Week 1

Introduction to Magnetism

Liceo Alfano

What is a Magnet?

• Applet

• A magnet is any piece of material that will attract iron

• Imagine living hundreds of years ago and discovering that certain materials can manipulate objects without touching them. How strange would that be?

What is Magnetism?• Magnetism is a force that exists naturally,

like gravity

• Have you ever played with two magnets to see how they react to one another? What happened?

• Like poles repel, opposite poles attract

What is Magnetism?

• What happens if you cut a magnet in half? Does it make a magnet with only one pole?

• No! It just makes two smaller magnets

• It is impossible to have a magnet with just one pole

• How could you make a really small magnet?

Experiments

• Magnetic field of a permanent bar magnet

• Compass’s reaction to being in the magnetic field

• Exploring magnetic fields worksheet

Magnetic Field of a Bar Magnet

• A magnet has two poles, N and S

• Magnetic field lines start from N and go to S

• Magnets in Motion

Concept QuestionThe picture shows the field lines outside a permanent magnet. The field lines inside the magnet point:

1) Up2) Down3) Left to right4) Right to left5) The field inside is

zero

Concept Question

1) Up

Magnetic field lines are continuous.There are no magnetic charges (monopoles) so B field lines never begin or end

How Does a Compass Work?

• Based on our experiment and the map of a bar magnet’s magnetic field, how does a compass work in a magnetic field?

• A compass always points north on Earth. What does this mean about Earth?

Earth’s Magnetic Field

• Our planet is a huge magnet!

• Compasses always align to point approximately north (but not exactly north, why not?)

Other Magnetic Field Geometries

Other Magnetic Field Geometries

Reminder: Electric Fields

• What kind of particle causes an electric field?

• Any positively or negatively charged particle

• What kind of particles can “feel” electric field?

• Any positively or negatively charged particle

Now: Magnetic Fields

• We will talk about what particles can cause a magnetic field later

• Any moving positively or negatively charged particle can “feel” a magnetic field

• How is a magnetic field different from an electric field?

Magnetic Field Force• Force of a magnetic field on a moving particle:

• F = q v x B

• F is the force on the particle (Newtons)

• q is the charge of the particle (Coulombs)

• v is the velocity of the particle (m/s)

• B is the strength of the magnetic field (Teslas)

Cross Product• In the equation F = q v x B, v x B is a

cross product

• Cross product means the magnetic force is perpendicular to both the velocity of the charge, v, and the magnetic field, B

Direction of Magnetic Force

• Right hand rule = USE YOUR RIGHT HAND – works for small positive charges

• Thumb in the direction particle is moving

• Other fingers in the direction of the magnetic field

• Force comes straight out of your palm

Practice Question

• What direction is the force on a positive charge when entering a uniform B field in the direction indicated?

A) Up

B) Down

C) Left

D) Right

E) Into page

F) Out of page

Practice Question

• What direction is the force on a positive charge when entering a uniform B field in the direction indicated?

A) Up

B) Down

C) Left

D) Right

E) Into page

F) Out of page

Practice Question

• What direction is the force on a positive charge when entering a uniform B field in the direction indicated?

A) Up

B) Down

C) Left

D) Right

E) Into page

F) Out of page

Practice Question

• A particle with a positive charge of 0.001C is moving perpendicular to a magnetic field of strength 0.3T. The particle has a speed of 400 m/s. Calculate the magnitude of the magnetic force exerted on the particle

• The particle is moving to the right and the magnetic field points into the page. Find the direction of the force on the particle

Practice Question

• A particle with a positive charge of 0.001C is moving parallel to a magnetic field of strength 0.3T. The particle has a speed of 400 m/s. Calculate the magnitude of the magnetic force exerted on the particle