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13.3
13.3
Oersted’s Discovery
N
For centuries, people believed that electricity and magnetism were somehow
related, but no one could prove a connecting link between them. Then, in 1819,
the Danish physicist Hans Christian Oersted (1777–1851) discovered the connection by accident while lecturing on electric circuits at the University of
Copenhagen. Oersted noticed that a compass needle placed just below a wire carrying a current would take up a position nearly perpendicular to the wire while
the current was flowing (Figure 1). When the direction of the current was
reversed, the compass needle again set itself at right angles to the wire, but with
its ends reversed. The effect lasted only while the current flowed. Much to his
own surprise, Oersted had discovered the basic principle of electromagnetism.
S
no electric current
Principle of Electromagnetism
Whenever an electric current moves through a conductor, a magnetic
field is created in the region around the conductor.
The Magnetic Field of a Straight Conductor
The magnetic field lines for a straight conductor are concentric circles around
the conductor (Figure 2). As the distance from the conductor increases, the field
gets weaker and the lines become more widely spaced. There are no poles; the
field lines are continuous and give the direction of the plotting compass at every
point.
Reversing the direction of electric current through the conductor causes the
field lines to point in the opposite direction, though their pattern remains the
same. To help you remember the relationship between the direction of the magnetic field lines and the direction of electric current there is the right-hand rule
for a conductor (Figure 3).
electric current
Figure 1
When there is no electric current, the compass needle points to the north. When there
is a current, the needle turns so that it is perpendicular to the wire.
principle of electromagnetism:
Whenever an electric current moves through
a conductor, a magnetic field is created in the
region around the conductor.
Right-Hand Rule for a Conductor
If a straight conductor is held in the right hand with the right thumb
pointing in the direction of the electric current, the curled fingers will
point in the direction of the magnetic field lines.
electric current
Figure 2
The field around a long straight current
carrying conductor is three-dimensional in
nature.
direction of magnetic
field lines
electric current
conductor
right hand
magnetic
field lines
right-hand rule for a conductor: If a
straight conductor is held in the right hand
with the right thumb pointing in the direction
of the electric current, the curled fingers will
point in the direction of the magnetic field
lines.
Figure 3
The right-hand rule for a straight conductor
Electromagnetism 479
(a)
Rather than drawing the conductor as a cylinder and using an arrow to indicate direction, it is often more convenient to use a two-dimensional picture, as in
Figure 4. A circle is used to represent a cross-section of the conductor. A circle
with an X in it represents an electric current moving into the page. A circle with
a dot in it represents an electric current moving out of the page.
current into page
(b)
Activity 13.3.1
Magnetic Field of a Straight Conductor
Question
What are the characteristics of the magnetic field around a straight conductor?
Materials
current out of page
Figure 4
Models of the magnetic field of a straight
conductor
(a) Imagine the X as being the tail of an
arrow moving away from you.
(b) Imagine the dot as being the tip of an
arrow facing you.
One of the wires from the
conductor should be connected firmly to one of the
terminals; the other wire
should be touched momentarily to the other terminal.
The resistance of the bare
wire is very low. As a result, it
draws a large current. This
will cause the wire to get hot
and the battery to discharge
quickly if the terminals are
connected for too long a time.
Be careful not to get iron
filings in your eyes. Wash
your hands after handling
iron filings.
480 Chapter 13
20 cm of bare 12-gauge copper wire
piece of stiff cardboard, 15 cm × 15 cm
battery (6 V–12 V) or DC power supply
iron filings
connecting wires with alligator clips
four compasses
Procedure
1. Push the short piece of bare copper wire through the middle of the cardboard square and support the cardboard in a horizontal position, as shown
in Figure 5.
2. Connect the upper end of the copper wire to either terminal of the battery,
using a wire with an alligator clip. Connect another wire with a clip to the
bottom of the copper wire, but do not connect it to the battery.
3. Lightly sprinkle iron filings on the piece of cardboard. Momentarily touch
the loose wire to the other terminal of the battery, and tap the cardboard
gently. Once the iron filings have assumed a pattern, disconnect the battery
and sketch the pattern. Be sure to include the copper wire in your sketch.
From the battery terminals used, determine the direction of the electric
current and mark it in your sketch.
4. Place four plotting compasses on the cardboard, as shown in Figure 5.
Connect the battery and note the directions in which the compasses point.
