PHYS 221 Recitation

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