Physics 106 Lesson #22 Magnetism: Speakers and Microphones

1. Physics 106 Lesson #22Magnetism: Speakers and Microphones Dr. Andrew Tomasch 2405 Randall Lab atomasch@umich.edu

2. Review: The Magnetic Force on a Wire • 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! Fingers point in the direction of the current flowing in the wire RightHand

3. Review: Motors Exploit 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 loopcausing it to rotate. A rotating switch (commutator) switches the current direction to maintain the torque in the same direction as the loop spins.

4. 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 throughthe 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! Review: A Changing Magnetic Flux Induces a Voltage Demonstration

5. Review: Quantifying Induced Voltage (EMF) With Magnetic Flux • Define Magnetic FluxΦas the product of the area of the loop and the component of the magnetic fieldperpendicular to the loop. • Φ isquantitativelyequivalent to the number of field lines passing through the loop. NLoops

6. Review: How to Generate EMF • Change the magnetic field with time. • Change the loop area with time. • Change the angle between the loop and the field with time (spin the loop). • Any combination of 1 – 3. To change the magnetic flux with time you can: NLoops

7. Review: Generating Alternating (ac) Current Rotating a coil of wire in a magnetic field at a constant rate produces alternating current that varies with time as a sine function

8. Loudspeakers: Another Type of Motor Applying an alternating voltage to the coil of a loudspeaker causes an alternating current to flow in the coil (I = V/Rcoil) which in turn causes an alternating magnetic force on the coil and speaker cone. This alternating force causes the cone to vibrate. The vibrating cone produces a sound wave which is a copy of the alternating input voltage (the input signal). Note that the simple speaker we built today has the magnet attached to the cone and the coil stationary, but it functions in exactly the same way.

9. Microphones: Another Type of Generator A microphone generates an alternating voltage across a coil of wire by exactly the same process of Faraday Induction that generates electrical power for the power grid. In this case the vibrations from a passing sound wave cause a magnet to vibrate near a coil (or vise-versa) to produce an alternating voltage signal which is a copy of the original sound. The signal is tiny compared to the AC EMF from a generator, but the physics is identical—changing magnetic flux induces a voltage across a coil.