Magnetic FieldsMagnetic Fields

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

moving charges and magnetic fieldsmoving charges and magnetic fields

Magnetic materialsMagnetic 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?

MagnetismMagnetism

• 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.

MagnetismMagnetism

• 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 Moving charges create Bmagnetic Fields (B fields)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.

Force on an Electric Current (or a Force on an Electric Current (or a moving charge) in a magnetic fieldmoving 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) )

Force on a Moving charge crossing Force on a Moving charge crossing a Magnetic Fielda 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.

Right Hand Rule #1Right 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 Motion of a charged particle moving across a magnetic fieldacross 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.

Magnetic Field Due to a Long Magnetic Field Due to a Long Straight WireStraight 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