29.4 - Physics and music: electric guitars
Electric guitars use electromagnetic induction in a
component called a pickup to produce music. In the
pickup, the vibrations of the strings are converted into
an electrical signal so they can be amplified and then
played over speakers.
The pickup consists of a permanent magnet
surrounded by a coil of wire. You see a conceptual
diagram of a pickup in the illustration to the right.
Guitars typically have two or three pickups under
every string, each pickup designed to be maximally
sensitive to a particular frequency.
The permanent magnet in the pickup serves to
magnetize the nearby guitar string. A musician plucks
the string, making it vibrate. With this vibration the magnetized string moves back and
forth near the coil, inducing an emf in the wire. This emf causes a current in the wire.
The emf and the current oscillate at the same frequency as the string.
The signal is transmitted through a circuit to an amplification system, which increases
its strength, or energy. The system then sends the signal to a loudspeaker and the
audience hears the music.
The pickups (raised black modules) on this electric guitar convert
the vibrations of the strings into a varying electric current.Physics and music: electric
guitars
String magnetized by permanent
magnet in pickup
Guitar player makes string vibrate
Motion of magnetized string induces
current in coil
Current flows to amplifier/speaker29.5 - Motional induction: calculating the potential difference
The diagram to the right shows a vertical wire segment moving through a uniform
magnetic field. This field has a constant strength and points into the screen. The wire
moves at a constant velocity to the right, which means its motion is perpendicular to the
magnetic field. In this section, we show how to calculate the potential difference induced
between the ends of the wire segment by its lateral motion.
The potential difference is caused by the force of the magnetic field on mobile electrons
in the moving wire. The electrons move together with the wire as it is pushed through
the field. Because charged particles experience a force when they move through a
magnetic field, the mobile electrons get pushed to one end of the wire, leaving a
positive charge on the other end. (Which end the electrons move to can be determined
by a right-hand rule.) As more and more electrons accumulate on one end, leaving
more and more positive charge on the other, there is an increasing potential difference
across the wire.
The wire’s motion causes the formation of two charged regions, one positive and the
other negative. Now, let’s consider the electron that is shown in the center of the wire in
Concept 1. The motion in the magnetic field causes it to experience a downward
magnetic force FB, and the charged regions cause it to experience an upward electric
force FE. When these two forces balance, the electron experiences no net force and
stays in place. The system is in equilibrium and the equation on the right can be used to determine the potential difference across the wire.
In a simulation a few sections ago you saw how, when a moving wire segment like this is part of a circuit, it becomes an emf source that can be
used to illuminate a light bulb. The amount of the potential difference in this scenario equals the amount of the motional emf in the former one.
To derive an expression for the equilibrium potential difference, we start with an equation stating that the magnetic force on the charged
particle equals the electric force. The other variables we use are shown in Equation 1 or defined in the strategy steps below.
Strategy
- State that the forces are in equilibrium: FB = FE.
- Substitute expressions for FB and FE, the magnetic and electric forces respectively, in terms of the quantities they depend on.
Calculating potential difference
Potential difference induced across wire
moving in a magnetic field
Electric, magnetic forces act on charge
·Forces balance