1. A –Level Physics: Magnetic Fields Generating Alternating Current

2. Objectives:

3. FLASHBACK
FLASHBACK: Explain how you could experimentally determine the viscosity of a fluid (6 marks)

4. What factors effect the EMF induced by the motor effect?
Starter Activity
What factors effect the EMF induced by the motor effect?
Current, Magnetic flux density, Length of wire within the field,
Angle of the wire,

5. 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”
5.1 x N
Convert this definition into a simpler one. Remember what flux linkage actually is!

6. Record what you observed in this video
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.
5.1 x N
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”

7. 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
5.1 x N

8. 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)
5.1 x N

9. 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
Area= 7.85x10^-3 ……Quantity of flux=BA, = 20x10^-3 x Area…..Flux linkage = BAN = 1.88x10^-2 Wb0turns, Emf= 0.94V

10. 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
5.1 x N

11. 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 (constant)
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.

12. Root Mean Square Value
To calculate the RMS for current (replace V for I) or voltage:
RMs= 325/sqrt2 = 230V
What is the RMS voltage of the AC mains electricity in the UK?

13. Independent Research
Use the textbooks provided to write notes on electromagnetic induction involving transformers. Notes need to be made answering these questions:
What do ‘primary coil’ and ‘secondary coil’ refer to?
How would you calculate the input or output voltage in a transformer?
Give an example calculation of the question above
Would a transformer function with a DC?

14. 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
Mass spectrometer and Flux Linkage Questions