Electric cells as energy systems
An electric cell is a device that converts stored chemical energy into electrical energy. Inside a cell, chemical reactions between the electrodes and the electrolyte push electrons around a circuit when it is connected.
- A single cell has a positive terminal and a negative terminal, and provides a fixed voltage (e.g. 1.5 V for a standard AA cell).
- A battery is technically two or more cells joined together.
- When cells are connected in series (positive to negative, end to end), their voltages add up โ e.g. two 1.5 V cells in series give 3 V.
EMF (electromotive force) is the energy the cell gives to each unit of charge that passes through it โ this is what we measure as the cell's voltage when no current is flowing.
Resistance
Resistance is the opposition a material offers to the flow of electric current, measured in ohms (ฮฉ). Resistance is caused by collisions between the moving electrons and the particles of the conductor, which convert some electrical energy into heat.
Several factors affect the resistance of a conductor:
- Length: a longer conductor has more resistance โ electrons collide with more particles along the way.
- Thickness (cross-sectional area): a thicker conductor has less resistance โ there is more space for current to flow.
- Material: different materials conduct differently. Copper is an excellent conductor (low resistance); nichrome and other alloys have higher resistance and are used in resistors and heating elements.
A resistor is a component deliberately placed in a circuit to control the amount of current flowing. In circuit diagrams, a resistor is drawn as a rectangle (or zig-zag in some notations).
Series circuits
In a series circuit, components are connected one after another, forming a single loop or path for current:
- There is only ONE path for current to flow.
- The current is the same at every point in the circuit.
- The total voltage of the source is shared (divided) between the components.
- If ONE component breaks or is removed, the circuit is broken and everything stops working.
Parallel circuits
In a parallel circuit, components are connected across separate branches between the same two points:
- There are multiple paths for current to flow.
- The voltage is the same across each branch (each branch gets the full supply voltage).
- The current is shared between the branches โ the total current from the source equals the sum of the currents in each branch.
- If ONE component breaks, current can still flow through the other branches โ they keep working.
Series: one path, current constant, voltage shared | Parallel: many paths, voltage constant, current shared
Circuit diagram symbols
| Component | Symbol description | Function |
|---|
| Cell | One long line (+) and one short line (โ) | Provides electrical energy (EMF) |
| Battery | Two or more cell symbols side by side | Several cells joined together |
| Bulb (lamp) | Circle with an X inside | Converts electrical energy to light (and heat) |
| Resistor | Rectangle | Opposes current flow, controlling its size |
| Switch | A break in the line that can open/close | Opens or closes the circuit |
| Wire (conductor) | Straight line | Connects components with (almost) no resistance |
Home wiring uses parallel circuits โ this way, each appliance receives the full mains voltage and can be switched on or off independently without affecting the others.
Safety with electricity
Electricity can be extremely dangerous if not treated with respect. Grade 9 learners must understand the following safety principles:
- Insulation: materials like rubber and plastic do not conduct electricity. Wires are coated in insulation to prevent electric shock and short circuits.
- Fuses: a thin wire designed to melt and break the circuit if the current becomes too large, preventing overheating and fires.
- Circuit breakers: a switch that automatically "trips" (opens) when it detects too much current, protecting the wiring โ unlike a fuse, it can be reset and reused.
- Earthing (earth wire): provides a safe path for current to flow into the ground if a fault occurs, rather than through a person.
Water and electricity: water (especially with dissolved salts/minerals) conducts electricity much better than dry skin. This is why using electrical appliances near water, or touching switches with wet hands, is extremely dangerous โ the risk of severe or fatal electric shock is much higher.
Safe practices at home: never overload a plug socket with too many appliances; never touch damaged or exposed wires; switch off and unplug appliances before cleaning them; never insert objects into electrical sockets; keep electrical appliances away from bathrooms, sinks, and swimming pools.