Studio Session 6

Faraday's law

Changing magnetic fields are a source of electric fields.  Faraday's law states that the emf induced in a loop is equal to the rate of CHANGE of the magnetic flux through the area enclosed by the loop.  The equation below expresses Faraday's law in mathematical form.

ΔΦB/∆t (through a fixed area) = -emf

In this studio session you will use a simulation to explore consequences of Faraday's law, and another simulation to explore the inductance of circuit elements.

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.


Activity 1

Your instructor will ask students to perform simple experiments demonstrating induction and Lenz's law.  You will be asked to answer questions and provide explanations.  Record your answers or explanations in your log.


Exploration 1

Use an on-line simulation from the University of Colorado PhET group to explore the Faraday’s law.
Link to the simulation: http://phet.colorado.edu/en/simulation/faraday

Explore the interface!

Click on the "Pickup Coil" tab.

What happens when a magnet moves through a coil in which a current can flow?
(Note:  The flow of electrons is shown.  The direction of current flow is opposite to the direction of the flow of electrons.)

Move the magnet at a relatively constant frequency back and forth through the coil.  Connect the voltmeter to the coil.  The voltage displayed is proportional to the current flowing in the coil.  Watch the reading of the voltmeter.  What happens

Record your observations for each type of change.

Click the "Transformer" tab. 

In a transformer, a current in one coil creates a magnetic field.  When the flux of this magnetic field through the second coil changes, an induced current flows in the second coil.

Click the "Generator" tab

In a generator, an external force moves a magnet in a magnetic field.  The external force does work and this work is converted into electrical energy.  This simulation provides a simple model for the generators in a hydroelectric plant.

Imagine you were actually turning the magnet by hand to generate a current. 


Exploration 2

Use an on-line simulation from the University of Colorado PhET group to explore the inductance of circuit elements.
Link to the simulation: http://phet.colorado.edu/en/simulation/circuit-construction-kit-ac

Explore the interface!

Construct a circuit.

circuit

Use a 50 V battery in series with a switch, connected to  a 10 Ω resistor R1 and a light bulb with resistance R2 = 100 Ω in parallel.  Use a current chart to monitor the current through the bulb and a voltage chart to monitor the voltage across the switch.

Make predictions before you play the animation:

Play the animation.

Stop the animation.

The circuit you have constructed could be a model for a low resistance device, such as the coil of an electric motor, in parallel with a high resistance device, such as an indicator light.  But a coil has self-inductance.  For a better model, place a 50 H inductor in series with the resistor R1.

circuit

Play the animation.

A spark across the switch can be prevented, by connecting a diode in parallel with the inductor.  A diode is an electronic component that lets current flow through it in only one direction.  It has near zero resistance when "forward biased", and very high resistance when "reverse biased".  When the switch is closed, the diode is reverse biased and affects the circuit minimally.  When the switch is opened, the diode become forward biased and current flows through the near-zero resistance diode instead of arcing across the switch.  Unfortunately the simulation does not let us put a diode into the circuit.

curcuit

Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.