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Magnetism and electricity

0. Abbreviations

B-field     magnetic field
E- field    electric field
I               electrical current

 

I. Electric currents create B-fields

Ørsted’s law: a steady electric current creates a magnetic field around it.

Discovered in 1820 by Danish physicist Hans Christian Ørsted (1777-1851)

the needle of a compass spun perpendicular  to a current-carrying wire

Oersted magnetic field long wire

What does this mean? A magnetic field is produced by the current!

Let’s see this when the current is moving to the right

Let’s see this when the current is moving to the left

 

Let’s draw this: Current is in direction of the blue arrow

Magnetic field is around the wire, in red.

current thru wire Wikipedia Electromagnetism

This was a huge mystery

Electrical current is INSIDE the wire, not outside it.

So how could it affect things OUTSIDE the wire?

And even if could affect things outside the wire, shouldn’t current align the compass needle with it ?!

Why did the needle align perpendicular to the current?

This mystery was the first connection found between electricity and magnetism.

The next connection found between them was Faraday’s law of induction (see next section)

 

II. Changing B-fields create currents

Faraday’s law of electric induction: changing magnetic fields cause current to flow in conductors

Here we have coiled wire (a conductor) around a cardboard tube

Both ends of the wire are attached to an ammeter (current detector)

We then move a magnet in, and out of it:
what happens to the ammeter as this occurs? (Be very specific.)

faraday law induction

Moving magnet into coiled wire increases B-field through the wire.
This changing B-field creates a current in the wire.
_____

Moving magnet out of coiled wire decreases B-field through the wire.
This changing B-field creates a reverse current in the wire.

_____
When magnet doesn’t move -> B-field stays constant -> So  I = 0

_____

Faraday law 1

III. Creating an electromagnet

electromagnetic-field-mechanism ThomThom

Wrap an insulated copper wire around a nail.
Connect the ends to a single 1.5 v battery.
It will become magnetized; try picking up staples!
Caution – the resistance is low so the current will be high.
Much of the current’s energy turns into waste heat.
After a few seconds it might burn one’s fingers.

Electromagnet simple

IV. Creating a generator

Two equivalent ways to make an electrical generator

A. Rotate a magnet through a coiled wire (less common)

Example “A dynamo is an energy-generating hub built into the front wheel of a bicycle that typically powers lights. Dynamos can also power USB ports and all manner of fun things, if you’re interested.”
~ https://momentummag.com/what-you-need-to-know-about-dynamo-lighting/

B. Rotate wire through a permanent magnet (more common)

AC generator Wire through magnet

PBS American Experience series: Inside the AC Generator

 

But how do we rotate any of these things to begin with?
Won’t happen by itself
We could study the generation of power, to learn how this happens.
Common sources of power to do this include:
Human powered: Check out the bicycle power dynamo, above!
Fossil fuels: Burning oil, coal, natural gas
Nuclear fission: urnanium nuclear reactors
Renewable energy: solar, wind, geothermal, tidal power, hydropower

PhET app on this topic

to be added

MCAS questions

Questions assume that you know the names of common electrical equipment!

Galvanometer Voltmeter multimeter table

Questions assume that you know what a circuit schematic looks like
Here we see a circuit:
A wire was broken, and reconnected with an ammeter inside the wire – so the ammeter feels the current flowing through it.
We also see a voltmeter being used. Nothing needs to be broken. We just put one lead on side of a component, and the other lead on the other side. The voltmeter then tells us the potential difference (voltage drop) across the component.

circuit with ammeter and voltmeter Schematic

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Which of the following forces allow a battery-powered motor to generate mechanical energy? (2014)

A. magnetic and static             B. electric and magnetic

C. static and gravitational       D. electric and gravitational

———————————————————

Which of the following statements describes an electric generator? (2013)

A.  A magnet is rotated through a coil of wire to produce an electric current.

B.  Electric potential in a rotating coil of wire creates a permanent magnet.

C.  An electrical current causes a coil of wire to rotate in a magnetic field.

D.  Forces from a permanent magnet allow a coil of wire to rotate.

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2012 magnet wire question MCAS

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This next one is from 2010

2010 MCAS galvanometer magnetic

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Which of the following would cause the galvanometer needle to move?

A. wrapping additional wire around the tube

B. uncoiling the wire wrapped around the tube

C. moving a magnet back and forth inside the tube

D. moving an aluminum block up and down inside the tube

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This next one is from 2009

Precise measuring instruments require shock absorbers to eliminate small vibrations that can affect the results of an experiment. One type of shock absorber that can be used is an electromagnet that repels a magnetic platform placed above it. Which of the following setups would provide the greatest lift to the platform?

2009 MCAS magnetic platform

Electromagnet conceptual exercises

Questions & animations from Dynamic Science (Australia)  http://www.dynamicscience.com.au/tester/solutions1/electric/electromagexe.htm

A wire was spotted protruding from a wall. Both Jonathon and Stephen wanted to know if a current was flowing through the wire. They placed two magnetic compasses near the wire as shown in the animation on the right. Jonathon concluded that the wire carried a steady current. Stephen thought that someone was turning the switch off and on and did not agree with Jonathon. Who is right? Explain.

Electromagnets exercise 1

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Irene bought a new house and found that an unknown wire was coming up through a hole in the floor. She wanted to know if it had a current running through it. She placed two magnetic compasses around the wire and noticed the deflection of the needles.

Is there a current flowing through the wire? How can you tell?
If a current is flowing how is it changing? Explain.

Electromagnets exercise 2

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An electrician was working on a building site when he came upon an insulated wire. He was told that all the power to the site was turned off, but he wanted to make sure. He pulled out a magnetic compass and placed it near the wire as shown in the animation on the right. What can he conclude from his observations?

Electromagnets exercise 3

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A magnetic compass was brought close to two wires as shown on the left. What can you conclude from your observations?

Electromagnets exercise 4

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Textbook/resources: Glencoe Physics

Chapter 24, Section 1, Section 2. Chapter 25 section 1

Additional resources Sdavies.com MCAS Review

Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

7.MS-PS2-5. Use scientific evidence to argue that fields exist between objects with mass, between magnetic objects, and between electrically charged objects that exert force on each other even though the objects are not in contact.

7.MS-PS3-2. Develop a model to describe the relationship between the relative positions of objects interacting at a distance and their relative potential energy in the system. {Examples could include changing the direction/orientation of a magnet.}

HS-PS2-4. Use mathematical representations of Newton’s law of gravitation and Coulomb’s law to both qualitatively and quantitatively describe and predict the effects of gravitational and electrostatic forces between objects.

HS-PS2-5. Provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.

HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields [e.g. magnetic fields.]

HS-PS3-5. Develop and use a model of magnetic or electric fields to illustrate the forces and changes in energy between two magnetically or electrically charged objects changing relative position in a magnetic or electric field, respectively.

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