Electromagnetic Induction - ASKnLearn

Dunman High School (Senior High Physics Department)Experimental ObservationWhen the lower loop is movedupwards, there is a current in the lowerloop. The faster the motion of the lowerloop, the larger the current. If the lowerloop motion is reversed, then thecurrent reverses.ExplanationWhen the lower loop is moved upwards, themagnetic flux linkage through the lower loop dueto the field from the upper loop changes. FromFaraday’s law, an e.m.f. is induced in the lowerloop. A faster motion gives rise to a morerapidly changing magnetic flux linkage throughthe lower loop and hence a larger current.When the lower loop is moved upwards, themagnetic flux through the wire increases, and byLenz’s law, the current flows in a direction todecrease the flux linkage through the loop.However, when the motion is reversed, themagnetic flux linkage through the loopdecreases and current flows in the oppositedirection so as to increase the flux linkagethrough the loop.16.3 Applications of Electromagnetic Induction16.3.1 MicrophonesThere are various types ofmicrophones that operate on theprinciple of electromagneticinduction. In one form, there is astationary permanent magnet and awire coil attached to a movablediaphragm as shown. When asound wave strikes the microphone,it causes the diaphragm to oscillate,moving the coil alternately closer toand further from the magnet. Thismovement changes the magneticflux through the coil, which in turnproduces an induced e.m.f.Connecting the coil to an amplifierincreases the magnitude of the induced e.m.f. enough that it can power a set of speakers.16.3.2 SeismographA seismograph measures the intensity of earthquakewaves using a magnet and a coil of wire. Either themagnet or the coil is fixed to the case and the other issuspended by a spring. The relative motion of the magnetand coil when the Earth shakes induces an e.m.f. output.9646 Physics (2012)Topic 16: Electromagnetic Induction 16-9