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)

5. 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: 1.This is not a force, it has units of volts 2.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

10. 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

14. Why should I care? – This is the sister component to the capacitor making it one of the most fundamental electronic components CapacitorInductor Depends on geometry and material between the plates Depends on geometry and material in intervening space Proportionality between charge and voltage Proportionality between flux and current Stores energy in an electric fieldStores energy in a magnetic field Causes current to lag voltageCauses current to lead voltage Current starts at maximum and drops to zero Current starts at zero and increases to maximum