Over 2,500 years ago, ancient Chinese civilization discovered that certain rocks – now called lodestones –

will attract each other, as well as pick up small bits of iron.

They soon learned that these rocks could be hung from a string and used for navigation.

Empirical Observations on Magnets

Opposite poles attract Like poles repel

What happens if you break a magnet?

If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two bar magnets, each of which has both a north and south pole.

N S

N S N S

Another way of stating this is that

there can be no magnetic monopoles

The physical laws of magnetism prove that a single magnetic pole cannot exist.

All magnets will always have both a North and a South pole.

Magnetic FieldsJust as electrically charged particles create an electric

field in the space around them, magnets create a magnetic field that distorts space around them.

E B

Magnetic Fields

Symbol: B

Units: Tesla (T)

Vector quantity

Nikola Tesla(looking mysterious)

How can we detect a magnetic field?

We need a test magnet.

A small magnet that has the ability to rotate - will always align with the magnetic field.

A compass measures the direction of a magnetic field!

The north side of a compass needle will point in the direction of the magnetic field

Magnetic Field Linesshow the magnitude and direction of the magnetic field.

The magnetic field of a bar magnet points from North to South.

Easy way to remember: “Magnetic fields fly South” © C. Rawding

Hans Christian Øersted (1777 – 1851)

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A current-carrying wire creates a magnetic field that is in the shape of concentric circular loops around the wire!

Magnetic fields are created by ELECTRICALLY CHARGED PARTICLES IN MOTION!

What actually creates a magnetic field?

The fundamental nature of all magnetism is the motion of charged particles.

Warning: This is very different from electrostatics!

• Magnet poles are not the same as electric charges, and magnetic forces are distinct from electric forces.

• A charged object will not deflect a compass needle, unless the charged object is in motion!

It’s all about the Øersteds.

A current-carrying wire contains electrons in motion, and thus creates a magnetic field!

Right Hand Rule # 1Point the thumb of

your right hand in the direction of the

current

Your fingers will wrap around the way

that the magnetic field wraps around

the wire!

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B

Working in Three Dimensions

This is an arrow pointing out of the page.

(Think of the tip of an arrow pointing at you)

This is an arrow pointing into the page.

(Think of the tail of an arrow pointing away from you)

Whiteboard: Cross-section of a current-carrying wire

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Draw the magnetic field lines that show the B field surrounding the current-carrying wire. Remember that the

spacing of the lines represents the strength of the field!

Result

Notice: The field gets weaker as you get

further from the wire (magnetic field lines are

spaced further apart)

The magnetic field will be tangent to the magnetic field lines

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Whiteboard: Side view of a current-carrying wire

What does the magnetic field look like on each side of

the wire?

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Think of slicing the magnetic field lines in half and looking at them from the side!

B B

Whiteboard: Superposition Edition!

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What is the direction of the magnetic field at a point directly between two wires carrying current in the same direction?

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Superposition II!

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What is the direction of the magnetic field at a point directly between two wires carrying current in opposite directions?

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1 2

B1

B2

Bnet = 0 Tesla!

1 2

B1 B2

Bnet = 2B downward!