Download Electricity from magnetism

ELECTRICITY FROM
MAGNETISM
CHAPTER 20, SECTION 1
OBJECTIVES
• Recognize that relative motion
between a conductor and a
magnetic field induces an emf in the
conductor.
• Describe how the change in the
number of magnetic field lines through
a circuit loop affects the magnitude
and direction of the induced electric
current
OBJECTIVES
• Apply Lenz’s law and Faraday’s law of
induction to solve problems involving
induced emf and current.
ELECTROMAGNETIC INDUCTION
Electromagnetic Induction – the process
of creating a current in a circuit loop by
changing the magnetic flux in the loop.
Figure 1
When the circuit loop
crosses the lines of the
magnetic field, a
current is induced in the
circuit, as indicated by
the movement of the
galvanometer.
ELECTROMAGNETIC INDUCTION
* Consider a closed circuit consisting of
only a resistor that is in the vicinity of a
magnet. There is no battery to supply a
current. If neither the magnet nor the
circuit is moving with respect to the other,
no current will be present in the circuit. But,
if the circuit moves toward or away from
the magnet or the magnet moves toward
or away from the circuit, a current is
induced.
The separation of charges by the
magnetic force induces an emf.
• The separation of positive and
negative moving charges by the
magnetic force creates a potential
difference (emf) between the ends of
the conductor. (See Figure 2, p.709)
The angle between a magnetic field
and a circuit affects induction.
• One way to induce an emf in a closed
loop of wire is to move all or part of the
loop into or out of a constant
magnetic field.
• No emf is induced if the loop is static
and the magnetic field is constant.
The angle between a magnetic field
and a circuit affects induction.
• The induced emf and current are:
a) largest, when the plane of the loop is
perpendicular to the magnetic field;
b) smaller, when the plane of the loop is
tilted into the magnetic field;
c) zero, when the plane of the loop and
the magnetic field are parallel
Change in the number of magnetic
field induces a current.
• Changing the size of the circuit loop or
rotating the loop changes the number
of field lines passing through the loop,
as does changing the magnetic field’s
strength or direction.
WAYS OF INDUCING A
CURRENT IN A CIRCUIT
• (Copy Table 1, page 710)
1. Circuit is moved into or out of
magnetic field (either circuit or
magnet moving).
2. Circuit is rotated in the magnetic field
(angle between area of circuit and
magnetic field changes).
3. Intensity and/or direction of
magnetic field is varied.
CONCEPTUAL CHALLENGE
• Homework: Answer in your notebook,
page 711.
LENZ’S LAW
• Heinrich Emil Lenz’s law states that:
“The magnetic field of the induced
current is in a direction to produce a
field that opposes the change causing
it.”
Note: The field of the induced current
does not oppose the applied field but
rather the change in the applied field.
FARADAY’S LAW
• Michael Faraday’s law of induction
predicts the magnitude of the induced
emf.
• Faraday' s law is not an explanation of
Induction but merely a description of
what induction is.
FARADAY’S LAW OF MAGNETIC
INDUCTION
average induced emf = – the number
of loops in the circuit x the time rate of
change of the magnetic flux
FARADAY’S LAW OF MAGNETIC
INDUCTION
Alternative formula:
or
Where: A = circuit area
B = applied magnetic field
strength
θ = angle of orientation
SAMPLE PROBLEM
• A coil with 25 turns of wire is wrapped
around a hollow tube with an area of 1.8
sq.m. Each turn has the same area as the
tube. A uniform magnetic field is applied
at a right angle to the plane of the coil. If
the field increases uniformly from 0.00 T to
0.55 T in 0.85 s, find the magnitude of the
induced emf in the coil. If the resistance
in the coil is 2.5 Ω, find the magnitude of
the induced current in the coil.
SAMPLE PROBLEM
• Answer:
emf = -29.12 V , approx. 29 V
I = -11.65 A, approx. 12 A
PRACTICE EXERCISES
1. A coil with 25 turns of wire is moving in
a uniform magnetic field of 1.5 T. The
magnetic field is perpendicular to the
plane of the coil. The coil has a crosssectional area of 0.80 sq.m. The coil
exits the field in 1.0 s.
a. Find the induced emf.
b. Determine the induced current if the
coil’s resistance is 1.5 Ω.
PRACTICE EXERCISES
• Answer:
a. 30 V
b. 20 A
PRACTICE EXERCISES
2. Solve Practice A exercises, page 714.
3. Solve Section Review, no. 3, page
715.
PRACTICE EXERCISES
• Answers:
Practice A
1. emf = 0.30 V
2. I = 14 A
3. emf = 0.14 V
4. B = 4.83 x 10^(-5) T
Section Review
3. emf = 0.21 V