Transcript Outline - University of British Columbia

Introduction to electricity and
electric circuits
electric charges, currents and voltage
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.
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.
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.
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.
conduction electrons in a conductor
Electric current=Q/t
Conservation of current
Q1.
1) 3A in;
2) 2A out;
3) 1A in;
4) 1A out.
?
Another example on
Page 214, STT 8.2
How to create an electric current
What is the time-dependence of current in the wire?
A Battery needed to keep currents flowing !!
Wchem
V 
q
Voltage of a battery
Electric potential
Gravitational Potential Energy mgh
Electrical potential Energy qV
Gravitational Potential gh
Electrical potential V
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
Some Typical Voltages
Voltage Source
Thundercloud to ground
High-voltage power line
Power supply for TV tube
Automobile ignition
Household outlet
Automobile battery
Flashlight battery
Resting potential across
nerve membrane
Potential changes on skin
(approx.)
108 V
106 V
104 V
104 V
120 V
12 V
1.5 V
10-1 V
10-4 V
Resistance/conductance
• Valid for “ohmic” devices mainly metallic
conductors at constant temperatures.
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
Resistivity of materials
Material
Resistivity
Copper
Iron
Seawater
Blood
Fat
Muscle
1.7 X 10^{-8}
9.7 X 10^{-8}
0.22
1.6
25
13
Pure water
2.5 X 10^5
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?
Q2
A and B conductors have same cross
sections. But A is longer than B. Which
is the correct graph?
Electrical Hazards
Electrical Hazards
Feel: 1mA
pain: few mA
deadly: over 70mA
Estimate the resistance of a human body !!
Grounded High Voltage Lines,
1 000 000 V Lightning Strikes
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.
0V
Electrical Power
• P = VQ/t = VI
• Unit: Watt (W) = VA
• kW, MW, GW
Electrical power on the ohmic
device
• P = VI
• V = RI (Ohm’s Law)
• P = V2/R = I2R