Transcript L27_EM - barransclass

Board Work
Which direction is
the flux change?
B
What current
would oppose it?
1. If the magnetic field B intensifies, in
what direction will a current be
induced around the loop?
AC Transformers
Source: OSHA
Objective
• Relate input and output power, voltage,
current, and number of windings in an AC
transformer.
Flux Change Creates Potential
Rapidly changing field
high induced potential
unchanging field
zero induced potential
How a Transformer Works
• Alternating current in the primary coil
creates a changing magnetic field.
• The changing field induces an electric
potential in the secondary coil.
Potential Proportional to Loops
• Same flux F and area A through both sets
of windings
• Each loop adds potential
• Potentials V are proportional to the
number of loops N
V1 N1
=
V2 N2
Energy is Conserved
• Ideally: power in = power out
V1I1 = V2I2
• Realistic: power in > power out
• Efficiencies usually around 95%
Transformer Summary
• Power in  power out
•  loops  higher V, lower I
•  loops  lower V, higher I
Poll Question
A “step-down” transformer converts input at
120 V to output at 20 V. If the input circuit
has 1100 W, how much power is available at
the output?
A.
B.
C.
D.
200 W.
660 W.
1100 W.
2400 W.
Board Work
2. A transformer with 1000 primary
windings converts AC at 110 V (primary)
to 220 V (secondary).
a. Which is greater: the potential in the primary
circuit or the potential in the secondary
circuit?
b. Where will the number of loops be greater:
in the primary or in the secondary?
Board Work
2. A transformer with 1000 primary
windings converts AC at 110 V (primary)
to 220 V (secondary).
c. What is the voltage ratio V2/V1?
d. What is the loops ratio N2/N1?
e. How many loops are in the secondary circuit
(N2)?
Board Work
2. A transformer with 1000 primary
windings converts AC at 110 V (primary)
to 220 V (secondary).
f. What is the current ratio I2/I1?
Formulas for Transformers
V1
N1
=
• Power: V1I1 = V2I2 • Loops:
V2
N2
• Potential: V2 = V1
• Current: I2 = I1
I1
I2
= V1
V1
N1
= I1
V2
N2
N2
N1
Example
A transformer with 5000 primary coils and
100 secondary coils has an input voltage of
50 kV. What is the output voltage?
Vout = Vin Nout/Nin
= (50 kV)(100/5000)
= (50 kV)/50
= 1 kv = 1,000 V
Light
Chapter 26
Objectives
• Describe the transverse “medium” of
electromagnetic waves.
Last Piece of EM Theory
Maxwell’s pivotal insight
• A changing electric field acts as a current
• Thought experiment: charging parallel plates
B
+
E
–
I
I
• Charges accumulate on the plates
• The E field between the plates increases
• The changing E field is sort of a virtual current
• A changing E field creates a B field!
Electromagnetic Fields
• Faraday’s law: a changing magnetic field
creates an electric field
• Virtual Current: a changing electric field
creates a magnetic field
DB
DE
What They Mean
• Faraday’s Law = Lorentz Force
– A transversely-moving B field makes an E
field
DB
– The electric field is proportional to the
magnetic field strength
What They Mean
• Virtual current works the same way
– A transversely-moving E field makes a B field
– The magnetic field is proportional to the
electric field strength (note the directions)
Electromagnetic Field Interplay
• A moving B field creates an E field
– E direction = B  v direction
• A moving E field creates a B field
– B direction = v  E direction
• The fields are self-propagating
– v direction = E  B direction
E
v
B
E
v
B
E
v
B
AC Creates Oscillating B Field
Observe here
The End Result
Electric and magnetic fields are perpendicular:
• to each other, and
• to the direction of propagation.
Poll Question
Are electromagnetic waves transverse,
longitudinal, or a combination of both?
A. Transverse.
B. Longitudinal.
C. A combination of transverse and
longitudinal.
Speed of Light
(in vacuum)
c = 2.9979  108 m/s
Electromagnetic Wave Energy
Non-classical result:
E = hf
E = energy
f = frequency
h = Planck constant = 6.621  10-34 J s
Electromagnetic Spectrum
Temp Influences Spectrum
• Higher T  greater
power
– P/A = sT4
– s = 5.6710–8 W m–2 K–4
• Higher T  higher
peak frequency
– lmax = b/T
– b = 2.898 106 nm K
Source: M. A. Seeds, Exploring the Universe
Poll Question
How do “warm” colors (red, orange, yellow)
and “cool” colors (green, blue) relate to
temperature?
A. At higher temperatures, more of the light
emitted is “warm”-colored.
B. At higher temperatures, more of the light
emitted is “cool”-colored.
Something to Ponder
Why do hotter objects emit a greater fraction
of their energy at short wavelengths (high
frequencies)?
Reading for Next Time
• Color
– How we see color
– Why things are colored