Magnetic Fields

1. Magnetic Fields Ch 20 is about Magnetism, magnetic fields, and interactions between moving charges and magnetic fields

2. 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?

3. 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.

4. 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.

5. 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.

6. 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) )

7. 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.

8. 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.

9. 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.

10. 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