1. Physics 106 Lesson #21 Magnetism: Motors and Generators Dr. Andrew Tomasch 2405 Randall Lab atomasch@umich.edu

2. Fingers point in the direction of the current flowing in the wire Right Hand The force on a wire carrying current is perpendicular to both the wire and the magnetic field The magnitude of the force is the product of the current, the wire length and the component of the field perpendicular to the wire The force on a wire carrying current parallel to the magnetic field is therefore zero! Review: The Magnetic Force on a Wire

3. Motors: Exploiting the Magnetic Force on Current-Carrying Wires A current is established in a loop by a battery. When the current-carrying loop is placed in a magnetic field the field exerts a torque on the loop causing it to rotate.

4. Generating Electric Power: Faraday’s Discovery  Changing magnetic fields induce voltages in loops and therefore drive currents! Michael Faraday 1791-1867

5. An Experiment: A Changing Magnetic Flux Induces a Voltage Faraday demonstrated that a voltage was induced if the magnetic flux through a loop changed with time Qualitatively, magnetic flux is the number of field lines passing through the loop Move a magnet near a loop or a loop near a magnet and you can generate electricity! Faraday Induction turned on the lights for all mankind! Demonstration

6. Faraday Induction: Basic Observations 1.Constant magnetic fluxes induce no voltage. 2.The magnitude of the induced voltage is proportional to how fast the magnetic flux is changing with time. 3.When there are many loops, the total induced voltage increases proportional to the number of loops. 4.The induced voltage is also called an “Electromotive Force” or EMF

7. Quantifying Induced Voltage (EMF) With Magnetic Flux Define Magnetic Flux Φ as the product of the area of the loop and the component of the magnetic field perpendicular to the loop. Φ is quantitatively equivalent to the number of field lines passing through the loop. N Loops

8. How to Generate EMF 1)Change the magnetic field with time. 2)Change the loop area with time. 3)Change the angle between the loop and the field with time (spin the loop). 4)Any combination of 1 – 3. To change the magnetic flux with time you can: N Loops

9. The Alternating Current (ac) Generator Rotating a coil of wire in a magnetic field at a constant rate produces alternating current that varies with time as a sine function