1. Magnetic Field and Magnetic Forces
Chapter 27
Magnetic Field and Magnetic Forces

2. How does a magnetic field differ from an electric field?
Question of the day
How does a magnetic field differ from an electric field?

3. Learning Goals for Chapter 27
Looking forward at …
the properties of magnets, and how magnets interact with each other.
how to analyze magnetic forces on current-carrying conductors and moving charged particles.
how magnetic field lines are different from electric field lines.
some practical applications of magnetic fields in chemistry and physics, including electric motors.
how current loops behave when placed in a magnetic field.

4. Magnetic poles
If a bar-shaped permanent magnet, or bar magnet, is free to rotate, one end points north; this end is called a north pole or N pole.
The other end is a south pole or S pole.
Opposite poles attract each other, and like poles repel each other, as shown.

5. Experiments with Magnetism: Experiment 1
Tape a bar magnet to a cork and allow it to float in a dish of water.
The magnet turns and aligns itself with the north-south direction.
The end of the magnet that points north is called the magnet’s north-seeking pole, or simply its north pole. The other end is the south pole.

6. Magnetism and certain metals
An object that contains iron but is not itself magnetized (that is, it shows no tendency to point north or south) is attracted by either pole of a permanent magnet. Magnetite is a naturally-occurring iron mineral that is magnetic.
This is the attraction that acts between a magnet and the unmagnetized steel door of a refrigerator.

7. Experiments with Magnetism: Experiment 2
Bring the north pole of a bar magnet near a compass needle.
When the north pole is brought near, the north-seeking pole of the compass needle points away from the magnet’s north pole. Apparently the compass needle is itself a little bar magnet.

8. Magnetic field of the earth
The earth itself is a magnet.
Its north geographic pole is close to a magnetic south pole, which is why the north pole of a compass needle points north.
The earth’s magnetic axis is not quite parallel to its geographic axis (the axis of rotation), so a compass reading deviates somewhat from geographic north.
This deviation, which varies with location, is called magnetic declination or magnetic variation.
Also, the magnetic field is not horizontal at most points on the earth’s surface; its angle up or down is called magnetic inclination.

9. Compasses and Geomagnetism
Apparently, the Earth is a large magnet, and one with a magnetic south pole near the Earth’s geographic north pole.
The reasons for the Earth’s magnetism are complex, involving standing currents in the Earth’s molten outer core. The Earth’s magnetic poles are separated somewhat from the geographic poles, and move around some as a function of time. There is evidence in the geological record that the Earth’s field has reversed directions at varying intervals spaced by millions of years. This is not well understood.

10. Although the Earth's core is iron, iron becomes nonmagnetic above 1043K (the Curie temperature).

11. Variations in the Earth’s surface magnetic field in 1980.

12. Differences between magnetic and electric fields
Magnets exist as dipoles (no magnetic “point charges”)
Magnetic force depends on velocity (no movement of test charge means no force exerted by field)
Magnetic force is exerted on a moving test charge perpendicularly to its motion

13. Magnetic monopoles Magnetic poles always come in pairs
There is no experimental evidence for magnetic monopoles.

14. Experiments with Magnetism: Experiment 3
Use a hacksaw to cut a bar magnet in half. Can you isolate the north pole and the south pole on separate pieces?
No; when the bar is cut in half two new (but weaker) bar magnets are formed, each with a north pole and a south pole. The same result would be found, even if the magnet was sub-divided down to the microscopic level.

15. Experiments with Magnetism: Experiment 4
Bring a magnet near the electrode of an electroscope.
There is no observed effect, whether the electroscope is charged or discharged and whether the north or the south pole of the magnet is used.

16. Electric current and magnets
A compass near a wire with no current points north.
However, if an electric current runs through the wire, the compass needle deflects somewhat.

17. Two Kinds of Magnetism?
We jumped from a discussion of the magnetic effects of permanent bar magnets to the magnetic fields produced by current-carrying wires.
Are there two distinct kinds of magnetism?
No. As we will see in the lectures that follow, the two manifestations of magnetism are actually two aspects of the same fundamental magnetic force.

18. Current Effect on Compass
Place some compasses around a wire. When no current is flowing in the wire, all compasses point north. When current flows in the wire, the compasses point in a ring around the wire.
The Right-Hand Rule: Grip the wire so that the thumb of your right hand points in the direction of the current. Then your fingers will point in the direction that the compasses point. This is the direction of the magnetic field created by the current flow.

19. The Discovery of the Magnetic Field
In April 1820, the Danish physicist Hans Christian Oersted was giving a evening lecture in which he was demonstrating the heating of a wire when an electrical current passed through it. He noticed that a compass that was nearby on the table deflected each time he made the current flow.
Until that time, physicists had considered electricity and magnetism to be unrelated phenomena. Oersted discovered that the “missing link” between electricity and magnetism was the electric current.
Hans Christian Oersted
( )

20. The magnetic field
A moving charge (or current) creates a magnetic field in the surrounding space.
The magnetic field exerts a force on any other moving charge (or current) that is present in the field.
Like an electric field, a magnetic field is a vector field—that is, a vector quantity associated with each point in space.
We will use the symbol for magnetic field.
At any position the direction of is defined as the direction in which the north pole of a compass needle tends to point.

21. The Magnetic Field Definition of the magnetic field:
The magnetic field at each point is a vector, with both a magnitude, which we call the magnetic field strength B, and a 3-D direction.
A magnetic field is created at all points in the space surrounding a current carrying wire.
The magnetic field exerts a force on magnetic poles. The force on a north pole is parallel to B, and the force on a south pole is anti-parallel to B.

22. Magnetic Field Lines B-field lines never cross.
The magnetic field can be graphically represented as magnetic field lines, with the tangent to a given field line at any point indicating the local field direction and the spacing of field lines indicating the local field strength. The field line direction indicates the direction of force on an isolated north magnetic pole.
Field Map
B-field lines never cross.
Field Lines
B-field line spacing indicates field strength
weak
strong
B-field lines always form closed loops.

23. Iron filings on a sheet of paper placed over a bar magnet show a representation of the magnetic field. The poles are where the field is the strongest.

24. Magnetic field lines are not lines of force
It is important to remember that magnetic field lines are not lines of magnetic force.
The force on a charged particle is not along the direction of a field line.

25. Ampere’s Experiment
When Andre Ampere heard of Oersted’s results, he reasoned that if a current produced a magnetic effect, it might respond to a magnetic effect. Therefore, in 1823, he measured the force between two parallel current-carrying wires.
He found that parallel currents create an attraction between the wires, while anti-parallel currents create repulsion.
André Marie Ampère
(1775 – 1836)