l19 ch30 inductors - faculty server...

Department of Physics and Applied PhysicsPHYS.1440 Lecture19 A.Danylov

Lecture 19

Chapter 30

Inductors

Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII

Physics II


Today we are going to discuss:

Chapter 30:

Section 30.8


Faraday’s law


Induced Electric Field in a Solenoid The current through the solenoid

creates an upward pointing magnetic field.

As the current is increasing, B is increasing, so it must induce an electric field.

We could use Lenz’s law to determine that if there were a conducting loop in the solenoid, the induced current would be clockwise.

The induced electric field must therefore be clockwise around the magnetic field lines.


Induced Electric Field in a Solenoid

ConcepTest Faraday’s Law

The magnetic field is decreasing. Which is the induced electric field?

E. There’s no induced field in this case.

The field is the same direction as induced current would flow if there were a loop in the field.

CCWImagine a loop

CCW


InductorsInductors (solenoids) store potential energy in

a form of a magnetic field.


Inductance (definition)Consider a solenoid of N turns with current I.

The total magnetic flux is I

Φ ~

The coefficient of proportionality is called inductance, L

The SI unit of inductance is the henry, defined as:1 henry = 1 H = 1 Wb/A = 1 T m2/A

The circuit symbol for an ideal inductor is .

The solenoid’s magnetic field passes through the coils, establishing a flux.


Let’s find solenoid inductanceConsider a solenoid of N turns with current I.

Recall (Lecture 14)

Φ

You see, solenoid inductance depends only on its geometry

Φ Φ ∙ ∥


Potential Difference across an Inductor

Φ

Δ

Potential difference across an inductorΔ


Potential Difference across an Inductor (cont)

• If current increases, 0 Δ 0

• If current decreases, 0 Δ 0

• If current is constant, I=const Δ 0

The induced V decreases across L if the current increases

The induced V increases if the current decreases _

_ind ind

So, L generates additional pot.difference in the “opposite direction“ relative to the original battery and, as a result, additional current Iind flows in the opposite direction to the original current reducing I0 growth in time (a system does not like changes).

• The magnitude of I has no effect on ΔV, only the rate of change of I counts.

+i

0

ind indSo, L generates additional pot.difference in the “same direction“ relative to the original battery and, as a result, additional current Iind flows in the same direction as the original current supporting decreasing I0 in time (a system does not like changes).

t

I

Turning current off

t

I

Turning current on

Expect Real

Expect Real

0

‐+fi

f+_

Δ


=ΔV across a solenoid when the current increase

(increases)

Magnetic filed created by I

Induced Magnetic filed

I

is in the opposite direction to a direction of I

+ _

(Physics)

" “which can create this induced current

A. Current I1

B. Current I2

C. They are changing at the same rate

D. Not enough information to tell

Which current is changing more rapidly?

ConcepTest Inductor

41

22


Thank you

l19 ch30 inductors - faculty server...

Documents