EXPLORING MAGNETISM What is a Magnet? What Materials are Magnetic?

EXPLORING MAGNETISMEXPLORING MAGNETISM

What is a Magnet?What is a Magnet?

What Materials are Magnetic?What Materials are Magnetic?

What Materials are Magnetic?What Materials are Magnetic?

What Do Magnets What Do Magnets Do?Do?Attract or repel other magnets (exert a force)

Attract other magnetic metals

Have at least 2 distinct ends (poles) each

Like poles repel, opposite poles attract

Notes:Do not need to touch to exert force (can exert force through empty space)Can turn other magnetic metal objects into temporary magnets

What is a Force?What is a Force?Force is a push or pull that causes a change in motion.

A Force Field is the pattern in space of how that force is felt by other objects.

Fundamental Forces arise from fundamental properties of matter:

Gravity arise from Mass: mass attracts mass

Electric force arises from Charge: two kinds of charge (positive and negative), like charges repel and opposite charges attract

Gravitational field of a point of mass Electric field of 2 opposite charges

Electric field of 2 like charges

Magnetic FieldMagnetic FieldMagnetic field of a Bar Magnet: 2 poles, called North and South

Dipole field

Field has direction: lines point away from N and toward S

Definition of a Pole: Where lines meet (converge)

e.g.: lines of longitude on a globe meet at poles

Moving molten iron in Earth’s outer core causes most of Earth’s magnetic field.

Magnetic field poles are NOT aligned with geographic poles. They also wander and flip (500,000 yrs between flips, 10,000 yrs to complete flip).

Earth: A Huge Bar Earth: A Huge Bar MagnetMagnet

Electric Current (Electricity) is moving charge.

In an electric circuit, batteries provide an electric field to push charges through a wire, which provides a path for them to flow. Unless there is a complete circuit the charges cannot flow.

Compasses around a circuit with flowing electric current detect a magnetic field.

Magnets can push or pull on a wire with current flowing through it.

Electricity and MagnetismElectricity and Magnetism

Electricity and MagnetismElectricity and MagnetismThe shape of the magnetic field around a long, straight wire carrying electric current

Looping the wire turns the magnetic field into a toroidal (donut) shape.

Many loops in a solenoid shape creates a dipole-shaped magnetic field, similar to that of a bar magnet

Electricity and MagnetismElectricity and MagnetismA dynamic electric field creates a magnetic field.

A dynamic magnetic field creates an electric field.

The basic principle behind electric generators is creating relative motion between a magnet and a wire to create an electric field that will push current through a circuit.

Converts kinetic energy into electric energy.

Electromagnetic RadiationElectromagnetic Radiation

A dynamic electric field creates a magnetic field.

A dynamic magnetic field creates an electric field.

Wiggle a charge and it will create waves in the electric field around it. Those electric field waves will create magnetic field waves, which in turn create more electric field waves, …

Wiggle a magnet and the same thing happens, just the first waves are in the magnetic field.

Electromagnetic RadiationElectromagnetic RadiationElectromagnetic waves travel through empty space at a speed of 300,000 km/s (186,000 miles/sec).

EM waves with wavelengths 400-700 nm are seen by the human eye as Visible Light.

Current in a Magnet?Current in a Magnet?What causes magnetism in a magnet?

Atoms are the basic building blocks of the Elements. They are neutrally charged, but composed of smaller charged particles: Quarks - combine to form neutrally charged neutrons and positively charged protons in the nucleusElectrons – negatively charged particles that orbit the nucleus in variously shaped “shells” or energy levels.

Electrons’ orbital motion as well as quantum mechanical spin produce an electric current and hence a magnetic field in the atom.

Current in a Magnet?Current in a Magnet?What causes magnetism in a magnet?

In most atoms, the magnetic fields generated by each electron cancel each other out.

In an atom two electrons can pair up and occupy an energy level, but their spins are opposite of each other, canceling their magnetic field.

But in a few atoms (like Fe, Co, and Ni) there are unpaired electrons in different energy levels whose spins can align and give the atoms an overall magnetic field.

Electron Structure of Iron

4 unpaired electrons

Magnetism in a MagnetMagnetism in a Magnet

When elements like Fe form solids, they form crystalline structures with little domains of many, many atoms all aligned. Each domain is like a mini-bar magnet. When all the domains are allowed to align, the whole object takes on a large-scale magnetic field.

For example:

B = 0.1 Tesla

Magnetic Fields have units of Tesla. Magnetic Fields have a symbol (B)

B= 0.1 Tesla

B = 3x10-5 Tesla

https://www.youtube.com/watch?v=Roq20IV4yP0

In the year 1819

Hans Christian Oertsed produced magnetism by using electricity.

The Magnetic Field (B) Circulates Around the Current

Like This

I

Right Hand Rule #1

Use: Direction of B Field produced by a current flow.

I BPoint your right thumb in the direction of the current and your fingers will curl in the direction of the magnetic field.

