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  • Only a few elements can be permanent magnets - iron is one. Copper is not. But if you pass

  • an electric current through any metal it becomes a magnet - an electromagnet. But how does

  • this work? Well strangely enough, it's a consequence of special relativity.

  • Special relativity is the fact that in our universe, length and time aren't absolute;

  • they're perceived differently by observers moving relative to each other (hence, "relativity").

  • For example, if you measure carefully enough, you'll find that time passes slower for observers

  • moving relative to you.

  • Hey Derek, when did you last shave? Derek1: Six hours ago.

  • MovingDerek: Actually it was five hours, 59 minutes and 59.99999999999 seconds

  • And moving objects are also contracted in their direction of motion.

  • You're looking slim. Only in your frame of reference.

  • So when an object is moving relative to you, it actually takes up less space than when

  • it's not moving. And even though this effect is obviously way tinier than we've shown,

  • length contraction IS what makes an electromagnet work.

  • Picture a copper wire - it consists of positive metal ions swimming in a sea of free negative

  • electrons. Now the number of protons is equal to the number of negative electrons so overall

  • the wire is neutral. So if there were a positive charged, err... positively charged cat nearby,

  • it would experience no force from the wire on it at all.

  • And even if there were a current in the wire, the electrons would just be drifting in one

  • direction, but the density of positive and negative charges would still be the same,

  • and so the wire would be neutral, so no force on the kitty.

  • But what if the cat starts moving? Imagine for simplicity that that the cat is moving

  • in the same direction as the electrons with the same velocity. Well now in my frame of

  • reference, the wire is still neutral and so there should be no force on the cat, but consider

  • the same situation in her frame of reference.

  • In the cat's frame of reference the positive charges in the wire are moving, so according

  • to special relativity their separation will be ever-so-slightly contracted. Also, from

  • this perspective the electrons aren't moving so they'll be more spread out than before

  • - remember, objects take up more space when they're not moving than when they are. These

  • two changes together mean there's a higher density of positive charges in the wire, so

  • it's no longer neutral - it's positively charged! Which means that the positively charged cat

  • will feel a repulsive electric force from the wire.

  • But in my frame of reference this seems mysterious: there's no force on a stationary charged cat,

  • but a moving cat is somehow repelled from this neutral wire.

  • How do you account for this force? Well we say it is the magnetic force, and that's mainly

  • because a wire with current in it deflects nearby magnets. So really, what this experiment

  • shows is that a magnetic field is just an electric field viewed from a different frame

  • of reference.

  • In the cat's frame of reference, it is repelled from the wire due to the electric field created

  • by the excess positive charges produced by the effects of length contraction. In MY frame

  • of reference, the cat is repelled from a neutral wire due to the magnetic field generated by

  • current flowing in the wire.

  • So whether you see it as an electric or a magnetic field just depends on your frame

  • of reference, but in either case the results are the same. So an electromagnet is an everyday

  • example of special relativity in action.

  • Now that might seem crazy since electrons drift through wires at about .0000000001%

  • the speed of light - so how can special relativity have anything to do with it? Well the truth

  • is there are enough electrons in a wire, and the electric interaction is so amazingly strong

  • that even the minuscule effects of length contraction can produce significant charge

  • imbalances that produce a noticeable force.

  • So special relativity explains electromagnets - but what about permanent magnets?

  • Yeah! I mean there can't be electrical currents flowing around inside lumps of rock, can there?

  • Click here to go to MinutePhysics where we'll explore magnetite, compasses and all the wizardry

  • of permanent magnets.

