Introduction to electricity and electric circuits

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Introduction to electricity and electric circuits

• electric charges, currents and voltage

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Example He-Atom

• 2 protons 2e
• 2 neutrons no charge
• 2 electrons - 2e
• elementary charge, e 1.6 x 10-19 C
• Atoms are neutral, unless electrons are removed
  (or added)
• Protons and neutrons consists of 3 quarks each.

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Properties of Electric Charge

• Charges are due to elementary particles Protons
  carry ( e), electrons carry (- e).
• Charges are quantized Q n e with n 1,
  2, 3, and e 1.602 x 10-19 C
• Charges are conserved.
• Charged objects exert a force onto each other
  like charges repel each other, unlike charges
  attract each other.

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Insulators versus conductors

• Insulators (plastic, glass, air) All electrons
  are tightly bound or localized and cannot move.
• No conduction electrons (or only very few) no
  transport of charge, i.e. electric currents.

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Conductors (Metals)

• Transport electric charges well.
• There are freely moving conduction electrons and
  bound electrons that remain bound to the nucleus
  of each atom.
• ions (nucleus bound electrons) remain in place
  and form a crystal lattice (chemical bonds).
• Electric current Net motion of charges (free
  electrons in a metal).
• Positive charges (ions) can only move in a liquid
  or a gas.

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conduction electrons in a conductor
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Electric currentQ/t
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Conservation of current
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Q1.

• 3A in
• 2A out
• 1A in
• 4) 1A out.

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Another example on Page 214, STT 8.2
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How to create an electric current
What is the time-dependence of current in the
wire?
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A Battery needed to keep currents flowing !!
Voltage of a battery
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Electric potential
Gravitational Potential Energy mgh Gravitational
Potential gh
Electrical potential Energy qV Electrical
potential V
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• The work done by the charge escalator or
• chemical forces W (chem) defines the voltage of a
  battery
• W(chem) /q terminal voltage of a battery

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• Some Typical Voltages
• Voltage Source (approx.)
• Thundercloud to ground 108 V
• High-voltage power line 106 V
• Power supply for TV tube 104 V
• Automobile ignition 104 V
• Household outlet 120 V
• Automobile battery 12 V
• Flashlight battery 1.5 V
• Resting potential across
• nerve membrane 10-1 V
• Potential changes on skin
• 10-4 V

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Resistance/conductance

• Valid for ohmic devices mainly metallic
  conductors at constant temperatures.

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Ohms Law
Current I V / R, 1/R sA V/d G A
is a cross section area of a wire, d is length.
Conductivity s. V is the voltage across the
wire. Inverse of s is called resistivity
?, ? 1/s R ? d/A
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Resistivity of materials

• Material Resistivity
• Copper 1.7 X 10-8
• Iron 9.7 X
  10-8
• Seawater 0.22
• Blood 1.6
• Fat 25
• Muscle 13
• Pure water 2.5 X 105

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• 
  Q1
• Two copper conductors, A and B, are of same
  lengths and are connected to two identical
  batteries. A has a bigger cross section than B.
  Which is the right I versus V graph?

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Q2

• A and B conductors have same cross sections.
  But A is longer than B. Which is the correct
  graph?

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Electrical Hazards
Electrical Hazards Feel 1mA pain few
mA deadly over 70mA Estimate the resistance of
a human body !!
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Grounded High Voltage Lines, Lightning Strikes
1 000 000 V
0 V
50 000 V
Even if you are not directly hit by a lightning
strike or a hot power line, there is danger The
potential decreases with distance from the
location of the impact (potential gradient). If
you take a step there may be a large potential
difference between your feet.
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Electrical Power

• P VQ/t VI
• Unit Watt (W) VA
• kW, MW, GW

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Electrical power on the ohmic device

• P VI
• V RI (Ohms Law)
• P V2/R I2R