introduction to electricity and electric circuits electric charges, currents and voltage
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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 current=Q/t
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Conservation of current
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Q1.
Another example on Page 214, STT 8.2
1) 3A in;
2) 2A out;
3) 1A in;
4) 1A out.
?
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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 !!
q
WV chem
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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Ohm’s Law
Current I = V / R, 1/R= σA V/d =G
A is a cross section area of a wire, d is length. Conductivity σ.
V is the voltage across the wire.
*** Inverse of σ is called resistivity ρ, ρ = 1/σ
R= ρ d/A
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Resistivity of materials Material Resistivity
Copper 1.7 X 10^{-8}Iron 9.7 X 10^{-8}Seawater 0.22Blood 1.6Fat 25Muscle 13
Pure water 2.5 X 10^5
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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: 1mApain: few mAdeadly: over 70mA
Estimate the resistance of a human body !!
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Grounded High Voltage Lines, Lightning Strikes1 000 000 V
50 000 V 0 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 (Ohm’s Law)
• P = V2/R = I2R