Download A: The magnetic field cancels outside!

Objectives:
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Magnets and Magnetic Field
EM Induction and Generators
AC Currents
Transformers
Two Poles: North and South
Same poles repel
Opposite poles attract
Magnetic Field Lines
Outside:
From North to South
Inside:
From South to North
Magnetic Field: B measured in Tesla [T]
Van Allen Belts
Protect us from the
Solar Radiation
On Earth’s Surface
B ≈ 0.0005 T
Electric Currents create Magnetic Fields
Question: Why do we use coaxial cables?
A: The magnetic field cancels outside!
Electric Current can be induced if the amount of
magnetic field lines through the coil changes with
time
The changing electric current in one coil leads to
changes in the amount of magnetic field lines
through the secondary coil.
The resultant change in the amount of magnetic
field lines in the secondary coil lead to induced
currents there
Question: Where have you seen
This principle in action?
A. Induction stoves?
Maximum Induced EMF
(in Volts) depends on:
• Number of Turns, N
• Area of coil, A
• Magnetic Field, B
• Angular speed, ω
EMF = N A B ω
Question: How much is the
max induced EMF in Volts
• Number of Turns, N = 100
• Area of coil, A = 0.0025 m2
• Magnetic Field, B = 0.005 T
• Angular speed, ω = 1 rpm
EMF = N A B ω
ω = 30 rpm ≈ 3.14 rad/s
EMF = (100) (0.0025) (0.05) (3.14) = 0.004 V = 4 mV
Effective (RMS) Current and Voltage
Ieffective = 0.707 Imax
Veffective = 0.707 Vmax
Question: How much is the max voltage in the
European Union where the effective (rms) voltage is
220 V?
EMFsecondary
EMFprimary
=
Nsecondary
Nprimary
Step-up Transformers
Question: A
Nsecondary
Nprimary
20
P1. How much max. EMF can you induced by rotating a coil at 100 rpm, that
has an area of 0.05 m2 and 100 turns, in the presence of 0.05 T magnetic
field?
Ans.
P2. If you triple the number of turns and double the area of a coil, by what
factor will you increase the induced EMF? (everything else kept the same)
Ans.
P3. The 110 V-value of the current in the US outlets is actually the effective
voltage. What is the actual maximum voltage?
Ans.
P3. A step-up transformer has 30 turns on its primary coil and 300 turns on
its secondary. If plugged into 110 Volts, what will be the voltage on the
secondary coil?
Ans.