Electricity

Atoms and Electrons

  • Electrons orbit the nucleus
  • Some electrons break free of the nucleus and can move freely through the metal.
  • Conduction electrons

image.png

Electric Circuits

image.png

  • Current moves from + to - (positive to negative)
  • Actually electrons travel from - to +

Electric Circuits and Resistance

image.png

  • Ohms Law: \(V=IR\)
  • Voltage = current times resistance
  • If you have constant voltage:
    • small resistance gives big current
    • big resistance gives small current

Power

  • Power is energy released per time
    • Big power: lots of energy released per second
    • Small power: little energy released per second
  • Formula for Power
    • \(P = VI\) - Power is the product of voltage and current.
    • Remember:
      • \(V = IR\) (Ohm's Law) - Voltage is equal to current times resistance.
      • \(P = IRI\) - Power can also be expressed as the current squared times resistance.
      • \(P = I^2R\) - Another expression for power.
  • Units of Power
    • Watts (\(W\)), where 1 watt = 1 volt times 1 amp

Thicker conductor → Lower resistance. Longer conductor → Higher resistance.

Direct/Alternate Current

image.png

Magnets

image.png

image.png

Magnets and Electricity

image.png

  • Electric current makes a magnetic field
  • Strength of field depends on current and number of windings
  • Iron bar has magnetic domains that are normally randomly oriented when there is not external field.
  • Domains oriented with the magnetic field from the coil grow (the others shrink)
  • This amplifies the total magnetic strength of the electromagnet

image.png

Concept Overview

  • Magnetic Field Change Induces Electric Field: A change in the magnetic field within a coil of wire induces an electric field across the coil. This principle is known as electromagnetic induction and is fundamental to the operation of transformers, motors, and generators.

How It Works

  • Coil and Magnet Interaction: A coil of wire is positioned in such a way that a permanent magnet can be moved in and out of the coil.
  • Changing Magnetic Field: As the magnet is moved, the magnetic field inside the coil changes. This changing magnetic field through the loop induces a voltage across the coil due to Faraday's law of electromagnetic induction.
  • Induced Voltage and Current: The induced voltage creates an electric current if the coil is part of a complete circuit that includes a load (e.g., a light bulb).

Practical Application

  • Generating Electrical Power: This principle is used in power generation where mechanical energy, through the movement of magnets around coils of wire, is converted into electrical energy.
  • Operational Example: In a simple setup, moving a magnet back and forth inside a coil attached to a light bulb can illuminate the bulb, demonstrating how mechanical motion is converted into electrical energy.

Lenz’s Law

  • Lenz's Law: The current induced by a changing magnetic field will produce a magnetic field opposing the change.
  • Interaction: When you push a magnet into a coil, the coil opposes by pushing back. This is due to the magnetic field created by the induced current.
  • Work and Energy: Work is required to move the magnet against the coil’s opposing force. This work is converted into electrical energy.
  • Application: This principle is used in generators to convert mechanical energy into electrical energy.

image.png

image.png

Lenz’s Law: harder to turn when there is a large load

Transformer

image.png

  • The ratio of the two voltages is the same as the ratio of the number of turns on the coils
  • \(V_{in} / V_{out} = \frac{\text{Number of turns in primary}}{\text{Number of turns in secondary}}\)

House Power

image.png

image.png

Fuses

image.png

  • Fuse wire heats \((I^2R)\)

  • Eventually melts and breaks the circuit

  • Protects the light bulb

Circuit Breaker

  • Bimetallic strip: two strips of metal that thermally expands at different rates
  • Bimetallic strip heats \((I^2R)\) bends and pulls the contacts apart protects the light bulb

Ground Fault Circuit Interrupter (GFCI)

image.png

If the currents are not perfectly balanced (5mA difference), the coil will sense the difference and activate the solenoid, opening the switch and turning off the power. There are also mechanical pieces that make certain power cannot turn on again unless the reset button has been pushed.

image.png

Live and neutral wires pass through a coil. If there is no leakage current (no short), the two currents cancel out and no magnetic field is induced in the coil.