Unit 4 Lesson 1: Flow of Electrical Energy - Notes

Describing Electrical Energy

  • Electrical energy is the energy associated with charged particles due to their positions and motion in an electric field.
  • Electrons, being negatively charged, can move within certain materials, which facilitates the transfer of electrical energy in a circuit.
  • Electrical energy is measured in joules (J).
  • A dimmer switch alters the rate of electrical energy flow in a circuit; lowering the switch decreases the brightness of an incandescent light bulb, indicating a relationship between brightness and energy flow rate.

Using Electrical Energy

  • Electrical energy can be stored in batteries and transformed into radiant and sound energy.
  • Energy transformations enable various devices, from toy cars to electric vehicles, lighting, and entertainment.
  • Electrical energy can be transformed into sound energy through electric speakers.
  • Batteries store energy in chemical bonds but are described as electrical energy sources due to voltage generation.

Potential Difference / Voltage

  • A charge in an electric field moves from higher to lower potential.
  • Voltage is the electric potential difference between two points, representing the work needed to move a unit of electric charge between them.
  • Voltage is measured in volts (V).
  • One volt is equivalent to one joule per coulomb (J/C), expressed as: 1V = 1 J/C

Electron Motion

  • Free electrons move rapidly in various directions.
  • Under an electric field, electrons slowly drift in alignment with the field.
  • The strength of the electric field determines the drift velocity.
  • Drift velocity is the average velocity of free electrons in a conductor due to an applied electric field.
  • A stronger electric field accelerates free electrons more, causing all free electrons to drift in the same direction simultaneously.

Electric Current

  • An electric circuit is a closed loop allowing continuous charge movement.
  • Applying voltage to a wire generates an electric field, exerting force on the electrons within.
  • Net movement of charge in the wire affects a circuit's behavior.
  • Electric current is the rate of flow of electric charge.
  • Current is measured in amperes (A), or amps, using an ammeter.
  • One ampere equals one coulomb per second: 1 A = 1 C/s
  • Electrons in household circuits drift slowly (approximately 1 cm per hour), yet a lamp turns on almost instantly when the circuit is closed because the electric field is established quickly throughout the circuit.

Directions of Electrons and Electric Current

  • Electric current flows from positive to negative terminals, while electrons drift in the opposite direction.

Direct Current (DC)

  • Batteries provide voltage in a circuit with negative and positive terminals.
  • Direct current flows in one direction only.
  • Flashlights, portable electronics, and photovoltaic cells use direct current.
  • In a circuit, field lines point from the positive to the negative terminal.

Alternating Current (AC)

  • Alternating current changes direction at regular intervals.
  • Many electrical components function regardless of current direction.
  • In alternating current, electrons oscillate back and forth as voltage alternates.
  • Adapters are needed to convert alternating current from wall outlets to direct current for charging devices like laptops because their batteries require direct current.

Controlling the Flow of Electrical Energy

  • An electric field is produced by the difference in electrical potential energy.
  • Connecting a wire between battery terminals accelerates electrons, creating an electric current.
  • This current is measured with an ammeter and visualized with an incandescent light bulb.
  • An incandescent bulb's brightness indicates current flow.
  • Voltage and current are directly proportional.

Limiting Current

  • Conductors and insulators control electrical energy flow.
  • Conductors (e.g., copper, nichrome) allow free electron movement.
  • Insulators restrict electron movement, having infinite resistance.

Conductor Vs Insulator

  • Conductors permit electricity or heat to pass through. Examples include silver, aluminum, and iron. Electrons move freely.
  • Insulators do not permit heat and electricity to pass through. Examples include paper, wood, and rubber. Electrons do not move freely.

Electrical Resistance

  • Electrical resistance measures an object's opposition to electric current flow.
  • Resistance is measured in ohms (Ω), and is determined by material properties, length, cross-sectional area, and temperature.
  • Copper has low resistance, making it suitable for wires.
  • Longer wires have higher resistance; wider wires have lower resistance.

