Magnets & Magnetism Guided Notes Name:_____________________

What is a magnetic field?

What causes a magnetic field?

Compare and contrast magnetic fields and electric fields.

Types of Magnets

Magnetic fields are created by moving electric charges … where are the charges??

1) _____________________________________________

◦ Magnetic field created by ______________________

◦ Can be turned on/off

Examples: junk yard cranes, components of motors, transformers, generators, speakers, etc

2) ______________________________

The charges are the _______________________________ of the atoms!

◦ All objects have spinning electrons, and each spinning electron has a mini-magnetic field around it

◦ Usually, each mini-magnetic field points in a random direction

◦ _________________________________________________________________________________

Examples:

◦ Fridge magnets

◦ A compass needle

◦ The Earth!

Magnets are cool!

North (+) Pole and South (-) Pole Opposite Attract Likes Repel

We can NEVER pull apart magnetic dipole. When we cut magnet in two we end up with two smaller dipoles. If we keep on cutting, more magnets will be produced, EACH with north and south pole.

Magnetic monopoles have never been detected. Just for fun … Ever wonder why fridge magnets don’t seem to have poles … they do!

The poles received their names because of the behavior of a magnet in the presence of the Earth’s magnetic field. The pole of a magnetic needle that points to the north of the Earth is called north pole. So, magnetic pole which is in the geographic north is magnetically south pole.

The Magnetic Field

A magnetic field is said to exist at a point if a ________________________ placed there ____________________________________.

The _________________________ of the magnetic field at any location is the _________________________________ of the compass needle points at that location.

– Arrows give direction to which a north pole will orient – Density gives strength– Looks like dipole!

Let’s break a magnet!

A fridge magnet has poles, we just don’t feel them

Magnetic Force

Strength of Magnetic Force

A charged particle moving in a magnetic field experiences a force that is

__________________________to BOTH the ____________________

and to the _______________________

Lorenz Force Law

The magnitude of the magnetic force on a moving, charged particle is

(θ is the angle between the charge’s velocity and the magnetic field)

Example:

Sin 0, 180 = 0

If a charge has velocity in the same (or opposite) direction of the magnetic field, it experiences no force!

Sin 90 = 1 A charge that has velocity perpendicular to the magnetic field experiences the greatest force!

Question

The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. Which particle experiences the greatest magnetic force?

Direction of Magnetic Force – the Right Hand Rule

Point fingers in the direction in which __________________________is moving

Curve fingers in the direction of ____________________

Thumb points in the direction of the ______________________

IMPORTANT: The thumb points in the direction a POSITIVE charge (or current) would move. The force on a negativecharge goes opposite the thumber.

Right hand rule practice

Magnetic Force Problems

We do:

What is the minimum magnetic field necessary to exert a 5.4 X 10 -15 N force on an electron moving at 2.1 X 107 m/s?

You do:

What is the magnetic field necessary to exert a 5.4 X 10-15 N force on an electron moving at 2.1 X 107 m/s if the magnetic field is at 45 degrees from the electron’s velocity?

What is the magnitude of the magnetic force on a proton moving at 2.5 X 105 m/s in a magnetic field of 0.5 T …

(a) …if the velocity and magnetic field are at right angles?

(b) … if the velocity and magnetic field are at 30°?

(c) … if the velocity is parallel to a magnetic field?

Comparison of Electric and Magnetic Fields

Induced Magnetic Field Guided Notes Name:_____________

Adding magnetic fields

What is the direction of the force on the top wire? What is the direction of the force on the middle wire?

First, use _____________________ to find the direction of the _____________________ on each wire.

Then, use ________________ to find the _____________ on each wire

CONCLUSION: Wire with current in the same direction ____________

CONCLUSION: Wires with current in the opposite direction ____________

What is the force on the left wire?

Quick Review

• How do you determine the direction of a magnetic field induced by a current?

• How does magnetic field vary with distance from a wire?

• What is the equation for induced magnetic field?

• Opposite currents …

• Currents in the same direction ...

Solenoids

A solenoid consists of several current loops stacked together.Used to create a strong, uniform magnetic field-- electromagnets-- starter for car engine B=μ0nIn = number of windings per unit length,I = current in windings