Magnetic Fields

Ch 20 is about Magnetism, magnetic fields, and interactions between moving charges and magnetic fields

Magnetic materials

Magnetic materials: what are they? How do you make a magnet? What is a compass and how does it align with

a magnetic field? Why does a magnet “stick” to certain metals?

Magnetics

Magnetic materials: typically magnets are made up of iron, nickel, and/or cobalt

Materials which are “magnetic” allow the little magnetic “dipoles” to align together, creating a magneti

Magnetic materials

Magnetic materials with dipoles aligned

Magnets

Magnet showing field lines

Magnetism

Magnetic fields are not electric fields A magnetic field does not mess with a

stationary charge, but an electric field does. For every magnet, there is a North and South

pole which can never be “separated”. Ain’t no thing as a North by itself.

Magnetism

Magnetic field lines point in the direction that a north side of a magnet would align

A North on a compass aligns itself with magnetic field lines.

The North pole is the south pole For a magnet, field lines point away from N

and into the S.

Moving charges create Bmagnetic Fields (B fields) A wire carrying a current creates a circular magnetic

field around it (Use right hand rule #2 for orientation) RHR#2: point thumb in direction of current, the

fingers wrap in the direction of the magnetic field. If you coil a wire or wrap wire around a cylinder, the

magnetic field from each wrap adds, creating a North at one end of the cylinder and a South at the other end. This is an electro-magnet, sometime called a solenoid.

Magnetic field around a current carrying wire B (magnetic) field around a wire, this is what I

call Right Hand Rule #2

Force on an Electric Current (or a moving charge) in a magnetic field If a charge moves across a magnetic field, it

experiences a “strange” magnetic force which is oriented Perpendicular to its motion and the direction of B.

For a current in a wire, the Force = ILBsinө, I = current, L = Length of wire crossing the magnetic field, B = magnetic field strength (Teslas) and ө = angle between the wire and the magnetic field lines. Notice when the angle = 90, the Force is a maximum.

F = BILSinө, (force = bill(nye)Sin(guy) )

Magnetic Force on a Current carrying wire Magnetic Force, use Right Hand Rule #1

Force on a Moving charge crossing a Magnetic Field The same idea applies to a charged object crossing

a magnetic field (like a charged duck flying across the earth’s magnetic field)

F = qvBsinө, q = charge (coulombs), v = velocity, B = magnetic field (Tesla). Again, when the angle = 90, the charge is crossing the magnetic field lines and is a maximum.

If the angle = 0, the charge is moving parallel to the magnetic field lines and there ain’t not no Force no mo.

Magnetic Force on a moving chargeMagnetic Force on a moving charge

Right Hand Rule #1 For the Force on a moving charge you must use

RHR #1 to determine the direction of the force 1) Point fingers in direction of the moving charge or

current 2) Orient your Balm so it points in the direction of

the magnetic Field (B) 3) Extend you Fumb and it points in the direction of

the Force. 4) the above orientation is for a + (positive) charge,

for a – (negative) charge, the direction of the force is opposite.

Motion of a charged particle moving across a magnetic field The Force on a charge particle moving across a

uniform magnetic field is always perpendicular to its motion.

This force causes the particle to move in a circle (as in circular motion, dude)

F=qvB = ma =mv2/r which can be rearranged to yield r = mv/qB.

This is a fairly common equation which shows up, it’s a derived equation and not on the green equation sheet.

Charged particle moving perpendicular to magnetic field Particle moves in a cirlce

Magnetic Field Due to a Long Straight Wire For a long straight wire, a circular magnetic

field exists (use RHR#2 for its direction) B = (μ0I)/(2лr) μ= permeability of free space

Magnetic field around a wire

Magnetic field strength, B = (μ0I)/(2лr)