3: Exploring the field around a current
As well as finding magnetic fields around magnets, we also find magnetic fields around
currents. The movie below shows compasses initially aligned with the earthÕs field. The
current is turned on, then about 2 seconds after the compasses settle the current is slowly
increased. The compasses show the vector sum of the currentÕs field and the earthÕs
field. Play the movie now.
(a) "Drag" through the movie again and
watch the top and bottom compasses (in
the N and S positions). Stop about half
way through the movie, just after the
current is turned on.
Draw a vector diagram for the magnetic
fields at the location of the South
compass showing:
the earthÕs field vector,
and
the resultant (total) field vector
and hence deduce the direction of the
currentÕs field vector. (Remember that
the earth's field points from south to north
and the compasses align with the
resultant field.)
(b) Repeat the vector diagram drawing for the North compass.
(c) Draw a similar (rather simple!) vector diagram for the West (left) compass. Why has it
not moved?
(d) Now drag through to the end of the movie watching the East (right) compass. See how
it flips direction? See if you can do the following:
reason out how strong the current's field must be compared to the earth's field to
make that compass flip. Then reason out in what direction the South compass
should point for that strength field (do a mental vector addition!). Verify that the
South compass does point in this direction just at the time the East compass starts
its flip.
(e) By looking at your vector diagrams, sketch what the field pattern would look like if the
current were very strong, producing a field much stronger than the earth's field.
Your last sketch shows a situation
where the effect of the currentÕs field
completely swamps the earth's very
weak field. The diagram opposite
shows a more complete picture of the
field produced by a current-carrying
wire.
Use your right
hand rule to
determine the
direction of
current flow in
the wire in the
movie.
A simple rule using your right hand
helps you determine the direction of the
field for a given current direction. This
is called the Right Hand Grip Rule. (The
"grip" is included in the name to
distinguish this rule from another
right-had rule you will meet later.)
Study this diagram and write your own
description of this right hand grip rule
relating the positions of your fingers
and thumb, and the directions of the
magnetic field and current.
Bending the wire into a coil
If a straight wire is bent to form a coil, the right
hand grip rule can still be used to determine the
field direction. Just imagine moving your thumb
around the coil in the direction of the current, with
fingers pointing into the coil.
A more complete picture of the field
produced by a single loop of wire is
shown here.
In what part of the pattern does the field
tend to be uniform?
How could you produce an extended
region of uniform field using wires?
Are there currents in a magnet?
Yes! You guessed it! The reason that a magnet produces a magnetic field is that there are
many tiny currents, caused by electrons orbiting within atoms, each producing tiny
magnetic fields. In a magnet many of these fields become aligned to produce an overall
net field.
For more details see Giancoli 20.13.
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