Download CHAPTER 19

A. Manalaysay – PHY2054, Fall 2004
CHAPTER 19
This chapter’s an introduction to magnetic fields. Magnetic fields are similar (but a little
more complicated) than electric fields. You’re familiar with a bar magnet probably;
magnetic field lines point out of a north pole and into a south pole. The Earth’s magnetic
field at the surface points north, geographically (i.e. away from Antarctica). But this
means the field lines point out of Antarctica, which means that is where the magnetic
north pole is. If this is confusing, look at page 589 of your book for a diagram.
Magnetic fields can produce a force on a charged particle:
---Magnetic force on a charged particle (magnitude only)
“q” is the charge, “v” is the velocity, “B” is the magnetic field, and “ ” is the angle
between the magnetic field and the velocity.
To find the direction of the magnetic force, use the Right-Hand-Rule: with your right
hand, hold your fingers straight and in the direction of the B-field, your thumb in the
direction of the velocity. The direction your palm faces is the direction of the force.
Magnetic fields produce a force on a current-carrying wire:
---Magnetic force on a current-carrying wire (magnitude only)
“B” is the magnetic field, “I” is the current, “L” is the length of the wire, and “ ” is the
angle between the magnetic field and the current. The direction of this force is again
found by using the Right-Hand-Rule, except now you put your thumb in the direction of
the current.
A loop of current in a uniform magnetic field experiences a torque:
---Torque on loop(s) of wire
“N” is the number of loops (or coils), “A” is the area of the loop, and “ ” is the angle
between the B-field and the normal to the loop. This confuses most people.
If a charged particle moves under only the influence of a magnetic field, it travels in
circular paths (or helical paths—see the figure on page 599). The radius of this circle is:
---Radius of a charged particle’s path in a magnetic field
Wires produce magnetic fields themselves:
---Magnetic field produced by a long, straight wire
, “I” is the current in the wire, “r” is the distance from/to the wire.
Ampere’s Law: basically the equivalent of Gauss’s Law for currents and B-fields.
---Ampere’s Law, simply says add up the pieces around a closed
loop. “I” is the total current enclosed by the loop.
---Force between two parallel, current-carrying wires (magnitude only)
---Magnetic field inside a solenoid; it’s uniform everywhere inside. Look at
page 605 to see what a solenoid is. “n” is the number of turns per unit length (basically
n=N/L).