Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity...

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Magnetism Physics

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Transcript of Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity...

Page 1: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

MagnetismPhysics

Page 2: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

A little history…

Until the early 19th century, scientists thought electricity and magnetism were unrelated

In 1820, Danish science professor Hans Christian Oersted was demonstrating electric currents in front of a class of students

When electric current was passed in a wire near a magnetic compass, the compass needle movedElectricity and Magnetism are

related!

Page 3: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Comparing E&M

Magnets exert forces on one another

Similar to electric charges: Can attract and repel without touching Strength of interaction depends on the

distance of separation of the two magnets

Different from electric charges: Electric charges produce electrical forces Regions called magnetic poles produce

magnetic forces

Page 4: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Poles

All magnets have both a north and a south pole

Like poles repel

Opposite poles attract

Magnetic poles always exist in pairs

Page 5: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Fields

Magnetic Field (B): The space around a magnet in which a magnetic force is exerted [measured in Tesla – T]. Magnetic Field is a VECTOR (has magnitude

and direction).

The direction of the magnetic field outside a magnet is from the north to the south pole

NS

B Field

Page 6: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Domains

Magnetic Domain = large clusters of atoms lined up with each other

Domains start out randomly oriented in this piece of iron

Domains align in the direction of the magnetic field as they are brought closer to a magnet

Permanent magnets are made by placing pieces of iron or certain iron alloys in strong magnetic fields

Page 7: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

How are E&M related?

Charges in motion have both E (electric) fields and B (magnetic) fields associated with them

In a bar magnet, electrons inside are constantly moving

Moving charge = current magnetic field

Electrons also spin Spinning charge =motion magnetic field

Moving electric charges create magnetic fields

Page 8: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Electromagnets

If a current carrying wire is bent into a loop, the magnetic field lines become bunched up inside the loop

A current-carrying coil of wire with many loops is an electromagnet

Page 9: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

The Right Hand Rule

To find the direction of the magnetic field in a wire, point the thumb/fingers of your right hand in the direction of current flow.

Your fingers/thumb point in the direction of the magnetic field.

Out of the page

Into the page

Page 10: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

The Right Hand Rule

What is the direction of the magnetic field in this wire?

What is the direction of the magnetic field in this coil or wire?

Page 11: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

ANSWERS

What is the direction of the magnetic field in this wire?

What is the direction of the magnetic field in this coil or wire?

B Field

B Field

Page 12: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Force is greatest when the particle moves perpendicular to the magnetic field

Force becomes less at angles less than 90 and zero when the particle moves parallel to the field lines

When MOVING electric charges are placed in magnetic fields, they feel a FORCE.

Page 13: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Force

The force that acts on a moving charged particle depends on the particle’s charge, its velocity, and the strength of the magnetic field.

B = magnetic field [T]

v = charge velocity [m/s]

F = force [N]

q = charge [C]

F = qvB

Page 14: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

The Right Hand Rule

To find the direction of the magnetic force on a charge Take two pens Hold them perpendicular to each other as in

the picture Take your RIGHT hand Place your RIGHT hand at the point where the

two pens meet Push v towards B The direction your thumb points is the

direction of FOut of the page Into the page

Page 15: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Right Hand Rule Practice

vv

v

B B

B

Page 16: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Force cont’d

Now we know: a charged particle moving through a magnetic

field experiences a deflecting force

So… a current of charged particles moving through

a magnetic field also experiences a deflecting force

F = force (N)I = current (A)L = length of wire (m)B = magnetic field (T)F = ILB

Page 17: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Magnetic Force

When an electric charge moves in a magnetic field, it feels a force.

Single charge: Many charges (current):

F = qvB F = ILB

F = force (N)I = current (A)L = length of wire (m)B = magnetic field (T)

F = force (N)q = charge (C)v = velocity (m/s)B = magnetic field (T)

Page 18: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Earth’s B Field

A compass points northward because Earth itself is a huge magnet

The compass aligns with the magnetic field of the earth

Most geologists think that moving charges looping around within Earth create its magnetic field

The magnetic field of Earth is not stable It has flip- flopped throughout geologic time Studies of deep-sea sediments indicate that the field

was virtually switched off for 10,000 to 20,000 years just over 1 million years ago

Page 19: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Electromagnetic Induction

Physics

magnetism can produce electricity & electricity can produce magnetism

Page 20: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Electromagnetic Induction

Electric current can be produced in a wire by simply moving a magnet into or out of a wire

Movement of the magnet induces a voltage, which causes current flow

Voltage is induced whether the magnet is moved through the wire or the wire is moved through the magnet

Page 21: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Flux

Magnetic Flux = the number of magnetic field lines passing through a given area Measured in Webers (Wb)

If a loop of wire lies perpendicular to a magnetic field, the maximum possible number of lines of flux will pass through the loop.

If the loop of wire lies parallel to the field, the flux through the loop will be zero.

Φ = ABΦ = flux (Wb)A = area of loop (m2)B = magnetic field (T)

Page 22: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Example

Eleanor is undergoing an MRI procedure and is placed inside a chamber housing the coil of a large electromagnet that has a radius of 25.0 cm. A flux of 0.290 Wb passes through the coil opening. What is the magnetic field inside the coil?

Page 23: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

greater number of loops of wire = greater induced voltage = greater current

Faraday’s Law: Induced voltage in a coil is proportional to:

the product of the number of loops

the cross-sectional area of each loop

and the rate at which the magnetic field changes within those loops

€

V =NΔφ

Δt

Page 24: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Example

The hood ornament on Abe’s sedan is shaped like a ring 8.00 cm in diameter. Abe is driving toward the west so that Earth’s 5.00*10-5 T field provides no flux through the hood ornament. What is the induced voltage in the metal ring as Abe turns from this street onto one where he is traveling north, if he takes 3.0 s to make the turn?

Φ = AB

€

V =NΔφ

Δt

Page 25: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Lenz’s Law

An induced voltage always produces a magnetic field that opposes the field that originally produced it

In other words: If the original magnetic field, and thus the

flux, is going toward the north, the induced voltage will produce an opposing field and flux that goes toward the south

Page 26: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Generators & Motors

Generator = A machine that produces electric current by rotating a coil within a stationary magnetic field

A motor converts electrical energy into mechanical energy.

A generator converts mechanical energy into electrical energy.

Page 27: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.
Page 28: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Transformers

A transformer works by inducing a changing magnetic field in one coil, which induces an alternating current in a nearby second coil

Voltages may be stepped up or stepped down with a transformer

€

VIprimary =VIsecondary

Page 29: Magnetism Physics. A little history… Until the early 19 th century, scientists thought electricity and magnetism were unrelated Until the early 19 th.

Power Transmission

Power is transmitted great distances at high voltages and correspondingly low currents, a process that otherwise would result in large energy losses owing to the heating of the wires.

Power may be carried from power plants to cities at about 120,000 volts or more, stepped down to about 2400 volts in the city, and finally stepped down again by a transformer to provide the 120 volts in our houses


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