Unit Planner: Electromagnetic induction
Unit: Electromagnetic induction
x Paper 1
x Paper 2
Text book reference
Inquiry: Establishing the purpose of the unit
List here one to three big, overarching, long-term goals for this unit. Transfer goals are the major goals that ask students to “transfer”, or apply, their knowledge, skills, and concepts at the end of the unit under new/different circumstances, and on their own without scaffolding from the teacher.
- To know the the quantitative relationship between the EMF generated in electromagnetic induction and rate of change of flux linkage
- To understand how the charge on a capacitor varies with time during charging and discharging by consideration of time constant
- To be able to consider means to reduce energy losses in electricity distribution
List here the key content that students will know by the end of the unit.
- Using knowledge about the force on a moving charge in a B field deduce that the free electrons in a conductor moving in a B field will experience a force.
- Derive the formula for the PD across the conductor.
- State Faraday's law.
- Use Faraday's and Lenz's laws to solve problems involving conductors in changing B field including the straight conductor and rotating coil examples.
- Use Fleming's RHR to find the direction of current in a moving conductor.
- Draw a diagram showing the basic parts of a simple transformer.
- Apply the law of conservation of energy to an ideal transformer.
- Derive the equation Vrms = Vpeak/√2 and use it to calculate rms values.
- Introduce the parallel plate capacitor.
- Define dielectric constant and understand why the introduction of a dielectric will change the capacitance.
- Derive the equations for capacitors in series and parallel.
- Derive the equation for energy stored by considering the process of charging a capacitor by adding small charges.
- Know the difference between N and P type semiconductor.
List here the key skills that students will develop by the end of the unit.
- Using Fleming's LHR find the direction of the force and conclude that a PD will be set up across the conductor.
- Apply Faraday's law to solve simple problems.
- Apply Faraday's and Fleming's RHR to show that an AC signal will be induced in a rotating coil.
- Apply the formula Vp/Vs = Np/NS for to solve problems invloving sinusoidal currents in transformers.
- Apply the law of conservation of energy to an ideal transformer.
- Solve problems related to combinations of capacitors.
- Construct a half wave rectifier.
- Construct a full wave rectifier.
List here the key concepts that students will understand by the end of the unit.
- Revise magnetic fields and the force on a moving charge in a B field.
- Apply conservation of energy to deduce that if a current flows then an EMF has been induced.
- Understand how Lenz's law is a consequence of the law of conservation of energy.
- Use Faraday's law to explain the operation of a transformer.
- Understand why a transformer only works with AC.
- Understand how power is lost in a transformer.
- Understand why the power delivered by an AC signal is VrmsIrms.
- Understand how power is lost in the transmission of electrical energy and why it is reduced by stepping up the voltage.
- Understand why the capacitance is inversely proportional to separation of plates but proportional to area of plates.
- Understand the exponential nature of the change of V, Q an I as a capacitor is charged and discharged.
- Understand how the characteristic VI curve is related to the semiconductor properties.
- Understand the importance of rectification in domestic applications.
Examples of real world practical applications of knowledge.
- All transformers (mobile phone chargers) and generators use this principal.
- Induction braking
- Maglev. To slow down an electric car (and maglev) the motor is used as a generator. The induced current is in the direction so as to oppose the change producing it so slows down the car.
- Electrical power transmission, device charges, spark plugs, electric fences.
- The frequency and rms voltage of domestic electricity is not the same in all countries.
- Smoothing a rectified signal.
- Used to be used in photographic flash lights to store charge needed to make the flash. This meant that there was a time delay between flashes as the capacitance charged up.
- Used in any plugged in device that uses DC. For example, a computer power supply takes AC from the mains socket, steps down the potential and converts it to DC to power the computer.
- Converting AC generated electricity to DC after distribution.
- Converting DC generated electricity to AC from photovoltaic cells.
Action: teaching and learning through Inquiry
Approaches to teaching
Tick boxes to indicate pedagogical approaches used.
x Small group work (pairs)
x Hands on practical
Examples of how TOK can be introduced in this unit
- On youtube there are many examples of ways to generate electricity without doing work, e.g. Free energy
Is it worth spending any time discussing such examples? The law of conservation of energy tells us this must be a fake.
- Another example of the way laws are used in physics to solve problems.
- There are several interesting videos showing how energy can be created using magnets and coils. It's interesting to see if students can spot the flaws in the arguments presented, including breaking the law of conservation of energy.
- What to believe? After doing the theory behind the transformer I get students to calculate the secondary voltage from when a 9V battery is connected to a simple transformer. Using Vp/Vs = Np/NS the voltage is not very high so I ask if they think they will get a shock or not. Few will dare to touch the wires even though they have calculated that it shouldn't hurt. Quite right too, when the circuit is switched on the rise in current is very rapid so the output voltage is much higher than calculated resulting in a small shock. Moral: don't believe your physics teacher.
- When capacitors are joined in series the combined capacitance is given by 1/CT = 1/C1 + 1/C2 and when in parallel CT = C1 + C2 .
- This is the opposite of resistor combinations which makes it very difficult to remember.
- Rectify means "to put right"; why is the word also used for changing AC to DC?
Examples of how NOS can be introduced in this unit.
- The version of Faraday's law used in this course (E = dφ/dt) only applies to thin wires; for other examples the Maxwell-Faraday equation should be used.
- Oersted's discovery of the connection between electricity and magnetism in 1820 is an example of a serendipitous discovery.
- Lenz's law is actually the law of conservation of energy applied to electromagnetic induction. Different laws must be consistent.
- There is a lot of controversy around the effect of the changing E and B fields around high voltage cables and living nearby.
- In Norway there is a battle between conservationists and electricity companies about building power lines from the West coast to the East. It is possible to put these in the ground and under water but is this environmentally more friendly or does it just look nicer?
- By considering a capacitor to be charged by manually taking charge from one plate to the other we can derive an expression for the energy stored even though this is not the way it is charged in practice.
- Once you start adding diodes together to make circuits it is difficult to follow what is happening to the electrons in each junction. To build circuits you only need to know what the component does not how it works.
- Physics is concerned with the principles that govern the operation of e.g. a diode, then electrical engineers construct circuits (they will of course also understand how the diode works).
Tests, exams and marked labs
- MCQs: Faraday's law
- MCQs: Lenz's law and AC
- Problems: Induction
- MCQs: Transformers and power
- Problems: Transmission of power
- Test: Generators and transformers
- MCQs: Capacitors
- Test: Capacitance
- MCQs: Charging and discharging
- Test: Charging and discharging
- Practical: Rectifier circuit
- Test: Rectification
Worksheets and exercises
Video clips, simulations demonstrations etc.
- Faraday's Law
- Lenz's Law
- Optional Practical: Falling magnet
- Optional Practical: Transformer simulation
- Optional Practical: Parallel plate capacitors (PhET)
- Optional Practical: Charging and discharging a capacitor
- Optional Practical: Iterative capacitor charging (Excel)
What went well
List the portions of the unit (content, assessment, planning) that were successful
What didn’t work well
List the portions of the unit (content, assessment, planning) that were not as successful as hoped
List any notes, suggestions, or considerations for the future teaching of this unit