Do Now (1/23/14):

1. How do magnets work?2. What are some of their properties?

Welcome back!

Magnetism and Electro Magnetism

TimsonJanuary 2014

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

When an unmagnetized substance is placed in a magneticfield, the substance can become magnetized.This happens when the spinning electrons line up in thesame direction.

An unmagnetized substance looks likethis…

While a magnetized substance lookslike this…

How to break a magnet:

1. Drop it

2. Heat itThis causes the domains to become random again!

Review:

• Field due to current carrying wire:

r

IB

20

Ampere’s Law

For any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop

Ampere’s Law • The magnetic field in space around an

electric current is proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source.

• Which law is analogous to Ampere’s Law?Gauss’s Law:

Example:A long straight wire carries a current of 5 A. at one

instant, a proton, 4 mm from the wire, travels at 1.5x103 m/s parallel to the wire and in the same direction as the current. Find the magnitude and direction of the magnetic force that is acting on the proton because of the magnetic field produced by the wire.

6x10-20 N

Magnetic Force Between Two Parallel Conductors

Magnetic Field of a Current Loop

• Examining the direction of the magnetic field produced by a current-carrying segment of wire shows that all parts of the loop contribute magnetic field in the same direction inside the loop.

Magnetic Field of Current Loop

• Electric current in a circular loop creates a magnetic field which is more concentrated in the center of the loop than outside the loop.

• Stacking multiple loops concentrates the field even more into what is called a solenoid.

Magnetic Field of a Solenoid• A long straight coil of wire can be used to

generate a nearly uniform magnetic field similar to that of a bar magnet.

• The field can be greatly strengthened by the addition of an iron core.

Example:

• A certain solenoid consists of 100 turns of wire and has a length of 10 cm. Find the magnetic field inside the solenoid when it carries a current of 0.5 A.

6.28x10-4 T

Inductance, Inductors

Lseries = L1 + L2 + …. + Ln

Lparallel = (1/L1 + 1/L2 + … + 1/Ln)-1

Potential across inductor: vL(t) = L diL(t) / dt

Inductance, unit: henry [H] = ability to store magnetic energy

L = N2 μA / ℓ UM = ½LI2

A circuit element that has a large self-inductance is called an inductor. The circuit symbol is