Electricity + Magnetism =
INDUCTION !!
(Ch. 25)
PH 104 w/ dr. g
Lec 29
How to get electric current from a magnet:
y Move a magnet in and out of a conducting coil! (demo)
y Voltage (electric field) is induced.= electromagnetic induction
y Must have relative motion between coil and magnet.
y Need magnetic field to thread through a conducting loop.
y Current induced is higher when R of wire is smaller
yBasically a magnetic force on an electric current:
yMagnet “sees”: charges in conductor moving with conductor
yFree charges get a magnetic force: get pushed along coil!
How to get electric current from a magnet:
y Requires mechanical work
y “Inductance”: induced current opposes motion of magnet
y
Induced current’s magnetic field is in opposite direction as magnet
y More loops = more induced fields opposite the magnet’s field
y Moving the magnet more difficult with:
y
y
Larger loops and/or more loops
Higher resistance in loops
How to get electric current from a magnet:
y Faraday’s Law of Induction:
y Induced voltage ~ (number of loops) x (rate of change of the
magnetic field within the loops)
y So: more induced voltage when:
y
y
y
move the magnet faster
have more loops
use a stronger magnet
y Amount of current: depends on resistance.
y Direction of induced voltage:
y If positive while field in loops increasing (putting IN), then
voltage is negative while field in loops decreasing (taking OUT)
Where AC comes from: spinning coil in a magnet!
Where AC comes from: spinning coil in a magnet!
y Magnet in & out of coil: induced voltage changes sign:
y If positive while field in loops increasing (putting IN), then
voltage is negative while field in loops decreasing (taking OUT)
y Voltage is alternating : corresponding current = AC !
y Another way: generator: turning conducting loop in field:
y Turning: area presented to field: oscillates
y So amount of field threading to loop also oscillates,
y Induced voltage is oscillating : corresponding current = AC.
y
COIL = more loops => higher amplitude for induced voltage!
Demo…
1
Where AC comes from: spinning coil in a magnet!
y Another way: generator: turning conducting loop in field:
y Turning: area presented to field: oscillates
y So amount of field threading to loop also oscillates,
y Induced voltage is oscillating : corresponding current = AC.
y
Power production: coils spinning in magnet
y Need source of mechanical energy to turn the coils
y Turbines: turned by water current, wind, steam
y Steam: requires heat source: fuel = coal, oil, gasoline, etc.
COIL = more loops => higher amplitude for induced voltage!
Power production: coils spinning in magnet
y Need source of mechanical energy to turn the coils
y Turbines: turned by water current, wind, steam
y Steam: requires heat source: fuel = coal, oil, gasoline, etc.
Transformer: Energy transmitted thru induction
y AC in “Primary” coil induces AC in “secondary” coil
y Field produced by primary ~ Voltage in primary ~ #turns = N1
Voltage induced in secondary ~ #turns = N2 ; or:
Primary voltage Secondary voltage
=
N1
N2
yStep-up: secondary > primary: N2 > N1
yStep-down: secondary < primary: N2 > N1
y Power transmission:
y To minimize energy loss in current: minimize current in lines,
using transformer:
y
y
y
Decreases current upon leaving power station
On utility poles: re-increase current as it enters buildings
From power outlet: smaller transformer (adapter)
Transformer: Energy transmitted thru induction
Other applications of induction:
y The “shake flashlight”!
y Shaking: permanent magnet moves in and out of a coil
y Energy stored (capacitor, or charging a battery), for use by light
y Credit cards, ATM cards: swiping
y Swiping moves magnetic strip past a coil, generating a signal
y Electric guitar: pickup
y Coils with magnetic cores
y Magnets magnetized nearby string
y Oscillating string induces current in coil
= electronic signal with same frequency
Primary voltage Secondary voltage
=
N1
N2
y Step-up : not in energy, just voltage:
y Energy conserved:
Power input = power output
(voltage x current)primary
= (voltage x current)secondary
y Step-up: Secondary V higher, I is lower
y Power transmission: Use step-up to
decrease the current in lines
Step-down transformer
2