Adjustable magnets
In the early 1800s scientists discovered that electric currents could generate magnetic fields stronger than the fields of permanent magnets. Improvements in such devices were made by 1830, when the key variables determining the strength of a magnetic field were identified. A typical electromagnet is illustrated here, and those variables are:
- core and core material – the piece of metal that the windings are wrapped around
- number of windings – the wire (often a lot of it) wrapped around a metal core
- amount of electric current passing through the windings.
An electromagnet consists of a source of electric current (a battery in the diagram), a metal (usually ferromagnetic – magnetizable) "core" and wire windings looped snuggly around it.
Improvements in insulation of fine wire made it possible to wind thousands of loops around a core to produce strong magnetic fields.
The current that can be passed through any wire is limited by its size. Any conducting wire has some small internal resistance, and a large current can heat up and melt the wire, breaking the circuit.
Modern superconducting magnets have circumvented this problem so that we can now create very strong (10s of Tesla) electromagnets by passing extremely large currents through wire that has virtually no resistance.
Our fundamental understanding of why electromagnets and related devices work came first from Faraday's law, which is given below.
Later Richard Feynman and others developed the theory of quantum electrodynamics, which merged classical theories of electromagnetic phenomena (Maxwell's equations) with quantum behavior. We'll stick to Faraday's law in this section and keep the discussion qualitative.
Faraday's law
Faraday's law is the description of all induction phenomena, including electromagnetism, but its mathematics is beyond the scope of this page, so we'll settle for a qualitative description of the law. It says:
The induced potential (force that can move electrons) in a closed electrical circuit is equal to the negative of the rate of change of the magnetic field that lies inside of the circuit.
It says that moving charges (current) create a magnetic field, and that a changing magnetic field produces a potential difference (voltage) that can produce an electric current.
Static (not moving) electric and magnetic fields cannot produce magnetic / electric potentials, respectively.
Electromagnetic induction
- A changing magnetic field can produce a potential, and thus a current in an electric circuit that encloses it.
- An electric current (the movement of charges in a conductor) can produce a magnetic field in a nearby paramagnetic object. A paramagnetic object is something that can be magnetized.
Examples: some devices that use electromagnetism
Speakers
Common loudspeakers work by creating vibrations of molecules that propagate through the air, from the speaker to your ear, to produce music, voice or other sounds. Once a sound is converted into a fluctating electrical signal, that signal is sent through a wire coil attached to a paper or cloth cone. The cone is the part of the speaker that pushes and pulls on the air in the room.
That coil surrounds (but is not attached to) a fixed magnet. by changing the strength and direction of the current in the coil, the coil – and thus the cone – are vibrated. While there are a few other kinds of speakers, most work on this basic principle.
Microphones
A microphone works in just the opposite way that a speaker does. A thin, flexible diaphragm material, such as a very thin piece of metal or polymer film, is stretched across a frame. Sound is projected toward one side of the diaphragm, causing it to vibrate accordingly. those vibrations cause a coil attached to the diaphragm to vibrate, too.
The movement of the coil around a permanent magnet, fixed to the frame, causes a current to flow in the coil wire. That current can be amplified electronically, and transmitted to a speaker, producing an amplified sound. Microphones are also used to digitally capture sound, which is by nature an analog or continuous phenomenon, so that it can be manipulated before it is reproduced by a speaker.
Solenoids, actuators & relays
You can think of a solenoid as an electromagnet with a moveable, permanently-magnetic core. When the coil is energized (current is passed through it), the magnetic core can be drawn to one end or the other of the coil.
In this way we can make several kinds of devices. One is a switch or actuator, a device which can take electric current as an input and produce a physical motion that might achieve some goal, such as closing or opening a switch or relay.
For example, it could be dangerous to close a switch on a 10,000 V circuit (the kind of potential you might see at a transformer station near high-voltage power lines) by hand. We isolate humans from that danger by letting them switch a smaller voltage that powers a solenoid, and the motion of the actuator then moves the high-voltage switch.
There are a great many other uses for solenoids, including
- Valves for switching liquid flow
- Automated door locks (think hotel doors & cars)
- Electrical switching in cars & trucks
- Air flow control in home heating systems
- Automatic sprinkler systems
Direction of the magnetic field
The right-hand rule
If we know the direction of the current, and here we're talking about conventional current, or current flowing from (+) to (-), then we can predict the direction of the magnetic field. That is, we can predict which ends of an electromagnet will be north and south.
Benjamin Franklin and others set the standard very early in the study of electric charge, for the direction in which currents flow. While today we know that direct currents flow from the negative to the positive electrode of a battery in a circuit, we generally use the conventional current, which assumes the reverse. It's OK. That doesn't cause any real problems in circuit or device analysis.
The right-hand-rule (RHR) for wire coils works like this:
The
qualitative / quantitative
A qualitative description of a phenomenon uses words and ideas, like "The rock was heavy." A quantitative description involves numbers, usually of a precision appropriate to the thing being described: "The mass of the rock was 22.4 Kg."
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