Only a few elements can be permanent magnets - iron is one. Copper is not. But if you pass

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B2 wire special relativity relativity frame reference moving

How Special Relativity Makes Magnets Work

  • 17 0
    hzd_zw2004 posted on 2013/11/09
Video vocabulary

Keywords

negative

US /ˈnɛɡətɪv/

UK /'neɡətɪv/

  • adjective
  • Pessimistic or unfavorable.
  • Unfavorable or critical.
  • Harmful or undesirable.
  • Carrying a negative electric charge.
  • Expressing or containing a negation or denial.
  • Being harmful, unwanted or unhelpful
  • Less than zero.
  • In mathematics, being less than zero
  • Less than zero.
  • Focusing on the bad aspects; pessimistic
  • Indicating the absence of something, such as a disease or condition.
  • Expressing disagreement or refusal.
  • Indicating the absence of a particular condition, substance, or organism.
  • Expressing or signifying negation, refusal, or denial.
  • noun
  • The opposite to a positive electrical charge
  • In grammar, containing words such as 'no' or 'not'
  • Reply to a question or statement that means 'no'
  • Image on camera film used to make a photo
  • A reversed image on a transparent base, used to make positive prints.
  • A refusal, denial, or rejection.
  • A negative statement or response.
force

US /fɔrs, fors/

UK /fɔ:s/

  • other
  • To break open (something) using force.
  • To compel (someone) to do something.
  • To cause (a plant or crop) to develop or mature prematurely in a greenhouse or under artificial conditions.
  • To cause (a plant or flower) to grow or develop at an increased rate.
  • To use physical strength to break open or move (something).
  • other
  • Coercion or compulsion; strength or power exerted to cause motion or change.
  • Strength or energy as an attribute of physical action or movement.
  • other
  • Coercion or compulsion; strength or power exerted to cause or affect.
  • An influence or effect.
  • Physical strength or energy as an attribute of physical action or movement.
  • Strength or energy as an attribute of physical action or movement.
  • noun
  • Coercion or compulsion, especially with the use or threat of violence.
  • Group of persons trained for military action; army
  • A body of people employed and trained for a particular task or purpose.
  • An influence or effect.
  • A body of people employed and trained for law enforcement.
  • A body of soldiers or police.
  • An influence that can cause a body to accelerate.
  • Pressure; attraction
  • Strength or energy as an attribute of physical action or movement.
  • The use of physical strength/violence to persuade
  • Strength or power of expression or argument.
  • verb
  • To use physical strength or violence to persuade
positive

US /ˈpɑzɪtɪv/

UK /ˈpɒzətɪv/

  • adjective
  • Completely certain or sure that something is correct or true
  • Showing agreement or support for something
  • Favorable or advantageous.
  • Being sure about something; knowing the truth
  • Indicating the presence of a disease or condition.
  • Carrying a positive electric charge
  • Having the charge produced by electrons
  • Having a good effect; helpful or useful
  • Being good or useful
  • Expressing a quality without comparison
  • Greater than zero
  • Showing the presence of a particular substance, disease, or condition
  • Feeling sure that something good will happen; optimistic
  • Showing a photographic image in natural colors or shades.
  • Showing that a chemical is present in a test
  • noun
  • A photograph in which the tones correspond to those of the original subject.
  • A photograph in which light areas are light and dark areas are dark
current

US /ˈkɚrənt, ˈkʌr-/

UK /'kʌrənt/

  • adjective
  • In general use or accepted by most people.
  • Of or relating to the present time; up-to-date.
  • Happening or being in the present time
  • Happening or existing now; belonging to the present time.
  • Valid or up-to-date.
  • noun
  • Electricity flowing through wires
  • Currency in circulation.
  • Movement of water in a river, or air in the sky
  • A widespread feeling or opinion.
  • A general tendency or course of events.
  • other
  • A flow of electrical charge through a conductor.
field

US /fild/

UK /fi:ld/

  • noun
  • An area of open land, especially one planted with crops or pasture, typically bounded by hedges or fences.
  • Area of study, such as physics or biology
  • A battlefield.
  • A space in a form or record that is used to enter a particular item of information.
  • Piece of land used to grow crops/raise animals
  • Open area of land, especially without buildings
  • A region of space in which a force acts on a particular particle.
  • Grassed area where you play some sports
  • A piece of ground specially prepared and marked for sports.
  • other
  • To catch or stop (a ball) and prevent the batter or another runner from advancing.
  • To put (a team or player) into a game.
  • adjective
  • Used or done in the normal working environment rather than in a laboratory or office.
  • verb
  • To respond to something or answer a question
  • To catch or stop a ball during a game
charge

