Fields 4: Magnetism

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What is Magnetism?

• Another force that exists around moving charged objects.

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What is a Magnet?• A material in which the spinning electrons of its

atoms align with one another.

What are ferromagnetic materials?

• Permanent magnets are made of ferromagnetic material such as Iron, Nickel, Cobalt, or an alloy of Al, Ni, or Co

• When a magnet is placed near a metallic object: Pairs of spinning electrons align temporarily to induce a magnetic field.

What if we cut a magnet?

How do like and unlike poles interact with each other?

• Has two ends called poles (where the magnetic force is the strongest.) Like poles repel, unlike poles attract.

• Lines of force emerge from a North pole, curve around the magnet and go into the south pole.

Earth is like a giant magnet• Moving charged particles in the molten

core generate a magnetic field.

• Can be detected using a compass, which aligns itself with the Earth’s magnetic field.

Earth is like a giant magnet

• Moving charges particles in the molten core generate a magnetic field.

How are magnetic fields similar to gravitational and

electric fields?• It’s a field – objects in the area around

magnet experience a force without the need to touch.

• Like an E-field– It has polarity with attractive and repulsive

forces (grav. fields are only attractive.)– Exerts a force on a moving charge.

Representing Magnetic Fields• Imaginary lines that map out a magnetic field around

a magnet.• Also known as Magnetic Flux Lines• Always form closed loops and never cross.• Emerge from N-Pole and enter S-Pole.

Magnetic Field Strength

• The number of magnetic lines of flux per unit area passing through a plane perpendicular to the direction of the lines.

Magnetic Field Strength• The density of field lines

– Bunched = stronger field– Spread out = weaker field

• Vector quantity– Magnitude– Direction

Magnetic Field Strength• Vector quantity

– Magnitude– Direction

Sketching Magnetic Fields

Magnetic Fields and Conductors

Moving electrons in a conductor produce magnetic

fields

• How can we describe these fields?

Magnetic field around a straight conductor

Definitions:

X = into the page.

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Magnetic field around a straight conductor

First Left Hand Rule:

Thumb: direction of current flow.

Fingers: direction of magnetic field.

Magnetic field around a coil of wire

Second Left Hand Rule:

Fingers: Wrap around the coil in the direction of current.

Thumb: Point in the direction of North.

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Electromagnet

Force on a conductor in a magnetic field

Fields on top add, while fields on bottom cancel

Higher field strength on top, so conductor moves down.

Force on a conductor in a magnetic field

Current (I)

Field (B)

Force (F)

Third Left Hand Rule: (FBI)

Thumb: Direction of Current (I)

Pointer: Direction of Mag. field (B)

Middle Finger: Direction of Force (F)

Force on a conductor in a magnetic field

Third Left Hand Rule: (FBI)

Thumb: Direction of Current (I)

Pointer: Direction of Mag. Field (B)

Middle Finger: Direction of Force (F)

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