Electromagnetism is the combination of the electricity and magnetism phenomena.
All electromagnetic waves have 3 properties in common:
Are transverse waves; can travel through vacuum.
At exactly same speed in vacuum in light speed, 300,000,000 m/s.
Wavelengths range or frequencies over which electromagnetic radiation extends.
[1] Common electromagnetism units:
ohm (for resistance)
volt (for voltage)
ampere (for current)
ampere-turn
coulomb (for electric charge)
farad (for capacitance)
henry (for inductance)
siemens (for conductance)
tesla (for magnetic flux density)
watt (for power)
weber (for magnetic flux)
Flux formula (an analogy to Ohm's law for magnetic circuits): Φ = F/R, but reluctance always changes in ways making this relationship hard to use.
Reluctance (ℜ) is the opposition to magnetic fields, proportional to a coil's length, inversely proportional to its area, and the core material's permeability (μ).
Reluctance formula: ℜ =L/(μ*A)
Magnetomotive force (mmf or F) is the magnetic pressure setting up a magnetic flux in a magnetic circuit, measured in ampere-turn (A.t), and its result is the magnetic field.
Its unit is the , with many symbols being used for it:
A (confused with ampere)
At (Robbins & Miller)
A-t (these power points)
F = NI
F = mmf in ampere-turns
N = number of turn
I = current
Changing MMF A DC power supply and an electromagnet on a stand
E.g. 1# 20 turns and 200 mA supply makes enough mmf to hold 10 ball bearings (6.74861 balls per pounds of 1 inch each):
F = NI = 20 x 200 x 10⁻³ F = 4 A-t
E.g. 2# 20 turns and 160 mA make enough mmf to hold 8 ball bearings:
F = NI = 20 x 160 x 10⁻³ = 3.2 A-t
E.g. 3# Only 10 turns and 400mA make enough mmf to hold 10 ball bearings:
F = NI = 10 x 400 x 10⁻³ = 4.0 A-t
4# What is the magnetomotive force created by 1.2 amps through 75 turns?:
F = NI = 1.2 x 75 = 90 A-t
Downsides The current in an equipment piece based on an electromagnet reduces also reduces the mmf.
A motor may not have enough torque to start.
A coil may not pull in or “chatter.”
We often can control current (or the field)--seldom the number of turns ( as it is fixed)
A core has wrapped windings affecting a field's strength.
Magnetic field of conductors add up in the coils.
This is an electromagnet.
To increase the magnetic field here,
coils = electric wire or other conductor in spiral shapes
either air, iron, or other material)
purpose = to make magnetic field or give electrical resistance
Used for:
Inductors: Used in circuits to store energy in a magnetic field.
Transformers: Transfer electrical energy between circuits via electromagnetic induction.
Electromagnets: create controllable magnetic field for lifting, holding, or other functions.
Motors and generators: convert electrical energy to mechanical energy and vice versa.
Speakers: convert electrical signals into sound.
An air core is only used at high magnetic frequencies.
Tiny flux is made for a given magnetomotive force (mmf).
An iron core highly rises a magnetic field's strength, often by 500 times.
Superconducting coil: type of electromagnetic coil made via superconducting wire. When cooled below critical temperature, wire show zero electrical resistance = allow coil to carry very big currents without energy loss. They're for applications like MRI machines, particle accelerators, and magnetic levitation trains.
etm: in Latin “super” means "above/beyond" + “conducting” meaning "conducere" meams "lead together."
In physics, it refers to the ability to carry electric current + “coil” in Middle English "coilen" meaning "to gather in a ring."
ropulsion coil:
levitation guidance coil:
at slide 14 in Edx notes
[1.2] Hopkinson (by John Hopkinson)/Rowland's law (by Henry Augustus Rowland) says the bond between mmf, magnetic flux, and a magnetic circuit's reluctance--the magnetic circuit's counterpart to Ohm's law.
Thought it's by Hopkinson, it was earlier made by Rowland's in 1873.
Magnetomotive formula: F = ΦR
F = magnetomotive force in ampere-turns
Φ = flux
R = reluctance in ampere-turn-per weber
Note: A magnetic circuit isn't an electric circuit, but both are analogous.
[3] Magnetic circuit is (always) a closed path providing a magnetic flux (unlike electric circuits which make current), often made of ferromagnetic material/other magnetic high permeability material (iron, cobalt, nickel, soft steel) and often for transformers, generators, motors, relay, coils and solenoid, measuring tools. A basic inductor/coil connected to a voltage source makes magnetic fields.
A magnetic circuit is analogous to reluctance.
[3]
Faraday’s law of induction
1831, Michael Farday found the induced electromotive force in any closed circuit equals the magnetic flux change rate enclosed by the circuit.
Demonstration apparatus
[R1] Electricity
[R2] Magnetism
[R3]
[1] Wikipedia
[1.1] Ampere-turn
[1.2] Magnetic circuit
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[1.4]
[3] Difference Between Electric and Magnetic Circuit - Electrical Technology
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[Q1] Flashcards
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