Transcript Physics 102 Introduction to Physics

Physics 102-002
Announcements
• WebAssign –
– Chapter 22 due this Wednesday
– Chapter 23 due next Wednesday
• Exam #2 graded .. Pick them up.
• Exam #3 is on April 9 (next Monday)
Picture: Aurora Borealis (courtesy Jan Curtis, University of Alaska).
The electric currents that flow in the Earth’s vicinity from its
interaction with the solar wind drive the aurora. In the process, they
can create overloads on electric power distribution grids, creating
massive power blackouts.
Class Schedule
3/19
Chapter 10
Projectile and Satellite Motion (Pg 184-191)
3/21
Chapter 11
Atomic Nature of Matter (Pg 211-223)
3/26
Chapter 22
Electrostatics, Part 1 (Pg 410-419)
3/28
Chapter 22
Electrostatics, Part 2 (Pg 419-430)
4/2
Chapter 23
Electric Current, Part 1 (Pg 436-443)
4/4
Chapter 23
Electric Current, Part 2 (Pg 444-452)
4/9
Midterm Exam #3
Chapter 23
Electric Current, Part 1
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Flow of Charge
Electric Current
Voltage Sources
Electrical Resistance
Ohm’s Law
Direct Current and Alternating Current
Speed and Source of Electrons in a Circuit
Electric Power
Electric Circuits
Next time
Flow of Charge
In order for a charge to flow, it needs a push (a force) and it is supplied by voltage,
or potential difference. The charge flows from high potential energy to low potential
energy.
Suppose A has a potential of 12 V and B has a potential of 2 V. There is a potential
difference. A has higher potential energy than B, and it means there is voltage. The
potential difference is VA - VB = 12 - 2 = 10 V.
Water will flow from a high
reservoir to a lower one (high
potential to low potential). Water
will flow as long as the
difference in levels is maintained
– like with a pump.
What will happen if something pushes the charge from the bottom plate to the
upper plate E in the diagram? This will generate a potential difference, and
hence, there will be continuous flow of charge. This is how a battery works; it
takes "+" charge from bottom and push it to top.
Electric Current
Electric current is the flow of electric charge.
Electric current is the rate of charge flow past a given point in an electric
circuit, measured in coulombs/second which is named Amperes.
Remember the conduction electrons … the electron sea in metals … are free to
move around. They’re the charges that move in a current.
Be careful, though, because in liquids (like in a car battery), the moving charges
are the positive ions.
Note that although charge moves in a wire while a current flows, the net charge on the wire is
always zero.
Therefore, there are no potential difference between these two
feet.
Let's say the power line is very low, almost touching the ground,
and a chicken is trying to cross it. If one leg is on the ground and
the other one is on the power line, then there are potential
difference between these two legs.
Therefore, there is a flow of charge and eventually the chicken will
be barbequed like the
Physics Place video.
Why can a bird sit
on the power line?
Question 1
Is a current-carrying wire electrically
charged?
– Yes
– No
– Maybe
Question 1 Answer
Is a current-carrying wire electrically
charged?
– Yes
 – No
– Maybe
Voltage Sources
We need more than just a continuous path (circuit) before a continuous flow of electrons will occur:
we also need some means to “push” these electrons around the circuit. Just like marbles in a tube or
water in a pipe, it takes some kind of influencing force to initiate flow. With electrons, this force is the
same force at work in static electricity: the force produced by an imbalance of electric charge – or a
difference in the “electric potential”.
Examples of voltage sources:
• Batteries
• Electric Generators
• Automobile battery = 12 V
• Fuel Cells
• Solar Cells
• Piezo-electrics
• Electrets
• Lightning
• Biological voltage generators
• Thermo-electrics.
• Capacitors
Electrical Resistance
Voltage can be thought of as the pressure pushing charges along a conductor, while the electrical
resistance of a conductor is a measure of how difficult it is to push the charges along.
Using the flow analogy, electrical resistance is similar to friction. For water flowing through a pipe, a
long narrow pipe provides more resistance to the flow than does a short fat pipe.
The same applies for flowing currents: long thin wires provide more resistance than do short thick
wires.
In a hydraulic circuit, a
narrow pipe resists the
flow of water.
In an electric circuit, a
lamp or other device
(for example, a motor
or resistor) resists the
flow of electrons.
More water flows
through a wide hose
than through a narrow
one. Similarly, more
electric current flows
through a thick wire
than through a thin
one.
Ohm’s Law
In many materials, the voltage and resistance are connected by Ohm's Law:
Current 
Voltage
Volts
or Amperes 
Resistance
Ohms
(in units form)
The unit for resistance is the “Ohm” and the symbol is “” (the Greek letter “Omega”)
Higher resistance IMPEDES the flow of current.
The greater the voltage, the greater the current.
The greater the resistance, the SMALLER the current!
High current can cause injury.
High Voltage + High Resistance = Low Current
High Voltage + Low Resistance = HIGH CURRENT
(The moral: Insulate yourself if you’re around high voltage)
Different types of resistors.
Direct Current and Alternating Current
DC – Charges move in the same direction
all the time.
AC – Charges move in alternating
direction. Done using an electric
generator arranged as shown on the right.
RMS
AC current is commonly used for residential and commercial power circuits.
In North America, the voltage is 120V – an average (called the RMS average).
It oscillates at about 60 Hz (or 60 cycles per second), which you get by making the generator
go around at that speed.
To convert AC current to DC current, you use a diode (to let through only one direction of
current) combined with a capacitor (to store some energy and release it while the current isn’t
going through the diode) …. See the figure below.
Some Diodes
Effect of a capacitor
Physics Place video.
Diode lets
through current
in only one
direction
Capacitor
stores energy
and releases it
over the “gap”
in the AC
current
Using several
diodes/capacitor
s makes the
current more
constant