Relating Current, Resistance, and Voltage

  • Ohm's law: The electric potential difference (V) between two points in a circuit equals the current (I) multiplied by the resistance (R) between those points: V = IR
    • V is measured in volts.
    • I is measured in amperes.
    • R is measured in ohms.
  • I = V/R (amps = volts / ohms)
  • R = V/I (ohms = volts / amps)

Ohm's Law

  • Current is directly proportional to voltage; halving the voltage also halves the current.
  • Current varies inversely with resistance; halving the resistance doubles the current.
  • Ohm’s law accurately relates voltage, current, and resistance in most materials, but some non-ohmic materials (e.g., semiconductors, filament lamps, thermistors, gas discharge tubes, superconductors) do not follow this relationship.

Problem Solving

  • A circuit with a 1.5-volt battery and a 3.0-ohm resistor has a current of 0.5 amps, calculated using Ohm's Law: I = V/R = 1.5V / 3.0Ω = 0.5A

Practice Problems

  1. Voltage in a circuit with 15 ohms resistance and 3.0 amps current: V = IR = 3.0A * 15Ω = 45 volts
  2. Current in a circuit with 15 ohms resistance and 12 volts voltage: I = V/R = 12V / 15Ω = 0.80 amps
  3. Resistance in a circuit with 4.5 volts voltage and 1.5 amps current: R = V/I = 4.5V / 1.5A = 3.0 ohms
  4. To keep current constant when voltage doubles, resistance must also double.

Controlling the Flow of Electrical Energy

  • Current in a toaster's metal coils emits energy as heat and light.
  • Heat may or may not be a desired output and components with higher or lower resistance can be chosen depending on the needs of the application.

Influence of Engineering, Technology, and Science on Society and the Natural World

  • Modern homes rely on electrical energy.
  • Society's reliance on electrical energy affects society and the natural world.

Can You Explain the Phenomenon?

  • Electric current in a thin metal wire raises its temperature until it glows.
  • Incandescent light bulbs contain a thin tungsten filament which glows when electric current passes through it.
  • A typical 100W incandescent bulb's tungsten filament reaches temperatures over 2000°C.
  • The filament is encased in an inert gas to prevent combustion.

Incandescent vs. Fluorescent vs. LED

  • Incandescent bulbs are used in warming lamps and incubators, where heat is a benefit.
  • When warming is not wanted, fluorescent or LED bulbs are more desirable lighting choices.
  • LED:
    • Average Life: 25,000 hours
    • No Mercury
    • 6-8 Watts
    • Uses 84% less energy
  • CFL:
    • Average Life: 8,000 hours
    • Mercury
    • 13-15 Watts
    • Uses 75% less energy
  • Incandescent:
    • Average Life: 1,200 hours
    • No Mercury
    • 60 Watts
    • 90% energy lost to heat

Case Study: Developing the Light Bulb

  • Lanterns use fire as a portable light source.
  • Homes used gas lanterns or candles for nearly a century after arc lamps were invented due to obstacles which limited the adoption of electric lights in residential homes.
  • Arc lamps produce light by generating an arc of electrical energy between electrodes.
  • Incandescent light bulbs were a safer, dimmer alternative to arc lamps.
  • Modern incandescent bulbs use a long, thin, coiled tungsten filament to increase surface area.
  • Efficiency of light bulbs can be measured in lumens per watt.

Engineering Optimizing the Light Bulb

  • Changing its shape affects the circuit by increasing its resistance, thus affecting light and heat output. Increasing the surface area of the filament compared to using a single strand in a loop allows it to radiate more light and heat.

Case Study: Developing the Light Bulb

  • Based on the table, LEDs are most efficient, producing the most light using the least power.

Case Study: Developing the Light Bulb

  • The International Space Station (ISS) needs light sources with constraints and criteria related to lifespan, efficiency, safety, and price. Lighting on the ISS would require long lifespan, low power usage (due to limited power resources), safety (no mercury), and be lightweight. The initial price would likely be less of a constraint than the other factors.