Transcript File - The Physics Doctor

A –Level Physics: Magnetic Fields
Generating Alternating Current
Objectives:
FLASHBACK
FLASHBACK: Explain how you could experimentally
determine the viscosity of a fluid (6 marks)
Starter Activity
What factors effect the EMF induced by the motor
effect?
Faraday’s Law
In the previous lesson, we discovered that due to electrons
feeling a force due to movement of the wire through a
magnetic field, an EMF is induced.
Reversing the direction of the field or the motion of the
wire will reverse the polarity of the EMF (direction).
Faraday’s law states “the induced emf is
proportional to the rate of change of flux
linkage”
Convert this definition into a simpler one. Remember what flux linkage
actually is!
Lenz’s law
Record what you observed in this video
If you drop a magnet through a copper
tube, it will fall slower than a normal piece
of metal.
As a copper tube is essentially many
stacked coils, when the magnet falls, a
small EMF is produced in each coil.
The resulting current will generate an electromagnetic field
which interacts with the magnet, slowing it down.
“The direction of an induced emf is such as to
oppose the change causing it”
Lenz’s law- Fleming’s Right Hand
Make the same shape as the left hand rule but with your right hand
Before, we used Fleming’s left hand
rule to determine the motion of
the wire (with a flowing current)
once placed in a magnetic field
This time, if the wire is physically
moved within a field, we can
determine the direction of the
induced emf via Fleming’s right
hand rule
Calculating the induced EMF
This equation indicated that the EMF is proportional to the rate of
change of flux (Faraday’s) and the negative sign indicates that this
induced emf is in a direction that opposes its creation (Lenz’s)
Practice Question
Sarah had a coil of wire with 120 turns with a radius of 5cm held in
a magnetic field of 20mT. She removes it in 0.02s. What emf is
induced? Hint: Find quantity of flux first
Alternating Current
As described before, if a coil of wire is continuously turned
within a magnetic field, an emf is consistently induced.
The direction of this emf will also constantly change, thus an
alternating current results
Alternating Current- Measuring
We can look at the waveform of an alternating current to take
various measurements.
Frequency= 1/T
Peak Value= the
maximum voltage
(Vo) or maximum
current (Io) e.g. in
UK, the peak voltage
is +/- 325V.
RMS (root mean square) value: The true ‘mean’ value of
an alternating current’s voltage/current. These are the
values you directly compare with voltage/current for DC.
Root Mean Square Value
To calculate the RMS for
current (replace V for I) or
voltage:
What is the RMS voltage of the AC
mains electricity in the UK?
Independent Research
Use the textbooks provided to write notes on
electromagnetic induction involving transformers. Notes
need to be made answering these questions:
1) What do ‘primary coil’ and ‘secondary coil’ refer to?
2) How would you calculate the input or output voltage in a
transformer?
3) Give an example calculation of the question above
4) Would a transformer function with a DC?
Practice and I/S
Revise all magnetic fields and capacitors. AND
tomorrow I want to see your EXAM PRACTICE
complete (half a section), your astrophysics and
the Bev/Bil