Moving charges create magnetic fields. What do moving magnets do? Put a force on charges by creating electric fields.

Electromagnetic induction In this section, we will discuss the induction of voltage by changing the magnetic field in loops of wire. We have seen that moving a magnet through a coil of wire induces a current to flow in the wire. (demo?) http://phet.colorado.edu/sims/faradays-law/faradays-law_en.html

PhET simulations http://phet.colorado.edu/sims/faradays-law/faradays-law_en.html http://phet.colorado.edu/en/simulation/faraday

Magnetic Induction related to Lorentz force Wire moving in a magnetic field gets an electric field induced in it – electromotive force or emf. More generally, a changing magnetic field induces an electric field whose strength is related to the rate of change of the magnetic field. This can be thought of either as a Lorentz force on the charges in the wire moving through a magnetic field, or the emf generated by the changing flux through the loop.

Lenz’s law The direction of any magnetic induction effect must oppose the cause of the effect. The change in magnetic flux in through a loop induces an emf in the loop. The emf in the loop creates a current, which creates a magnetic field, which opposes the original change in magnetic field.

A changing magnetic flux induces a current in a metal ring A changing magnetic flux induces a current in a metal ring. In order to conserve energy, the induced magnetic field is in the opposite direction from the original field. This is the mechanism involved with the Ring Flinger demo. The opposing magnetic fields repel each other, flinging the ring into the air. If the ring is not complete, a current cannot flow and no opposing magnetic field is created.

Magnetic Induction http://phet.colorado.edu/en/simulation/faraday Generator – demo with hand cranks. Animation below http://new.wvic.com/index.php?option=com_content&task=view&id=9&Itemid=46 http://phet.colorado.edu/en/simulation/faraday

Magnetic Induction and displacement current Displacement current - If a current in a wire is carrying charges from one metal plate to another (charging a capacitor), the magnetic field around the capacitor portion of the circuit is the same as the magnetic field around the current carrying portion of the circuit. Stated in another way, a changing electric field creates a magnetic field. Magnetic induction – we have learned that a changing magnetic field creates an electric field. An accelerated charge creates a magnetic field that is changing, so that creates an electric field that is changing, so that creates a magnetic field that is changing, so that creates an electric field that is changing, so … Let there be light (or at least electromagnetic radiation). http://www.walter-fendt.de/ph14e/emwave.htm

Transformers When two coils are placed near each other, alternating current in one (the primary, which is the one connected to the power source) induces a changing magnetic field. The second coil is within that field, so it has a current induced in it. If the first coil has fewer loops than the second, the second will experience a higher induced voltage than the first. This is called a step-up transformer. If the first coil has more loops than the second, the second will experience a lower induced voltage than the first and it is called a step-down transformer. Placing an iron core in the coils will intensify the magnetic field in each.

Heat and energy loss in wires is much higher when the current is high Heat and energy loss in wires is much higher when the current is high. For power transmitted through wire, using P = IV, we can see that higher voltage results in lower current for the same power. Thus, we step up the voltage for tranmission and then step it down again where we want to use the electricity. Since alternating current is more easily stepped up and stepped down than is direct current, electricity is transported over long distances via AC rather than DC.

Rewriting this in terms of voltage and current yields: The relationship between the voltages of the primary and secondary coils is given by: The transformer transfers energy from one coil to the other. The rate at which the energy is transferred is the power. Since energy is conserved, the power of the first coil equals the power of the second coil. Rewriting this in terms of voltage and current yields:

Online induction demos from Eugenia Etkina http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=91 http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=123 http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=122 http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=92 http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=93