electicity

Chapter 15-17: Electricity

Electrical Charges

Learning Objectives

  • Understand what happens when insulating materials are rubbed together.

  • Learn about the transfer that occurs when objects become charged.

  • Explain what happens when charged objects come together.

Key Words

  • Static Electricity (静电)

  • Repel (排斥)

  • Attract (吸引)

  • Insulator (绝缘体)

  • Electric Charge (电荷)

  • Proton (质子)

  • Neutron (中子)

  • Electron (电子)

  • Atom (原子)

  • Nucleus (核)

  • Perspex Rod (有机玻璃棒)

  • Polythene Rod (聚乙烯棒)

  • Electroscope (验电器)

  • Conductor (导体)

Electrical Charges (电荷)

Static Electricity Examples

  • Balloon Experiment: When you rub a balloon on your clothing and then touch it to the ceiling, it sticks due to static electricity. This sticking occurs because the balloon becomes electrically charged, resulting in an attractive force with the ceiling.

Inside the Atom

  • Structure of the Atom:

    • Protons and neutrons are located in the nucleus.

    • Electrons orbit around the nucleus.

  • Charge Characteristics:

    • A proton has a positive charge.

    • An electron has a negative charge of −1.6imes10−19-1.6 imes 10^{-19} coulombs.

    • A neutron has no charge.

Atom Charge Balance

  • An uncharged atom contains equal numbers of electrons and protons.

  • Electrons can be transferred to and from atoms, resulting in charged atoms, known as ions (离子).

  • Negative Ion: Created by adding electrons to an uncharged atom (more electrons than protons).

  • Positive Ion: Formed by removing electrons from an uncharged atom (fewer electrons than protons).

Charging by Friction

  • Insulators:

    • Materials that do not conduct electricity well; for instance, rubber, glass, plastic, and ceramics.

  • Rubbing Insulators:

    • When a polythene rod is rubbed with a dry cloth, electrons transfer to the rod from the cloth, making the rod negatively charged (负电荷).

    • Conversely, rubbing a Perspex rod with a dry cloth removes electrons, resulting in a positively charged rod (正电荷).

Force Between Charged Objects

Key Observations

  • Charged objects exert forces on each other:

    • Objects with like charges (same type) repel each other.

    • Objects with unlike charges (different types) attract each other.

    • This behavior is summarized by the Law of Electrostatics: Like charges repel, whereas unlike charges attract.

Conductors and Insulators

Definitions

  1. Conductor: Materials that easily conduct electricity (e.g., metals, human body, earth, graphite, saltwater).

  2. Insulator: Materials that do not easily conduct electricity.

Electric Circuits

Learning Objectives

  • Recognize symbols used for electrical components.

  • Draw simple circuit diagrams using component symbols.

  • Describe the direction of electric current in a circuit.

  • Calculate electric current from charge flow and time taken.

Keywords

  • Charge (电荷)

  • Current (电流)

  • Electric Circuit (电路)

  • Open Circuit (开路/断路)

  • Short Circuit (短路)

  • Amperes (安培, A)

  • Coulombs (库仑)

Circuit Definition

  • An electric circuit is defined as a path for electrons to flow from a voltage or current source.

  • A circuit diagram visually represents how components in a circuit are interconnected.

Electrical Component Symbols

  • Cell: Pushes electrons around a complete circuit.

  • Battery: Comprises two or more cells.

  • Switch: Controls current flow in a circuit, allowing it to be switched on or off.

  • Indicator: Emits light to signal when a current flows or serves as a light source.

  • Diode: Allows current to flow in one direction only.

  • Light-Emitting Diode (LED): Emits light when current passes through.

  • Fixed Resistor: Limits current in a circuit.

  • Variable Resistor (Rheostat): Adjusts current in a circuit.

  • Fuse: Melts and breaks the circuit when current exceeds a certain level.

  • Heater: Converts electrical energy into heat.

  • Ammeter: Measures electric current.

  • Voltmeter: Measures potential difference (voltage).

Open Circuit and Short Circuit

Definitions

  • An open circuit occurs when a part of the circuit is disconnected; no current flows.

  • A short circuit occurs when the positive and negative terminals are directly connected, bypassing components in the circuit.

Electric Current (电流)

Definitions

  • Electric Current: The flow of charge. The rate at which electric charge flows in a conductor.

  • Measured in Coulombs (C) for charge and Amperes (A) for current.

  • Current (I): I=racQtI = rac{Q}{t} where:

    • II is the current in amperes,

    • QQ is charge in coulombs,

    • tt is time in seconds.

  • An electric current of 1 ampere is equivalent to a flow of 1 coulomb per second.

  • The direction of current is assumed to flow from positive to negative, opposite to the movement of electrons.

Conditions for Current Flow

  1. Directed movement of charge is necessary.

  2. A closed switch is required to complete the circuit.

Resistance (电阻)

Definition

  • Resistance is defined as the opposition faced by electric current in a conductor, represented as:

    • R=racUIR = rac{U}{I}
      where:

    • RR is resistance in ohms (Ω),

    • UU is voltage in volts (V),

    • II is current in amperes (A).

  • Resistance increases with longer wires and decreases with thicker wires.

  • Different materials have different resistivity. For most conductors, resistance increases with temperature.

Ohm's Law

  • Ohm's law states that the current through a resistor is directly proportional to the voltage across it while inversely proportional to its resistance.

    • Formula: I=racURI = rac{U}{R}

  • For components in series, total resistance is the sum of individual resistances.

  • For components in parallel, the voltage across each component is the same, and the total current is the sum of currents through each component.

Component Characteristics

Filament Lamp

  • Increases in temperature lead to an increase in resistance, as higher temperature causes metal filament atoms to vibrate, creating more hindrance.

Diode

  • Allows current to flow in one direction (forward) but blocks it in the reverse direction. Diodes are non-ohmic conductors.

Thermistor

  • Resistance decreases as temperature increases; used in thermostats for temperature control.

Light Dependent Resistor (LDR)

  • Resistance decreases as light intensity increases, often used in light sensing applications such as automatic lighting systems.

Sensor Circuits

Functionality

  • Designed to respond to changes in temperature or light intensity using components such as thermistors and LDRs.

  • They can switch devices on or off based on light or temperature thresholds, functioning via changes in resistance according to environmental conditions.