Adeline S2 Science Unit8.1-8.4 revision

8.1 Simple Circuits

A) Conditions for Electrical Appliances

  • Complete Path Requirement

    • The bulb only lights up when connected to the cell with wires to form a complete path.

    • This complete path is known as an electrical circuit (or simply circuit).

  • Conditions for Electrical Appliances

    1. Energy Source

    • An appliance must have a source of electrical energy which may include:

      • A cell (or battery)

      • A generator

      • A solar panel

    1. Closed Circuit

    • The energy source and appliance must create a complete path or closed loop.

    • This loop originates from one end of the source, passes through the appliance, and returns to the other end of the source.

    • Closed Circuits (or complete circuits) are those that form closed loops.

    • Open Circuits (or incomplete circuits) do not form closed loops.

    • Energy Conversion:

      • Chemical energy stored in the cell is converted to electrical energy.

      • The circuit conveys electrical energy from the cell to the bulb.

      • The bulb transforms electrical energy into both light energy and thermal energy.

B) Electrical Conductors and Insulators

  • Electrical Conductors

    • Materials that allow electricity to flow through them are called electrical conductors.

    • Examples:

    • Metals

    • Pencil lead (graphite)

    • Vinegar

  • Electrical Insulators

    • Materials that do not conduct electricity are referred to as electrical insulators.

    • Examples:

    • Plastics

    • Wood

    • Distilled water

C) Switch

  • A switch is utilized to either open or close a circuit.

    • Contains both conducting and insulating components.

  • Example: Torch

    • Switch Closed: Conducting parts touch, making the circuit complete, and the torch lights up.

    • Switch Open: Conducting parts do not touch, rendering the circuit incomplete.

D) Circuit Symbols

  • Standard circuit symbols are adopted to represent different components in a circuit:

    • Circuit Component Symbols:

    • Cell

    • Battery

    • Bulb

    • Switch

    • Connecting wire

    • Connector

8.2 Current

A) What is Current?

  • In a closed circuit, electric charges flow continuously.

  • The flow of electric charges is termed electric current, or current for short.

  • Current direction flows from the positive terminal of a cell to the negative terminal through an appliance (e.g., a bulb).

  • Continuous electric charge flow necessitates an energy source and a complete path (closed circuit).

B) Measuring Current

  • Ammeter

    • Current size is measured using an ammeter.

    • Unit of Current: ampere (A).

  • Connecting an Ammeter:

    • The red knob (+) of the ammeter should connect to the positive terminal of the cell.

    • The black knob (-) should connect to the negative terminal.

    • Incorrect connection may damage the ammeter.

  • Different Ammeter Knobs: Some ammeters feature multiple red knobs for various measurement ranges:

    • For currents under 1A, the 1-A knob should be used for precision.

    • For currents between 1A and 5A, use the 5-A knob.

  • Sizes of Current in Daily Life: Arrange the options in ascending order:

    1. Electric portable fan (c)

    2. Hairdryer (b)

    3. MTR train (e)

    4. Electric kettle (a)

    5. Air conditioner (f)

    6. Typical lightning bolt (d)

C) Analogy of Electrical Circuit

  • Water Flow Model vs. Electrical Circuit:

    • Water Flow Model Components

    • Water pump

    • Water pipe and water channel

    • Water flow turbine

    • Electrical Circuit Components

    • Battery (cell)

    • Connecting wires

    • Current

    • Bulb

  • Functions:

    • Water pump supplies energy; the cell provides energy for electric charges to flow.

    • Water pipes are pathways for water; wires are pathways for current.

    • Turbine consumes energy; the bulb also consumes energy to produce light.

D) Effects of Current

  • Heating Effect:

    • Thermal energy is generated when current flows through a conductor.

    • This effect amplifies with an increase in current.

    • Appliances such as electric kettles, irons, and hairdryers utilize the heating effect of the current.

    • The heating element of these devices becomes hot when current flows through.

  • Magnetic Effect:

    • Current flowing through a coil of wire paired with an iron nail creates an electromagnet, attracting paper clips.

    • Applications:

    • Electromagnetic Door Lock: An electromagnetic door lock consists of an electromagnet and a metal plate. When the electromagnet's circuit is closed, current flows through and attracts the metal plate, locking the door.

    • Electric Motor: Converts electrical energy to kinetic energy; flowing current turns wires into electromagnets, making the motor spin.

    • Electromagnetic Crane: Used to lift heavy magnetic objects when current flows through the electromagnet.

8.3 Voltage

What is Voltage?

  • Voltage indicates the energy supplied by the cell for electric charges to circulate within the circuit.

  • Analogous to the water flow model:

    • The height of the water level reflects the strength of the pump; a stronger pump elevates water higher.

    • A higher voltage cell functions as a stronger pump.

Measuring Voltage

  • Voltmeter: Instrument for measuring voltage.

  • Unit of Voltage: volt (V).

  • Circuit Symbol of a Voltmeter:

  • Connecting a Voltmeter:

    • Connect across the cell to measure its voltage.

    • The red knob (+) connects to the positive terminal; the black knob (-) connects to the negative terminal.

    • For additional components in the circuit, appropriate connections must be made (with validation).

  • Multiple Knobs on Voltmeters: Some voltmeters come with switches for various ranges:

    • Use the 5-V knob for measurements above or below 5 V for precision.

Practicing Voltage Summaries

  • Connecting multiple cells in the same direction adds voltage:

    • Overall voltage = sum of individual voltages.

  • If a cell is connected oppositely, the voltage across is less than the total voltage sum.

Voltage Measurements in Daily Life:

  • Item vs. Voltage

    • AA-cell: 1.5 / 5 / 10.8 V (most likely 1.5 V)

    • Smartphone: 1.5 / 3.7 / 8.9 V (most likely 3.7 V)

    • Mains Electricity: 10 / 55 / 220 V (most likely 220 V)

    • Overhead Power Cables in HK: 25,000 / 52,400 / 132,000 V (most likely 132,000 V)

  • Effect of Changing Voltage on Current: Increased voltage results in a larger current flowing through the circuit.

8.4 Resistance

  • All conductors exhibit some degree of electricity conduction, but efficiency varies.

  • Resistance measures how difficult it is for current to traverse a conductor:

    • Good conductors exhibit low resistance.

    • Unit of Resistance: ohm (Ω).

  • Resistors are specific circuit components with fixed resistance values; they do not have defined polarity, allowing connection in any orientation.

  • Effect of Resistance on Current:

    • As resistance rises, the current decreases because current flow is hindered by higher resistance.

    • Connecting a higher-resistance resistor results in a smaller current.

  • Resistors are often used to regulate current within a circuit.

Factors Affecting Resistance

  • Resistance Dependency:

    • The resistance of a wire relies on its length, thickness, and material:

    • Thinner wires have higher resistance than thicker wires of identical length.

    • Longer wires possess higher resistance than shorter counterparts with the same thickness.

Rheostats

  • These variable resistors allow for adjustable resistance.

  • Symbol of a Rheostat:

  • Use of Rheostats:

    • Commonly found in volume controls for audio equipment to manage volume.

    • Utilized in dimmer switches for adjusting lamp brightness.