Electrical Systems and Energy Consumption and Safety Study Notes

WHAT DOES AN ELECTRICAL SYSTEM COMPRISE?

Definition and Necessity of Electrical Systems
- Electrical appliances such as mobile phones and ECG machines cannot function autonomously; they must be integrated into an electrical system comprised of various components.
- Case Study: ECG Machine: This system consists of probes pasted on human skin, connected to the electrical mains or a portable battery pack. The closed circuit formed by the probes, the human body, and the power source allows the machine to function.
Standard Circuit Components and Symbols
- Cell: Represented by two parallel lines of unequal length (longer line is positive, shorter is negative).
- Battery: A series of cells connected end to end; represented by multiple cell symbols.
- Switch: Shown as a line that can be either touching another point (closed) or raised (open).
- Connecting Wires: Represented by straight lines.
- Bulb: Represented by a circle with an 'X' inside.
Open vs. Closed Circuits
- Open Circuit: The switch is open, meaning there is a break in the path. No electric charge flows, and the device (e.g., a bulb) does not function.
- Closed Circuit: The switch is closed, creating a continuous path. Electric charge flows, and the device (e.g., a bulb) functions/lights up.
The Nature of the Electric Cell
- Distinction: An electrical cell is distinct from a biological cell. An electrical cell stores chemical potential energy, which is converted to electrical energy within a closed circuit.
- Terminals: Labels include a positive ( $+$ ) end and a negative ( $-$ ) end.
- Battery Formation: Created when two or more cells are connected end to end.
Biological Connections: The Nervous System
- The human nervous system sends electrical signals throughout the body for communication and normal function.
- Disorders: Disruptions in these electrical signal transmissions can lead to medical conditions such as epilepsy and multiple sclerosis.
Water-in-Pipe Analogy for Electrical Systems
- Cell as the "Pump": Just as a pump provides energy to move water, a cell provides energy to drive electricity in a circuit.
- Electric Charge as the "Water": Negative electrons move within wires similarly to water moving in pipes.
- Wires as the "Pipes": Wires provide the path for charge; they are made of conductive materials rather than being hollow like pipes.
- Electrical Device as the "Water Wheel": Energy is transferred from the source to the device. In a bulb, chemical potential energy from the cell becomes electrical energy, then heat and light energy.

ELECTRIC CHARGE IN A CIRCUIT

Electric Current ( $I$ )
- Definition: The rate at which electric charge flows past a specific point in a circuit per unit time.
- Mechanism: In conductors, this flow is primarily the movement of electrons.
- SI Unit: The ampere ( $A$ ).
- Measurement: Measured using an ammeter, which must be connected in series. The positive terminal of the ammeter connects to the positive side of the cell, and the negative terminal to the negative side.
Potential Difference ( $V$ )
- Definition: The amount of energy required to move a unit charge from one point to another in a circuit. It is also known as voltage.
- Analogy: Similar to gravitational potential energy differences in a water tank system where water flows from a region of higher potential to lower potential.
- SI Unit: The volt ( $V$ ).
- Measurement: Measured using a voltmeter, which must be connected in parallel across the component being measured.
- Biological Example: The electric eel can produce shocks up to $600\,V$ for hunting and defense—over double the voltage of a standard wall socket in Singapore.
Electrical Resistance ( $R$ )
- Definition: A measure of how much a component opposes the flow of electric current.
- SI Unit: The ohm ( $\Omega$ ).
- Conductors: Materials with low resistance (e.g., copper, silver). Some conductors like tungsten and nichrome have higher resistance and convert electrical energy into heat and light.
- Insulators: Materials with extremely high resistance (e.g., plastic, glass) that prevent charge flow.

TYPES AND USES OF RESISTORS

Function: Resistors reduce the flow of charge to keep currents at safe levels and prevent overheating.
Types:
- Fixed Resistor: Has a non-adjustable resistance value, often identified by color bands.
- Variable Resistor (Rheostat): Has a range of resistance that can be adjusted using a slider.
- Thermistor: A specialized resistor whose resistance changes based on temperature; used in digital thermometers.
Practical Applications:
- Used in dimmers to control room light brightness.
- Used in appliances to convert electricity into light or heat.

ARRANGEMENT OF ELECTRICAL COMPONENTS (G2 SCIENCE)

Resistors in Series
- Provides only one path for the electric charge.
- Effect: Adding more resistors in series increases the overall resistance, which decreases the current ( $I$ ). Consequently, bulbs in this arrangement become less bright.
Resistors in Parallel
- Provides more than one path for the electric charge.
- Effect: Adding more resistors in parallel decreases the overall resistance, which increases the current ( $I$ ). Consequently, bulbs in this arrangement become brighter.
Rheostats in Circuits
- The resistance of a rheostat depends on which terminals the wires are connected to and the position of the slider.
- Slider Left: Minimum resistance; results in large current and bright bulb.
- Slider Right: Increased resistance; results in smaller current and dimmer bulb.

