Download Do Now (2/10/14)

Do Now (2/10/14)

An airplane traveling at 750 km/h passes
over a region where the Earth’s magnetic
field is 4.5 x 10-5 T. What is the voltage
induced between the plane’s wing tips if
they are 85m apart?
.796 V
 To
 Room
the Lab!!!
127, get in groups of 3-4
Do Now (2/11/14)

What did we observe yesterday when we
moved the magnet through the loop?
◦ Did changing the number of loops make a
difference?
◦ What about the speed that you moved the
magnet?
Faraday’s Laws
2/10/14
Magnetic flux – the magnetic field
through an area
  BA cos
Units: T*m2
Tesla per meter squared

A circular loop of wire has a radius of
.324m. It is in a uniform magnetic field of
.89 T. What is the flux through the loop if
it’s plane is perpendicular to the field?
A   * r  .33m
2
2
  BA
.89 * .324  .29 T m
Practice!!!
2
What causes an induced EMF?
A wire moving with respect to a
magnetic field
Remember this?
• The EMF (ε) is measured in volts and depends on:
ε =Blv
Where l is the length of the wire.
Faraday’s Law

Induced EMF with coils or loops
◦ N = number of coils/loops of wire

  N
t

If ε is proportional to the change in flux,
how do I calculate that?
   final   inital
  ( BA) final  ( BA)inital
 Let’s
try Number 1 on your
Faraday’s Law Worksheet!
Classwork

Use the rest of class time to work on your
homework.
‘Electromagnetic Induction’
or
‘Faraday’s Law’
Do Now (2/12/14)

Describe Flux in your own words.

What is Faraday’s Law?
 How
is power/energy transported
from power plants to our homes,
schools, and businesses?
The Transformer

A transformer is a device that uses induction
and ac current to step voltages up or down.
Transformer
a.c.
R
Nprimary
Input
Nsecondary
Output

EP   N P
t

ES   N S
t
Transformers (Continued):

EP   N P
t
Transformer
a.c.
Np
Ns
R

ES   N S
t
Recognizing that f/t is the same in each coil, we divide first relation by
second and obtain:
This is the ratio of turns or coils:
The transformer equation:
EP N P

ES
NS
Example 7: A generator produces 10 A at
600 V. The primary coil in a transformer has
20 turns. How many secondary turns are
needed to step up the voltage to 2400 V?
Applying the transformer
equation:
 P NP

 S NS
Ns 
N P s
P
(20)( 2400V )

600V
I = 10 A; εp = 600 V εS = 2400 V
a.c.
Np
20
turns
Ns
R
NS = 80 turns
This is a step-up transformer; reversing coils will make it a step-down
transformer.

A transformer has an output voltage of 8V
when connected to a 120V power-supply.
What is the turns ratio of the transformer?
15:1 Ratio
Is this a step-down or step-up transformer?
Step-up increases Voltage
Practice
Step-down decreases Voltage
Classwork

Use the rest of class time to work on your
homework.
‘Faraday’s Law’
or
‘Transformers’
Exit Question: