Electricity and Magnetism unit

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Electricity and Magnetism unit
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Concept Map
Electrostatics
Electricity
Electromagnetism
Magnetism
Electrostatics and Magnetism are both natural
phenomena which were manipulated by and for
technology
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Electrostatics - overview

• Natural phenomena
• Obeys law of opposites where the terms negative
  and positive were selected to indicate the two
  opposites
• Materials are divided into Conductors and
  insulators

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Electrostatics - charging

• By friction - electron affinity
• Induced Charge Separation (occurs before
  charging)
• Grounding - connecting to the earth
• contact

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Electrostatics - 3 keys

• size of the Charge difference
• Distance between opposite charges
• Conductivity of the medium between the opposite
  charges

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Electrostatics - pre 1900

• Charges either positive and negative - Ben
  Franklin
• positive charges flow to balance out the
  negatives
• negative charges arent bad

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Electrostatics - post 1900

• Protons and electrons are discovered
• negative electron is free to move and is the
  charge carrier (transfer)
• Positve proton is fixed in the nucleus of the atom

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Electrostatics - charge difference

• Charge difference (symbol Q) is measured in
  coulombs (Unit C) Charge difference is a
  reflection of the
• number of excess/deficit electrons in matter -
  QNe
• N - number of excess/deficit electrons
• e - charge of 1 electron - -1.6 x 10-19C
• charge of 1 proton - 1.6 x 10-19C

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Electrostatics to Electricity

• Electrostatics is unpredictable
• Volta was able to create a small charge
  difference that was predictable with a wet cell
  (primitive battery)in 1793
• moving electrons can now do work for some
  application (motor, light, heat)

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Electricity - Circuits

• A loop is created where the electrons can flow
  from the power supply (battery - origin of the
  charge difference) to one or more loads.
• A wire, a highly conductive medium, provides the
  path for the electrons

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Electricity - Circuits

• Battery/Power supply
• Q Ne represents the charge difference
• E VQ represents the work/ potential energy that
  these electrons possess
• E -Energy(J, Joules)
• V - Voltage, Electric Potential, Potential
  difference(V, Volts)

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Electricity - Clarification

• EVQ should really be ?E ?VQ
• ?E reflects the change in energy as the electrons
  do their work
• ?V is called potential difference or voltage,
  where V is just called electric potential. In
  Grade 12, we will clarify their differences.

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Electricity - Circuits

• Current (I measured in Amps,A) reflects the
  moving energy of the electrons. This energy is
  lost at the loads (applications)
• Instead of measuring their kinetic energy
  directly, current measures the amount of charge
  that moves past a given area in a second
• I Q/t

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Electricity - Circuits

• Resistance is a measure of how much opposition
  the electrons experience as they move throughout
  the circuit. Sometimes this resistance is desired
  as it does work on the load. Sometimes it is not
  desired as seen the energy loss within the wire
  itself

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Circuits - Resistance

• Resistance at the load (symbol R, unit - Ohm,O)
  - R V/I
• Resitivity in the wire

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Resistivity - factors

• the type of metal (gold is the best conductor)
• the length of the loop
• the cross sectional path (thickness)
• the temperature (higher the temperature of the
  wire the higher the resistance)

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Resistivity - Equation

• R resistance, L length, A cross sectional
  area
• The coefficient ? depends upon the material.
  Values can be found in Table 13.1 of your
  PRACTICE PROBLEMS sheet.

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Resistance at the load

• The total resistance in the circuit dictates the
  voltage and current based on the limits of the
  power supply.
• The greater the resistance, the greater the need
  for energy (EVQ) to pull one electron around the
  loop. This results in less electrons completing
  the loop (I ?). (and vice versa)

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Resistance at the source

• We first look at the total Resistance in the
  circuit at the power supply
• The Power supplys overall resistance RT controls
  the size of VT and IT
• Ohms Law applies RT VT/IT.

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Resistance at each load

• The total Resistance in the circuit, mirrored at
  the power supply, must be equivalent to the loads
• Each load will have its own resistance R
  controlling the size of V and I
• Ohms Law applies R V/I

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Resistance in the circuit

• Depends on how the loop is created
• Rules have been established to distinguish
  between a series circuit, parallel circuit and a
  mixed circuit. - tomorrows lesson

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Power

• Power is defined as the rate at which work is
  done.
• P W/t (1 W 1J/s) or P VI (1 W 1VA)
• Note W ?E

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Power in the home

• Homes use energy, not power. Power rates how fast
  energy is used. However, because the loads are
  often listed by their power requirements 60 W
  light bulbs Electrical companies will measure
  ones energy use through the KWh instead of J.
  Therefore , using an appliance for 1 hour might
  require 1 KWh or 1 KJ.