Add these directions to your sketch of the iron filings.
5. Without moving the compasses, reverse the connections to the battery.
Make another sketch. Show the direction in which the electric current is
moving.
Observations
(a) Describe the shape of the magnetic field lines produced by the electric
current in a straight conductor.
(b) Describe the spacing and clarity of the field lines farther away from the conductor. What does this indicate about the magnetic field in these regions?
(c) Compare the compass direction pattern obtained in step 4 with that
obtained in step 5. Does the right-hand rule provide an adequate description of these patterns? Explain.
13.3
copper
wire
Practice
compass
Understanding Concepts
1. Figure 6 shows three current-carrying conductors with their magnetic
fields. Copy the diagrams into your notebook and indicate the direction of electric current in each wire.
2. Figure 7 shows three conductors with the direction of the electric current. Copy the diagrams into your notebook and draw magnetic field
lines around each, indicating polarities where applicable.
battery
(a)
(b)
(c)
Figure 5
Setup for Activity 13.3.1
(a)
Figure 7
3. Choose the diagram from Figure 8 that best illustrates the strength of
the magnetic field surrounding a conductor. Explain your answer.
(a)
(b)
(c)
(b)
Figure 8
(c)
SUMMARY
Oersted’s Discovery
• The principle of electromagnetism states that whenever an electric current
moves through a conductor, a magnetic field is created in the region
around the conductor.
• The magnetic field lines around a straight conductor are in concentric
circles.
• The right-hand rule for a straight conductor states that if a conductor is
held in the right hand with the right thumb pointing in the direction of
the electric current, the curled fingers will point in the direction of the
magnetic field lines.
Figure 6
For question 1
Electromagnetism 481
Section 13.3 Questions
Understanding Concepts
1. An electric current is travelling southward in a straight, horizontal
conductor. State the direction of the magnetic field
(a) below the conductor
(b) above the conductor
(c) east of the conductor
(d) west of the conductor
2. What is the direction of the magnetic field lines around a conductor with the electric current travelling (a) away from you, and
(b) toward you?
Figure 9
For question 3
3. The diagram in Figure 9 represents two parallel current-carrying
conductors. Determine whether the conductors attract or repel
one another. Explain your reasoning.
Reflecting
4. Why do you think the connection between electricity and magnetism was not discovered until the early 19th century? (Hint:
Research when magnetism, static electricity, and current electricity were discovered.)
13.4
Figure 1
electromagnet: object that exerts a magnetic force using electricity
solenoid: a coil of wire that acts like a
magnet when an electric current passes
through it
uniform magnetic field: the magnetic
field is the same strength and acts in the
same direction at all points
right-hand rule for a coil: If a coil is
grasped in the right hand with the curled fingers representing the direction of electric
current, the thumb points in the direction of
the magnetic field inside the coil.
482 Chapter 13
The Magnetic Field of a Coil
or Solenoid
In a junkyard, a crane lifts a pile of scrap metal to be compressed and eventually
recycled (see Figure 1). How is the scrap metal held up by the crane? You might
say by a magnet, but it couldn’t be a permanent magnet—otherwise how would
the metal be released? It is held by an electromagnet, a device that exerts a magnetic force using electricity.
The magnetic field around a straight conductor can be intensified by
bending the wire into a loop, as illustrated in Figure 2. The loop can be thought
of as a series of segments, each an arc of a circle, and each with its own magnetic
field (Figure 2(a)). The field inside the loop is the sum of the fields of all the segments. Notice that the field lines are no longer circles but have become more like
lopsided ovals (Figure 2(b)).
The magnetic field can be further intensified (Figure 3) by combining the
effects of a large number of loops wound close together to form a coil, or solenoid.
The field lines inside the coil are straight, almost equally spaced, and all point in
the same direction. We call this a uniform magnetic field; the magnetic field is
of the same strength and is acting in the same direction at all points.
If the direction of electric current through the coil is reversed, the direction
of the field lines is also reversed but the magnetic field pattern, as indicated by a
pattern of iron filings, looks the same as it did before. To help you remember the
relationship between the direction of electric current through a coil and the direction of the coil’s magnetic field, there is the right-hand rule for a coil (Figure 4).
Right-Hand Rule for a Coil
If a coil is grasped in the right hand with the curled fingers representing
the direction of electric current, the thumb points in the direction of the
magnetic field inside the coil.