US /tʃɑrdʒ/

UK /tʃɑ:dʒ/

  • noun
  • A formal accusation made against someone
  • An attack by running quickly toward someone
  • An attack in which people rush forward
  • A price for a service or goods
  • When a battery has a certain amount of electricity
  • A feeling of excitement and enthusiasm
  • Amount of money you pay for a service
  • An official instruction or command
  • The amount of money you have to pay for something, especially a service
  • Control over someone or something; responsibility for something
  • other
  • To formally accuse (someone) of something, especially a crime
  • To ask (someone) to pay an amount of money for something
  • To put electricity into a device such as a battery
  • To entrust (someone) with a duty or responsibility
  • To fill (a battery or electrical device) with electricity
  • To ask someone to pay a particular amount of money for something
  • To accuse someone of something, especially a crime
  • verb
  • To run quickly toward someone to attack them
  • To ask for money as a price for a service or goods
  • To put electricity in a battery
  • To give responsibility to take care of or manage
  • To state that someone has committed a crime
  • other
  • To rush forward and attack
  • other
  • An amount of electricity stored in a battery or capacitor
permanent

US /ˈpɚmənənt/

UK /'pɜ:mənənt/

  • adjective
  • Lasting forever; not temporary or changing
  • Existing or intended to exist for a long or unlimited period
  • Remaining in the same state or condition.
  • Remaining in the same state or condition.
  • Intended to last or remain for an unlimited period
  • Lasting or intended to last or remain unchanged indefinitely
  • Designed to remain in a particular place or state for a long time.
  • Continuing or intended to continue without significant change.
  • noun
  • A person who is a permanent employee.
  • A chemical treatment to create lasting curls or waves in the hair.
contract

US /'kɑ:ntrækt/

UK /'kɒntrækt/

  • verb
  • To become ill as a result of getting a disease
  • To make an agreement for doing work for money
  • To reduce in size, length due to heat loss
  • To reduce the amount of
  • Decrease in size, number, or range.
  • noun
  • A written or spoken agreement, especially one concerning employment, sales, or tenancy, that is intended to be enforceable by law
  • Legal agreement, e.g. for doing work for money
  • Agreement to pay someone to kill a person
  • other
  • To enter into a formal and legally binding agreement
  • To catch or develop (a disease or infectious agent)
  • To shorten (a word or words) by omitting or combining some of the sounds or letters
  • other
  • To decrease in size, number, or range
frame

US /frem/

UK /freɪm/

  • verb
  • To make a person that is not guilty appear guilty
  • To put say or write something in a careful way
  • To put a picture in a protective structure
  • noun
  • The body or build of a person or animal.
  • A border or case for surrounding a picture, door, or window.
  • Structure that holds a picture or photo
  • A state of mind or attitude.
  • A single image in a film or video.
  • A person's body shape determined by their skeleton
  • Main part of structures supporting the other parts
  • other
  • To construct the basic structure of a building or object.
  • To produce false evidence against an innocent person so that they appear guilty.
  • To present or express something in a particular way.
  • To surround or enclose something.
direction

US /dɪˈrɛkʃən, daɪ-/

UK /də'rekʃn/

  • other
  • The control or management of someone or something.
  • The act of directing a film or play.
  • Guidance or supervision of action or conduct; management; control.
  • The act or power of controlling; management.
  • The act or skill of directing something, such as a play or film.
  • The management or guidance of someone or something.
  • The act or function of managing or guiding.
  • noun
  • When someone tells people what to do
  • Describes the way you are going, e.g. North
  • The way that someone or something moves, faces, or is aimed.
  • The line or course on which something is moving or pointing; aim; orientation.
  • Guidance or supervision of action or conduct; management; control.
  • A statement of instructions for doing something.
  • The point to which someone or something is moving or facing.
  • A general tendency or course of events.
  • A course along which someone or something moves.