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Electricity and Magnetism unit

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Resistance at each load The total Resistance in the circuit, mirrored at the power supply, ... Electricity and Magnetism unit Concept Map Electrostatics ... – PowerPoint PPT presentation

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Title: Electricity and Magnetism unit


1
Electricity and Magnetism unit
2
Concept Map
Electrostatics
Electricity
Electromagnetism
Magnetism
Electrostatics and Magnetism are both natural
phenomena which were manipulated by and for
technology
3
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

4
Electrostatics - charging
  • By friction - electron affinity
  • Induced Charge Separation (occurs before
    charging)
  • Grounding - connecting to the earth
  • contact

5
Electrostatics - 3 keys
  • size of the Charge difference
  • Distance between opposite charges
  • Conductivity of the medium between the opposite
    charges

6
Electrostatics - pre 1900
  • Charges either positive and negative - Ben
    Franklin
  • positive charges flow to balance out the
    negatives
  • negative charges arent bad

7
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

8
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

9
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)

10
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

11
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)

12
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.

13
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

14
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

15
Circuits - Resistance
  • Resistance at the load (symbol R, unit - Ohm,O)
    - R V/I
  • Resitivity in the wire

16
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)

17
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.

18
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)

19
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.

20
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

21
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

22
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

23
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.
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