinning motion) into electrical energy. Generators use the principle of electromagnetic induction: moving a conductor (like a wire coil) through a magnetic field generates an electric current.
Cooling: The steam exiting the turbine is cooled and condensed back into water, which is then pumped back to the boiler to repeat the cycle. Large cooling towers are often used to cool the water.
Environmental Impact: Burning coal releases greenhouse gases (like carbon dioxide), which contribute to climate change. It also releases pollutants like sulfur dioxide and nitrogen oxides, which can cause acid rain and respiratory problems. Ash (the non-combustible residue of coal) must also be disposed of. 2.
3. Other Electricity Sources in South Africa While coal dominates, South Africa also uses other energy sources: Nuclear Power: Koeberg Nuclear Power Station near Cape Town uses nuclear fission to generate electricity.
Advantages: Low greenhouse gas emissions during operation.
Disadvantages:* Nuclear waste disposal, potential for accidents.
Hydroelectric Power: Dams like the Gariep Dam use the potential energy of water to turn turbines and generate electricity.
Advantages: Renewable energy source.
Disadvantages:* Dependent on rainfall, can disrupt ecosystems. Renewable Energy (Solar, Wind): Solar photovoltaic (PV) panels convert sunlight directly into electricity. Wind turbines use wind to turn turbines and generate electricity.
Advantages: Renewable, low emissions.
Disadvantages:* Intermittent (dependent on weather), can require large land areas. 2.
4. Electricity Transmission: Transformers and High Voltage After electricity is generated, it must be transmitted over long distances. This is done using high-voltage transmission lines. Why High Voltage? Transmitting electricity at high voltage reduces energy loss due to resistance in the wires. The power lost due to resistance is given by the formula P = I²R, where P is power loss, I is current, and R is resistance. By increasing the voltage, the current can be reduced for the same amount of power transmitted, thus reducing power loss.
Transformers: Transformers are used to step up (increase) the voltage for transmission and step down (decrease) the voltage for distribution.
Step-up transformers: Increase the voltage from the power station to the high voltage required for transmission.
Step-down transformers: Decrease the voltage from the transmission lines to the lower voltage needed for distribution to homes and businesses.
Suppose a power station generates 100 MW (megawatts) of power at 11 kV (kilovolts). It needs to transmit this power over a long distance with a transmission line that has a resistance of 10 ohms.
Without a transformer: The current would be I = P/V = 100,000,000 W / 11,000 V = 9090.9 A. The power loss would be P = I²R = (9090.9 A)² 10 ohms = 826,446,281 W = 826.4 MW. This is more power lost than generated!
With a step-up transformer: Let's say the transformer steps up the voltage to 400 kV. The current would be I = P/V = 100,000,000 W / 400,000 V = 250 A. The power loss would be P = I²R = (250 A)² 10 ohms = 625,000 W = 0.625 MW. This is a significantly smaller power loss.
2. 5.
Electricity Distribution: Substations and Power Lines
After being transmitted at high voltage, electricity is stepped down to lower voltages at substations for distribution to homes and businesses.
Substations: Substations contain transformers that reduce the voltage to levels suitable for distribution.
Distribution Lines: These lines carry electricity from substations to homes, businesses, and other users. Local transformers, often mounted on poles, further step down the voltage to the standard voltage used in homes (typically 230V in South Africa).