Illustrating Magnetic Fields

B Field Towards You

B

Illustrate Magnetic Fields

X X X X

X X X X

X X XX

B Field Towards YouB Field Away

from You

Q

1. Right

2. Left

3. Up

4. Down

5. In the page

6. Out of the page

A ? I

Find the direction of the B field at point A created by the

electric current flowing out of the page.

Up

Down

Left Right

A I

Find the direction of the B field at point A created by the

electric current flowing out of the page.

1. Right

2. Left

3. Up

4. Down

5. In the page

6. Out of the page

Current (I)

X

X

X

X

X

.

.

.

.

.

Current (I)

X

X

X

X

X

.

.

.

.

.

The magnetic field varies with distance

• The further from the magnetic field source the location is, the weaker the magnetic field.

• The magnetic field at a location twice as far from a magnet or current carrying wire will only be half as strong.

Right Hand Rule #2 – Given the magnetic field around a wire, determine the direction of the current

Essentially, this is exactly the same as right hand rule #1.

What is the direction of the current to create the B-field shown?

B-Field

1. Object must have a charge (electron, proton, -1 ion)

2. Object must not be moving parallel to the magnetic field lines.

a. The magnetic force on a stationary charge is zero.

b. The magnetic force on a charge that is moving in the direction of the magnetic field is also zero.

c. The magnetic force is largest on a charge that is moving perpendicular to the magnetic field.

An object in a magnetic field will experience a force as long as two conditions are met:

The force on a charged particle in a magnetic field can be found using …

. . . .

. . . .

....

+

F = QvB sin

Follows a Circular Path

Where: Q = Charge of the particle (C)

v = the velocity of the particle (m/s)

B = the magnetic field (T)

y

x

z

B

F

v

Charge Velocity, B Field, and The Force Produced, are all at right angles to each

other.

• Straight fingers in the direction of the magnetic field.

• Thumb in the direction of the particle’s velocity (or wire’s current).

• Your palm will face in the direction of the force

Right Hand Rule #3: Direction of force on a moving positive charge/current in a magnetic field

B

v or I

F

Another way to do Right Hand Rule #3er

The direction of the force on a negative charge is opposite that for a positive charge.

Index finger = B-fieldThumb = Velocity or currentMiddle Finger = force

Example: At one instant a +2 ion moving with a velocity of 1000.0 m/s in a magnetic field strength of 5.0x10-6T caused by a current carrying wire.

(A) What is the charge of a +2 ion?

(B) What is the force on the ion from the wire?

+2.0m

Atoms have equal numbers of protons and electrons. A +2

ion has lost two electrons (or has two more protons than

electrons)

Q 2(1.610 19C)



50A

+2m

BvQF

N21106.1

X

X X X X



50A

+2m

BvQF

N21106.1

X X X X

X X X X

x x x x x

x x x x x

x x x x x

-

What is the direction of the force?

1. Up

2. Down

3. Into the page

4. Out of the page

5. Right

6. Left

Bv

Find the direction of the magnetic F acting on an electronelectron moving inside the uniform B.

v B

1. Up

2. Down

3. Into the page

4. Out of the page

5. F=0

Find the direction of the magnetic F acting on an electronelectron moving along the magnetic field lines

Charged Particles Moving In a Magnetic Field

Motion of a Particle in a Magnetic Field

X X X X X X X

X X X X X X X

X X X X X X X

X X X X X X X

X X X X X X X

F+

V

FF

V

+

V+

A charged particle is moving an a magnetic field. Determine the direction of the particle’s velocity compared to the direction of the magnetic field that would produce the following movement:

Circle -

Straight Line -

Helix -

Velocity is entirely perpendicular to the B-field

Velocity is parallel or anti-parallel to the b-field

Velocity is at an angle to the b-field

Force on a Current Carrying

Conductor

Consider a current carrying wire

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

I

…in an external magnetic field

+ + + + + +

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

I

Because the current is just flowing charges, each charge will have a force exerted on it.

+ + + + + +

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

I

L

F

These individual forces, add together to be one big force….Given by F = BIL sin

1. See Speaker Diagram

2. See Speaker Working

How does a doorbell work?

1. Doorbell Buzzer

2. Doorbell Ding-Dong

Bells and buzzers are electrically the same.

1. See Doorbell - Buzzer

2. See Doorbell - Chime

SOLENOIDS

A solenoid is basically a coil of wire.

It might have one turn (a loop), or it might have many turns (like a spring).

What does the magnetic field of a solenoid look like?

What does the magnetic field of a solenoid look like?

Electromagnets

The magnetic field inside a solenoid has to do with:

B oN IL

•number of turns in the solenoid (N)

•current going through the solenoid (I)

•Length (L) in meters of the solenoid.

Example: What is the magnetic field inside a laboratory solenoid with 200. turns in a length of 30.0 cm when it carries 2.0 A?

B onIL

B (4 10 7 )(200)(2)

0.30

B 1.710 3T 1.7mT

Electric Motors

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EXPLORING MAGNETISM What is a Magnet? What Materials are Magnetic?