













Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
Key points in this lecture are: Magnetic Induction, Electromagnetic Induction, Faraday’s Law, Alternating Current, Generators, Power Production, Transformers, Power Transmission, Self-Induction, Field Induction Topics covered in this course "Basic Concepts of Physics" are: Newton’s Laws of Motion, Linear Motion, Momentum, Energy, Rotation, Gravity, Liquids, Gase, Plasmas, Heat, Waves, Sound, Electrostatics, Electric current, Magnetism, Electromagnetic Induction, Color, Light, Atom and Quantum.
Typology: Slides
1 / 21
This page cannot be seen from the preview
Don't miss anything!
if magnet is moved near stationaryconductor,
or
magnet
-^
Recall ch. 24
The key point is that the conductor lies in a region where the magnetic fieldchanges
http://www.youtube.com/watch?v=sPLawCXvKmg
Eg. Idea behindtransformers, seeshortly:
Evenjust 1loopherewillwork.
-^ Recall that induced voltage (or current)
direction
changes as to
whether magnetic field is increasing or decreasing (eg magnet beingpushed in or pulled out). In fact:frequency of the alternating voltage = frequency of changing magneticfield.
Generator:
when coil is rotated in a stationary
magnetic field: ac voltage induced by thechanging field within the loop.Note similarity to motor from Ch. 24: the onlydifference is that in a generator, the input isthe mechanical energy, the output is electrical.(other way around for motor).
Note, change in # field linesintersecting the loop area,as it rotates.
Fundamentally, induction arises because of the force on movingcharges in a magnetic field (recall Ch.24):
Motor: currentalong wire,means movingcharges in magfield. Soexperienceforce perp tomotion and tofield, ie. upward.
Generator: wire(no initial current)moved downward,so electrons aremoving down infield, so feel forceperp to motion andto field ie alongwire, i.e. a current.(+ ions also feelforce, in opp dir.but not free tomove).
Consider first the following arrangement of side-by-side coils:
The primary coil has a battery, sowhen switch is closed, current flowsin it, creating a sudden magneticfield that threads the secondary coil– inducing current pulse in it too. (Note no battery in secondary coil). Only brief though, since current insecondary only flows at the time theswitch in primary is opened or shut.
Question: Say the switch in primary coil is closed at time 0 and then openedagain after 5 seconds. What is (roughly) the behavior of the current in theprimary coil? the secondary coil?
Primary: current begins to flow at time 0, is constant for 5
seconds, and then drops to zero.
Secondary: current pulse at time 0 flows in one direction,
then goes to zero while the primary current is constant. Then pulse flows inopposite dir. when the switch is opened, and again goes to zero afterwards.
-^ To maintain current flow in the secondary coil, need always
changing
magnetic field
, i.e. always changing current in the primary coil –
use ac
iron core
through the coils, as this intensifies the
field (recall Ch.24) and so amplifies the current through the secondary,i.e. simple transformer looks like:
-^ Recall dependence on # coils (called #
turns
-- the field generated by the primary coil is greater if there are more
loops in it (Ch24, property of electromagnets)
-- the voltage induced in the secondary coil is greater if there are
more loops in it (Faraday’s law)
So…..
docsity.com
Because of energy conservation, if the voltage in the secondary isstepped up, the current must be correspondingly lower:
Power into primary = power out of secondary,
so
(voltage x current)
primary
= (voltage x current)
secondary
Recall: rate ofenergy transfer•
Transformers are behind the main reason why most electric power is acrather than dc: easy way of stepping up and down.• To transmit across large distances (i.e. cities…), want to minimize energyloss due to wire heating i.e. want
low currents
, so correspondingly
high
voltages
i.e. step up for transmission
If 120 V of ac are put across a 50-turn primary, what will be thevoltage and current output if the secondary has 200 turns, and isconnected to a lamp of resistance 80
(120 V)/50 = (?V)/(200), so? = 480 VCurrent = voltage/resistance = 480/80 = 6 A
What is the power in the secondary coil?
Power = voltage x current = 480 V x 6A = 2880 W
Can you determine the current drawn by the primary coil? If so, whatis it?
current = power/voltage, and power input = power out = 2880W
so, current = 2880/120 = 24 A
Current-carrying loops in a coil interact with magnetic fields ofloops of other coils, but also with fields from loops of the same coil– called
self-induction
Get a self-induced voltage, always in a direction
opposing
the
changing voltage that creates it. - called
back emf
(= back
electromotive force)
-^
We won’t cover this much, except to say that this is behind thesparks you see if you pull a plug out from socket quickly, whiledevice is on:
Consider here a long electromagnetpowered by a dc source. So have strongmag field through coils. If suddenly open aswitch (e.g. pulling the plug), the currentand the large field go to zero rapidly. Largechange in field
large induced voltage
(back emf) – this creates the spark (zap!).
Fundamentally, a changing mag field produces an
electric field
, that
consequently yields voltages and currents.
-^
You don’t need wires, or any medium, to get fields induced.
-^
Generally, Faraday’s law is An electric field is created in any region of space in which amagnetic field is changing with time. The magnitude of the inducedelectric field is proportional to the rate at which the magnetic fieldchanges. The direction of the induced electric field is perpendicularto the changing magnetic field. Complementary to Faraday’s law (due to Maxwell):
just interchange
“electric” and “magnetic” in the law above!
i.e.
A magnetic field is created in any region of space in which anelectric field is changing with time. The magnitude of the inducedmagnetic field is proportional to the rate at which the electric fieldchanges. The direction of the induced magnetic field isperpendicular to the changing electric field.
-^
This beautiful symmetry is behind the physics of light andelectromagnetic waves generally!