How Things Work II
(Lecture #16)
Instructor: Gordon D. Cates
Office: Physics 106a, Phone: (434) 924-4792
email: cates@virginia.edu
Course web site available through COD and Toolkit
or at http://people.virginia.edu/~gdc4k/phys106/spring08
February 22, 2008
Announcements
• The midterm will be postponed until Wednesday,
March 12th.
• Problem Set #3 will be posted sometime on Sunday.
• Solutions to Problem Set #2 will be also be posted
sometime on Sunday.
• Problem sets #2 and the quiz will be graded before
the break.
Important Idea I
Static Charge
Does not produce
magnetic field
Moving charge
(current)
Does produce
magnetic field
Important Idea II
Static
magnetic fields
No forces on
charges, i.e.,
no electric fields
Changing
magnetic fields
Do produce forces
on charges, i.e.,
electric fields
A CHANGING magnetic field
produces electric fields.
(Which in turn produces voltages.)
For example, moving a magnetic in and out of a
wire loop “induces” a current around that loop.
We were discussing how do
transformers work.
Transformers
Current in primary
winding produces
changing magnetic field
Transformers
Iron core carries
magnetic field
Current in primary
winding produces
changing magnetic field
Transformers
Changing magnetic field
produces electric field in
coils of wire, thus
producing current.
Iron core carries
magnetic field
Current in primary
winding produces
changing magnetic field
Current and Voltage an Power
in transformers
Power in .....
Must equal....
Power Out.....
Transformer
Power in = voltagein·currentin
•
Power out = voltageout·currentout
A transformer changes the voltage and
current while keeping power unchanged !
How voltage and current change in
transformers
voltagein
voltageout
•
•
•
# turns in primary
=
# turns in secondary
A transformer changes the voltage and current while
keeping power unchanged !
The voltage change is proportional to the ratio of the
number of turns between the primary and secondary.
The current changes inversely with the voltage to keep
the power constant.
Step Up Transformers
•
•
•
More turns in secondary
circuit so charge is pushed
a longer distance.
Larger voltage rise.
A smaller current at high
voltage flows in the
secondary circuit.
Step Down Transformers
•
•
•
Fewer turns in secondary
circuit so charge is pushed
a shorter distance.
Smaller voltage rise.
A larger current at low
voltage flows in the
secondary circuit.
Power transmission
Power transmission
•
•
By going to very high voltage, it is possible to transmit a given
amount of power with relatively low current.
In our demo, we can use the thin speaker wires, which
previously resulted in large losses, to power the light bulb.
Question:
Electric power reaches the University via high
voltage transmission lines. What fraction of
the electric charge in the electricity we are
using in this lecture hall traveled on those
transmissions lines ?
A. Roughly 100%
B. Roughly 10%
C. Roughly 1%.
D. Roughly 0.01%.
E. Exactly 0.0%.
Electric generators and
electric motors
Electric generator
Electric generator
•
•
The underlying idea is very similar to a transformer.
Here, a spinning magnetic rotor creates a changing magnetic
field instead of AC current running through the primary coil.