Generating Electricity & Logic Gates

Electrochemical cells

An electrochemical cell produces electricity from a chemical reaction between two different metals (electrodes) in an acidic or salty solution (electrolyte). You can build a simple cell using a lemon, potato, or salt water with a zinc and copper electrode pushed into it — the chemical reaction between the two different metals produces a small voltage.

Series vs parallel batteries

Photovoltaic (solar) cells

A photovoltaic cell converts light energy directly into electrical energy. Advantages: free and renewable fuel (sunlight), no moving parts, no emissions while operating, useful in remote areas without grid access. Disadvantages: only generates electricity in daylight, output drops in cloudy weather, needs batteries or backup for night-time use, and panels are relatively expensive and resource-intensive to manufacture and eventually recycle.

Generating electricity for the nation

South Africa (and most countries) generate large-scale electricity using several different methods, each with trade-offs:

• Thermal power stations: heat (from burning coal or gas, from nuclear reactions, or concentrated solar heat) boils water into steam, which spins a steam turbine connected to a generator. Advantage: reliable, constant output. Disadvantage: coal and gas release greenhouse gases and pollutants; nuclear waste must be safely stored for centuries.
• Hydroelectric power stations: falling or flowing water spins turbines directly. Pumped storage schemes pump water uphill to a reservoir when demand (and electricity prices) are low, then release it downhill through turbines to generate power quickly when demand is high — effectively "storing" electricity. Advantage: renewable, quick to switch on. Disadvantage: needs a suitable river/dam site, can disrupt ecosystems and communities.
• Wind-driven turbines: wind spins large blades connected to a generator. Advantage: renewable, no fuel cost. Disadvantage: only works when wind blows, can affect birds and landscapes, output is variable.

Energy access, ethics & equity

Not everyone in South Africa has equal, reliable access to the national grid. Many rural areas and informal settlements are not yet connected, and residents there often rely on burning wood, paraffin or coal for heating, lighting and cooking — options that are far more dangerous (fire and poisoning risk) and less convenient than mains electricity.

Where a legal connection isn't available, some people make illegal connections to nearby power lines. This raises serious ethical and safety issues: illegal connections are extremely dangerous (a major cause of electrocution and shack fires), and they also place extra, uncosted load on the grid, which can cause outages that affect the whole area, including houses that are legally connected.

Alternating current and the national grid

Power stations generate alternating current (AC) — current that regularly reverses direction — because AC can be efficiently changed to a different voltage using a transformer. A step-up transformer raises the voltage to very high levels (hundreds of thousands of volts) for transmission over long-distance power lines, since higher voltage means lower current for the same power, which means less energy is wasted as heat in the cables. A step-down transformer then reduces this high voltage back down to safe levels for homes, schools and factories. This network of power stations, transformers, and power lines that moves electricity across the whole country is called the national grid.

Ohm's Law — a qualitative introduction

At Grade 8 level, no calculations are required — only the general relationship. If you connect one cell, then two cells, then three cells in series to the same lamp (keeping the resistance of the circuit the same), the lamp gets progressively brighter. This is because more cells in series means more voltage, and if the resistance in the circuit stays the same, more voltage causes more current to flow. More current through the lamp's filament means a brighter light.

AND and OR logic gates

Logic gates describe how switches combine to control an output. Two switches can be arranged in two different ways:

• AND logic — switches in series: the lamp/buzzer only switches on if switch A and switch B are both closed. If either switch is open, the circuit is broken and nothing happens. This is useful when you want two separate conditions to both be true before something activates — like a panic button that needs both a key switch and a press-button activated together to reduce false alarms.
• OR logic — switches in parallel: the lamp/buzzer switches on if switch A or switch B (or both) are closed, because current can flow through either branch. This is useful when any one of several conditions should trigger the same result — like a panic-button alarm system with buttons in the bedroom, hallway and kitchen: pressing any one of them should sound the alarm.