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PHYS 221 Recitation

PHYS 221 Recitation. Kevin Ralphs Week 6. Overview. HW Questions Electromotive Force (EMF) Motional EMF Farraday’s Law Lenz’s Law Inductance. HW Questions. Ask Away…. Electromotive Force (EMF). What does it tell me?

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PHYS 221 Recitation

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  1. PHYS 221 Recitation Kevin Ralphs Week 6

  2. Overview • HW Questions • Electromotive Force (EMF) • Motional EMF • Farraday’s Law • Lenz’s Law • Inductance

  3. HW Questions Ask Away…

  4. Electromotive Force (EMF) • What does it tell me? • The change in potential energy per unit charge an object has when moved along a path • It can also refer to the voltage measured across two terminals • Why do I care? • So far we have considered conservative electric fields which have scalar potentials • For non-conservative fields, the change in potential energy becomes path dependent and EMF is accounting for that

  5. Electromotive Force (EMF) • Why do I care? • If a particle is free to move around in space, this is not all that helpful, but when they are constrained to move on a specified path (like an electronic circuit), it becomes well-defined. Note: • This is not a force, it has units of volts • This is not a potential, the path taken matters very much

  6. Motional EMF • When a conductor moves through a magnetic field, it acquires an EMF (this is more along the lines of the two terminal definition) • This happens because a Lorentz force from the magnetic field shuffles charges to opposite ends of the conductor • This sets up a voltage like a parallel plate capacitor bringing the charges into an equilibrium

  7. Motional EMF

  8. Faraday’s Law • Two earlier approximation schemes • Electrostatics • Stationary charges • Conducting charges at equilibrium • Magnetostatics • Steady Currents

  9. Faraday’s Law • In electrodynamics we allow non-steady currents • This causes time varying magnetic fields bringing Faraday’s law into effect

  10. Faraday’s Law • What does it tell me? • A changing magnetic field creates a non-conservative electric field • Anything that affects that flux integral induces an EMF in a loop • Why should I care? • Without this law, you could not see, there would be no cell phones or radio: electromagnetic waves exist because of this • Inductors and transformers exploit this phenomenon

  11. Lenz’s Law • What does it tell me? • When the flux through a loop changes, a current is produced that fights this change • Why should I care? • This principle is how you determine the direction of an induced current

  12. Lenz’s Law • If you are having problems with this, you are not alone • People spend thousands of hours researching this (no kidding) • The idea is to find the direction of the induced magnetic field and use the right hand rule to find the current • To find the direction of the induced field • Note the direction of the original field through the loop • Determine whether this field is getting stronger or weaker • The direction of the induced field will maintain the status quo

  13. Inductance • What does it tell me? • The flux through a loop depends linearly on the currents through conductors in the vicinity (including itself) • This is a direct consequence of the principle of superposition and magnetic fields being proportional to the currents that create them

  14. Inductance • Why should I care? • This is the sister component to the capacitor making it one of the most fundamental electronic components

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