Electrical Systems (G3 Lower Secondary)

Electrical charge

Property of protons (positive charge) and electrons (negative charge).

Current

Rate of flow of electric charge

(“How fast the water flows”)

A (Amperes)

SI unit for current

Ammeter

Device used to measure current

(Connect in series)
(Connect + to + terminal of cell, and - to - terminal of cell)

Potential Difference

The potential difference between two points is the amount of energy needed to move one unit of electric charge from one point to another.

(“How much energy you need to push a ball all the way up the hill” or “how steep the slope down which the river flows”)

Current usually will not flow if there is no potential difference.

V (Volts)

SI unit for potential difference

Voltmeter

Device used to measure potential difference

(Connect in parallel)
(Connect + to + terminal of cell, and - to - terminal of cell)

Resistance

How much a component resists (or opposes) the flow of electrical current.
or
The ratio of potential difference across the component to the current flowing through it (Resistance = Voltage / Current)

(“How skinny the river is”)

Ω (Ohms)

SI unit for resistance

Resistor

Device that restricts the flow of electrical current.

Low resistance: Copper wires
High resistance: Electrical insulators, devices that convert electrical energy to light and/or heat energy (e.g., components in light bulbs, irons)

Electrical Power

Rate of transfer of electrical energy
or
Amount of electrical energy converted to other forms of energy per unit time by the component (Power = Energy / Time)

W (Watts)

SI unit for power

1000 W = 1 kW

Converting watts (W) to kilowatts (kW)

Electrical energy

Energy derived from electric current

kWh (Kilowatt Hours)

Common (non-SI) unit for electrical energy

1 kWh = 1 Kw × 1 h

Finding kilowatt hours (kWh) from power (in kW) and time (in h)

Cell (Battery)

Component drawn with Long | (+) short | (-)

Light bulb

Component drawn as a circle with X inside

Closed circuit

Complete path for electricity to flow through

(“Closed loop”)

Fixed Resistor

Component drawn as a rectangle

Resistor whose resistance cannot be changed

Variable Resistor

Component drawn as a rectangle with diagonal arrow

Resistor whose resistance can be changed: The more to the right the slide is, the higher the resistance is.)
(Also known as a rheostat)

More current: Brightness increases

How current affects the brightness of light bulb

Chemical effect of electrical current

Electrical current causing chemical change

Examples: Electrolysis, Electroplating (e.g., plating an iron object with copper to prevent rusting)

Magnetic effect of electrical current

Effect of electrical current on magnets

Examples: Making electromagnets (by making current flow in a coil of wire called a solenoid), use in electric fans and telephones

Heating effect of electrical current

Electrical energy can be converted into heat energy when passed through some materials.

Examples: Heating elements in kettles and irons

Lighting effect of electrical current

Electrical energy can be converted into light energy when passed through some materials.

Examples: Filaments of light bulbs

Electrical fires

Happen when current is too large. This generates heat, which may melt the insulation and cause a fire.

Examples:
- Overloading (inserting too many plugs into the same socket)
- Damaged insulation
- Damaged electrical components

Electrocution

Happens when large currents pass through a person’s body.

Examples:
- Touching electrical parts with wet hands
- Touching electrical plugs without switching off the power first
- Damaged insulation
- Damaged electrical components

Earth

Wire found in the household which is connected to the ground (or earth)
Provides a path of low electrical resistance for the current to flow into the ground (or earth), preventing the current from reaching the user

In plug: Wire on top in the plug
In cable: Yellow and green

Live

Wire found in household and in plugs, which carries electrical current from power source to the device
Found next to the fuse in the plug, on the right
Brown wire in cable

Neutral

Wire found in household and in plugs, which carries electrical current from the device back to the power source
Found on the left in the plug
Blue wire in cable

Fuse

Component drawn as a rectangle with line

Contains a thin wire which melts when a large current passes through it (heating effect of electricity)
When the wire melts, the fuse is ‘blown’ and needs to be replaced.

Circuit breaker

Switch which turns off (‘trips’) when the current flowing through some part of the circuit is too large
Unlike a fuse, it does not need to be replaced. When a damaged part of the circuit is fixed, the circuit can be switched on again using the circuit breaker.