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Electricity and Magnetism
Lesson 37: Magnetic Fields
Lesson 37: Magnets & Magnetic Fields
What are some everyday things that we use magnets for?
1. Credit/ATM/Debit have magnetic strips that indicate information about the particular
card.
2. Most TV’s and computer monitors use what is called an electromagnet to guide
electrons to the screen. An electromagnet is created from coiled wire. It acts similar to a
switch turning on and off its magnetism by current. When a current is passing through
the coil, it becomes magnetized. When there is no current passing, it is not magnetic.
3. Sticking notes/pictures on the refrigerator
4. Motors
5. Speakers
Can you come up with some more? We all have some idea of what magnet is; as you can see
from the examples, we use them in many different applications. They are useful
instruments in our day to day life, but it is one thing to know what a magnet is, it is another
to understand how they work.
What exactly is a magnet? A magnet is a material or object that produces a magnetic field
where the object exerts attractive and repulsive forces on other materials or charged
particles. That being said, what is a magnetic field? A magnetic field is an invisible region of
forces that pulls on ferromagnetic materials (materials mostly associated with containing
iron, nickel, and cobalt), by causing attraction or repulsion to other magnets.
Let’s take a closer look at the magnetic field. The magnetic field is driven by the poles
(ends) of the magnet. Generally, we call these poles the north and south pole,
corresponding to the geographic location of the north and south poles of the Earth. The
names come from the attraction of the magnetic poles to these locations. The illustration
below shows the magnetic field on a bar magnet with the north and south pole indicated on
the picture. The iron filings surrounding the magnet emulate the magnetic field.
NASA-Threads
Electricity and Magnetism
Lesson 37: Magnetic Fields
A key concept of magnetic poles is evident in the picture above. The idea that for magnets
opposites attract is visible by the iron filings are looping over from the north pole to the
south pole. This means if you had two magnets, the north pole of the magnet will be
attracted to the south pole of the other magnet and vice versa.
On the other hand, the north pole of one magnet will repel the north pole of the other
magnet and vice versa for the south pole of the magnets. These concepts of attraction and
repulsion can be visualized with a compass. Why does the needle of a compass point north?
It is because the south pole of the magnet in the compass is being attracted to the
geographic north pole of Earth. A compass is designed to align a magnetic pointer with the
Earth’s magnetic field. Thus providing us with a good directional idea of where north is in
relation to a geographic location.
Say for instance, you had a wire that had a certain current passing through the wire. No
longer will the magnetic field be in the direction of the Earth’s geographic poles, but now a
localized magnetic field is formed from the current in the wire. The magnetic field will
move in a new direction. In order to predict the direction of a magnetic field, you can
implement the right hand rule.
Right Hand Rule Steps:
1. Imagine as though you grab the wire with your right hand.
2. Make sure your thumb is pointing in the direction of the current flow.
3. Your fingers will curl in the direction of the magnetic field.
Example: What is the direction of the magnetic field given the wire and the indicated
current direction?
Answers:
1.
2
.
1.
I
3
.
2.
I
3.
I
NASA-Threads
Electricity and Magnetism
Lesson 37: Magnetic Fields
Now that you are able to predict the direction of the magnetic field, you might also want to
calculate its magnitude. In order to calculate the magnitude of a magnetic field you could
use the following relationship:
magnetic force on a charged particle
magnetic field =
(magnitude of charge)(speed of charge)
or simply:
B=
Fmagnetic
qv
Where q is the magnitude of the charge and v is the speed or velocity of the charge. The
units associated with magnetic field are the Tesla (T). A Tesla is broken down to the
following units:
N
N
(V ∗ s)
T=
=
=
m
m2
(C ∗ s ) A ∗ m
The equation for magnetic force applies to a charged particle moving through a magnetic
field. How would the equation change so that we can apply it to the examples we did earlier
using a wire and current? When you look at the units of Tesla it contains a Newton over an
Ampere times a meter. Associate this back to our examples we have a current value which
is in amperes and the wire should have a length. Thus, we can obtain a new equation for the
magnitude of the magnetic field, where I is the current, and l is the length of the conductor
(wire):
Fmagnetic
B=
I∗l
Now, using the example problems from earlier, let’s see if we can calculate the magnitude
of the magnetic field.
1.
l=?
l=43m
I=21A
If a magnetic force on the wire
is pulling it at 4.32x10-2N,
what is the magnitude of the
magnetic field?
2
.
I=28A
If a magnetic force on the wire
is pulling it at 5.71x10-3N
and the magnitude of the
magnetic field is 4.98x10-6T,
what is the length of the
conductor?
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3
.
Electricity and Magnetism
Lesson 37: Magnetic Fields
If the magnitude of the magnetic field is
l=24m
I=35A
3.82x10-7T, what is the magnetic
force pulling on the wire?