Magnetism

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# Magnetism - PowerPoint PPT Presentation

Magnetism. Physics. A little history…. Until the early 19 th 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

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Presentation Transcript

### Magnetism

Physics

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 moved
• Electricity and Magnetism are related!
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
Magnetic Poles
• All magnets have both a north and a south pole
• Like poles repel
• Opposite poles attract
• Magnetic poles always exist in pairs
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

B Field

S

N

Magnetic Domains

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

• 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
How are E&M related?

Moving electric charges create magnetic fields

• 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
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
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

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?
• 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

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

• Force is greatestwhen 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
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

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
• 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 F

Out of the page

Into the page

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

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)

q = charge (C)

v = velocity (m/s)

B = magnetic field (T)

F = force (N)

I = current (A)

L = length of wire (m)

B = magnetic field (T)

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

### Electromagnetic Induction

Physics

magnetism can produce electricity & electricity can produce magnetism

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
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 = flux (Wb)

A = area of loop (m2)

B = magnetic field (T)

Φ = AB

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?

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
Example

Φ = AB

• 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?
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
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.
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
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