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The conductor is filled with mobile charges (by definition).
X X X X X
X X X X X
X X X X X
X X X X X
+ +++
This causes, (induces) a current to flow.
Each charge is a moving charge in a magnetic field, and will therefore have a force exerted on it. (RHR#3)
The conductor is filled with mobile charges (by definition).
X X X X X
X X X X X
X X X X X
X X X X X
+ +++
We call this phenomena
Electromagnetic Induction
An electromotive force (emf) is produced in a conductor whenever it cuts across magnetic field lines. No emf
arises from motion parallel to a magnetic field.
Coil moves Down
Coil moves Up
Magnetic Field Lines
Calculating EMF
EMF = Electromotive Force (Volts)B = Magnetic Field (T)L = Length of the wire (m)v = velocity of wire that is moving in the field (m/s) = angle between wire and magnetic field
Lenz’s Law:The direction of an induced current is
always such that its own magnetic field opposes the magnetic field responsible for producing it.
Lenz’s law is a statement of the Law of Conservation of Energy
Drop a magnet through a conducting ring.
N
SA current will be
induced in the ring.
Consider a charge Q in
the ring
Drop a magnetic through a conducting ring.
N
SThe
secondary magnetic
lines of force always
oppose the creating lines
of force.
• Usually it isn’t a single straight wire that is moving in a external magnetic field.
• Usually it is a coil of wires (like what you saw with the motor) that are rotating in the magnetic field.
Alternating Current (AC)
Remember, a coil is just a solenoid with one loop & a solenoid creates a magnetic field just
like a magnet
Alternating Current (AC)• This is how the electric
company generates the electricity we use.
• Mechanical energy (spinning the coil) turns into electrical energy (EMF)
• The catch is figuring out what will spin the coil in the external magnetic field.
Effective Current & Effective VoltageUnits: Amps
Ieff = effective current (average current)
Imax = maximum current
Units: VoltsVeff = effective voltage
(average voltage)Vmax = maximum voltage
In the USA, Veff = 120 V
Example: A generator is created by moving a 0.20 m long wire in an external magnetic field of 2.0 T at a speed of 15 m/s. (A) What EMF is produced by the generator?
EMF = (2.0 T)(0.20 m)(15 m/s)sin90o
EMF = 6.0 V
(B) If the 6.0V is the maximum voltage produced by the generator, what is the effective emf?
Veff = 0.707 (6.0 V)
Veff = 4.2 V
(C) If the generator is hooked up to a light bulb with a resistance of 2.5 , what is
the effective power of the the bulb?
Remember: P = IV = I2R = V2/R
Peff = (Veff)2/R
Peff = (4.2)2/2.5
Peff = 7.1 W
Mutual Inductance• Mutual inductance is a measure of the
ability of one circuit carrying a changing current to induce an emf in a nearby circuit.
• The coil carrying the current initially is called the primary coil.
• The coil in which the current is induced is called the secondary coil.
Transformers
• Transformers are devices that change one AC potential difference to a different AC potential difference.
• Real transformers are not perfectly efficient.– Efficiencies of real transformers typically range
from 90% to 99%.
• For this class, we will assume 100% efficiency. (We like living in an ideal world) :)
Residential Power Poles usually have 3 levels of wires.
High Voltage
120 –240 Volts
Telephone-Cable TV
Transformer
Step up vs Step down:Think of a staircase
• Step up: Start with a low voltage, end with a higher voltage
• Step down: Start with a high voltage, end with a lower one.
Vp
Vp
Vs
Vs
Transformerscoilssecondary
voltssecondary
coilsprimary
voltsprimary
##
Volts x amps = Volts x amps
in out
Transformer Equation
p
s
p
s
N
N
V
V
Where V = potential difference (voltage)
N = # of turns in coil
Subscript p refers to primary coil
Subscript s refers to secondary coil
Example: A step up transformer is used on a 120 V line to provide a potential difference of 2400 V. If the
primary has 75 turns, how many turns must the secondary have?
p
s
p
s
N
N
V
V
Vp = 120 V
Vs = 2400 V
Np = 75 turns
Ns = ?75120
2400 sN
Ns = 1500 turns
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