electromagnetic induction. electric fields electric fields are created bycharges a charge in an...
TRANSCRIPT
Electric Fields• Electric fields are created bycharges
• A charge in an electric field always has a force on it
+force
Magnetic FieldMagnetic fields are created by moving charges
This can happen inmagnets or current carrying wires.
Magnetic Flux
• Magnetic flux is the amount of magnetic field.
• It depends on the field strength and the area
NS
There is a large flux through this loop
And a smaller flux through this loop
Even smaller flux through this loop
Which ring has the biggest magnetic flux in it?
Which ring has the strongest magnetic field strength in it?
Magnetic field lines
• Year 12:A wire cutting across a magnetic field has an
induced EMF (or voltage)
Faraday’s LawWhen the magnetic flux through a loop
changes, there is an induced EMF (voltage)
The faster the change, the bigger the EMF
t
Changing the Flux
• You can change the flux by changing the field strength or the area perpendicular
B A
Close switch… current increases …field increasesFlux change through 2nd coil
Induced EMF in 2nd coil
Creates current in 2nd coil
Creates magnetic field that opposes the cause.
N N
Lenz’s Law Electron flow
Force on roller
1: electrons in roller are moving 2: causing them to be pushed 3: electrons in roller are now flowing 4: causing them (and roller) to be pushed
• Faraday: A flux change causes an induced EMF (voltage)
• Lenz’s Law states that the induced voltage opposes the flux change that caused it.
t
NFalling magnet creates a flux change in the pipe.
This creates induced EMF
This creates induced current
This creates induced magnetic field
magnetic field opposes flux change
Increasing current in Primary
Causes flux change in core Causes flux change in Sec Causes induced EMF in Sec Causes induced current in Sec
This is called Mutual Induction
P
S
AC Generator
• http://www.walter-fendt.de/ph11e/generator_e.htm
• A transformer is used to increase or decrease the voltage.
• Mains voltage in NZ is 240 V AC.
• Your cellphone charger needs about 4.0 V. It uses a transformer to reduce the voltage.
Increasing current in P
Causes flux change in P
Causes flux change in S
Causes induced EMF in S Causes induced current in S
This is called Mutual Induction
P S
The AC current in the primary coil is changing sinusoidaly
So the magnetic flux is changing sinusoidally
So the induced EMF is sinusoidal
• The output voltage (Vs) depends on:
the input voltagethe ratio of turns
orp
ss
p
VN
NV
s
p p
sN
N V
V
VsVpprimary secondary
Iron core
Np Ns
Mutual Induction
• This is when a changing current in one coil induces a voltage (EMF) in a second coil.
Transformers are very important for power transmission.
• The power transmitted along wires is given by:
Power = V x I• The power wasted as heat in the
wire is given by :
Power = I2 x R
• To minimise the power lost as heat, the wire’s resistance and the current must be a small as possible.
Power lost = I2 x R
• To transmit the same power, the voltage must be very large.
Power transmitted = V x I
Power station generators produce electricity at about 4000 V
Transformers increase it to about 400 000 V. (and reduce the
current)
• http://phet.colorado.edu/web-pages/simulations-base.html
An inductor is designed to oppose a changing current
This is because it can induce an EMF in itself. This is called Self
Inductance.
Increasing current
Causes increasing magnetic fieldThere is a flux change through coilCauses induced EMF
Direction of EMF opposes current change
Increasing current
Induced EMF
Current Increasing
Self Inductance
• A coil can induce a voltage in itself.
• L is called the self inductance
EMF in coil PI
tL
Self Inductance (L) is measured in Henries (H)
• This induced EMF (or back EMF) opposes the increase in current, so the current rises…
SLOWLY
This is called Self Inductance
(The coil induces an EMF in itself)
timeClose switch
Imax
L
R
Recall the meaning of time constant for a capacitor and resistor?
It means the same for an inductor and resistor.
0.63 Imax
L
R
Larger inductance, longer time to reach maximum current
Larger resistance, smaller current so shorter time to reach maximum current
current
Close switch
How would the graph change if:
Inductor had higher inductance?
Inductor had higher resistance (careful)
Decreasing current
Causes decreasing magnetic fieldThis is a flux change
Causes induced EMF
Direction of EMF opposes current change.
Current Decreasing
• This induced EMF (or back EMF) opposes the decrease in current, so the current drops…
SLOWLY
This is called Self Inductance
When the switch opens, the current drops to zero rapidly
This causes a large flux change
This induces a very large EMF
This causes a spark across the switch
Mutual Inductancehttp://
phet.colorado.edu/web-pages/simulations-base.html