USEFUL EFFECTS OF ELECTRICITY

Chemical Effect
- Occurs when current passes through certain chemical liquids/solutions, causing changes.
- Applications: Extraction of metals and electroplating.
- Medical Electroplating: Surgical instruments (scalpels, tweezers) are coated with gold, silver, or copper for antimicrobial properties and corrosion resistance. Orthopedic implants (e.g., titanium knee joints) are coated with biocompatible materials to prevent body rejection.
Heating Effect
- Occurs when current passes through high-resistance heating elements (usually nichrome).
- Household Items: Hairdryers, rice cookers, toasters, and kettles.
- Medical Items: Infant incubators (to maintain constant infant body temperature) and autoclaves/sterilizers (using high-pressure steam generated by heating elements to kill bacteria).
Magnetic Effect
- Discovery: Hans Christian Oersted (1819) observed that current deflects a compass needle.
- Electromagnets: Temporary magnets made by passing current through a coil of wire; strength is increased by adding an iron core.
- Medical Applications: Robotic surgery systems, removal of magnetic objects from eyes by adjusting magnetic force, and Magnetic Resonance Imaging (MRI) for soft tissue visualization.
- MRI Safety: Patients must remove metal items; those with pacemakers or hearing aids may need alternative tests to avoid injury from strong magnetic fields.

SCIENTIFIC MILESTONES

Alessandro Volta (1799): Invented the first cell, the voltaic pile, using alternating zinc and silver discs separated by brine-soaked cloth. The unit "volt" is named in his honor.
Michael Faraday (1820s-1830s): A self-taught scientist who discovered the laws of electromagnetism and invented the electric motor and electric generator.

HARMFUL EFFECTS AND SAFETY DEVICES

Electrical Hazards
- Electrical Fires: Caused by damaged circuits or overloading (too many plugs in one socket), leading to excessive heat and melting wires.
- Electric Shock: Can occur from lightning, worn insulation exposing conducting wires, or using appliances near water (a conductor). Safety measures include using dry hands, two-pin plug adaptors, and switching off power before cleaning.
Safety Devices
- Fuse: A device protecting appliances from excessive current. If current exceeds the current rating (e.g., $3\,A$ , $5\,A$ , $13\,A$ ), the thin, high-resistance wire inside melts, creating an open circuit. It must be replaced after the fault is fixed.
- Circuit Breaker: Found in the Distribution Board (DB) box. These lever-like switches trip to cut off supply when excessive current is detected. They can be reset after repair.
- Critical Backups: Hospitals use dual power systems connected in parallel to ensure life support machines such as ventilators remain powered if one system fails.

CONSUMPTION OF ELECTRICAL ENERGY

Power ( $P$ )
- Definition: The rate at which electrical energy is converted to other forms per unit time.
- SI Unit: Watt ( $W$ ). $1\,kW = 1000\,W$ .
- Energy Efficiency: Regulated by the Mandatory Energy Labelling Scheme (Singapore). More ticks on a label indicate better energy efficiency (e.g., an LED bulb at $5\,W$ vs. a filament bulb at $20\,W$ ).
Electrical Energy Calculation (G2 Science)
- Formula: $\text{Electrical energy (kWh)} = \text{Power (kW)} \times \text{Time (h)}$
- Kilowatt-hour ( $kWh$ ): The amount of energy used by a $1\,kW$ appliance running for $1\,h$ .
- Unit Conversion: $1\,kWh = 1000\,W \times 3600\,s = 3,600,000\,J$ .
Cost of Consumption
- Calculated by multiplying the energy in $kWh$ by the electricity tariff (regulated by the Energy Market Authority).
- Example Comparison ( $27\,cents/kWh$ for $8\,h$ ):
  - Fan ( $60\,W = 0.060\,kW$ ): $0.48\,kWh \times \$0.27 = \$0.1296$ .
  - Aircon ( $1500\,W = 1.5\,kW$ ): $12.0\,kWh \times \$0.27 = \$3.24$ .
Energy Conservation Tips
- Purchase appliances with $3$ or more ticks on labels.
- Switch off air conditioners after cooling and use fans.
- Switch off water heaters after use.
- Use vacuum flasks instead of electric dispensers.
- Address Standby Power (vampire power) by switching off power sockets or unplugging devices.

INFRASTRUCTURE: ELECTRICITY TRANSMISSION

Historical Development in Singapore: Transitioned from overhead power lines to underground cables in the 1970s (buried $3\,m$ deep).
Modern Transmission: Due to congestion from other utilities, Singapore utilizes deep cable tunnels built about $60\,m$ below ground level (completed in 2019). These tunnels protect cables and allow for maintenance without disrupting road traffic.