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12.1 Magnetic Forces and Fields

What is a magnetic field?

Electricity and magnetism were regarded as unrelated phenomena until it was noticed that an electric current caused the deflection of the compass needle. Then, magnets were found to exert forces on current-carrying wires. The stage was set for a whole new technology, which would eventually bring electric power, radio, and television.

3If you suspend a bar magnet from its center by a piece of string, it will act as a compass.The end that points northward is called the north-seeking pole.The end that points southward is called the south-seeking pole. More simply, these are called the north and south poles. All magnets have both a north and a south pole. For a simple bar magnet the poles are located at the two ends. Magnetic Poles4Magnetic Poles

5What are fields?A field is a region of space surrounding an object that can cause another object to experience a force (gravitational field, electric fields, magnetic fields)Magnetic field: a 3D region of space around a magnet that causes a magnetic force on magnetic objectsEarth has a magnetic field both inside and surrounding itA magnetic force is the force produced by the interaction of two magnetic fields.

The direction of the magnetic field outside a magnet is from the north to the south pole.Where the lines are closer together, the field strength is greater. The magnetic field strength is greater at the poles. If we place another magnet or a small compass anywhere in the field, its poles will tend to line up with the magnetic field.Magnetic Fields

7Magnetic field patterns for a pair of magnets when opposite poles are near each otherlike poles are near each otherMagnetic Fields

8How magnetic fields are created

MagnetismMagnetism is the attraction or repulsion of certain materials to a magnetic material.Early scholars were very interested with magnetic rocks and noticed that these rocks had 2 ends.When the rocks were suspended by a string, the two converging points aligned along Earths northsouth axis.These rocks were named lodestones, which contain magnetite (Fe3O4), the most magnetic of all minerals on Earth.

The Law of MagnetismAll magnets have a north pole (N pole) and a south pole (S pole).The law of magnetism:Like magnetic poles repel and unlike poles attract each other

Section 12.1Pole to PoleWhen two magnetic poles are brought near each other, the pattern of field lines reveals whether the poles are like or unlike. You can use iron filings to see this pattern.Click the screen to reveal the field lines. Then, label each image using the terms below.

like polesunlike poles


Section 12.1Pole to PoleWhen two magnetic poles are brought near each other, the pattern of field lines reveals whether the poles are like or unlike. You can use iron filings to see this pattern.Click the screen to reveal the field lines. Then, label each image using the terms below.

like polesunlike poles

13The Force of MagnetismIn 1600, it was found that Earth itself is a lodestone with a north and south magnetic pole. Magnets exert forces that seem to originate from the magnetic poles, and they can affect another magnetic object even without contact. This is an action at a distance force. For example, if you suspend a magnet on a string and bring another magnet close to one of its poles, the suspended magnet will rotate, even though there is no visible contact between the two magnets.Magnetic fields are vector quantities they have a magnitude (strength) and a direction. Since the magnetic field is a vector quantity, it is represented by a vector arrow. In diagrams, the length of the vector arrow represents the magnitude of the field, and the direction of the arrow represents the direction of the field at a point.The magnitude of the field is greatest near the magnet and diminishes with distance.Mapping Magnetic Field LinesTo represent the entire magnetic field surrounding a magnet, it would be necessary to draw arrows at an infinite number of points around the magnet. This is impractical. Instead, you can draw a few magnetic field lines with a single arrowhead indicating the direction of the magnetic field. It is important to remember that the magnetic field exists in three dimensions.

Magnetic Field PropertiesThe pole of a magnet that points towards Earths north magnetic pole is labeled north and the other pole is labeled south.Since the north pole of a compass needle points north Earths north pole must actually be a south magnetic pole since unlike poles attract (north and south)Similarly, Earths south pole must actually be a north magnetic polesee p. 548 for story

The north pole of a magnetic compass swings toward Earths north pole. What does this tell you about Earths north pole? How does this question relate to the direction of magnetic field lines?Section 12.1Discussion: Pole to Pole18

Section 12.1Answers for Discussion Questions:Sample answer: Earths north pole must really be a south magnetic pole. Field lines go from the compasss north pole to this south pole, which is why magnetic field lines are directed toward south.19Properties of Magnetic Field LinesA bar magnet creates magnetic field lines that are not parallel to each other and are closely spaced (dense) at the poles. The magnetic field is non-uniform because the lines are not parallel. If the field lines are parallel to each other, the magnetic field is uniform.

Magnetic field line diagrams have the following properties:Exist in 3D surrounding a magnet and are more intense at the polesmagnetic field lines are continuous loops that never crossinside the magnet, the magnetic field lines point from the south pole to the north poleoutside the magnet, the magnetic field lines point away from the north pole toward the south polethe closeness of the lines represents the magnitude of the magnetic field (the closer the lines are together, the stronger the magnetic field)if the field lines are parallel, the magnetic field is uniform

Magnetic Field LinesExist in 3D surrounding a magnet and are more intense at the polesThey are invisible but can be represented using magnetic field line diagrams which:Point from the north pole to the south pole outside a magnet and from south to north within a magnetNever cross one anotherAre closer together where the magnetic field is stronger

Maglev train support mechanism, the red support magnet attracts up toward bottom of steel rail, levitating the train at all times. Guidance magnet used to keep train along a specific path as it move above track

Tesla vs Edison

Domain TheoryA material that is made from a magnetized material and creates a magnetic field is a permanent magnet. Some materials become magnetized when they are placed in an external magnetic field. These are called ferromagnetic materials, and include iron, nickel, and cobalt. The outermost electrons of the atoms of ferromagnetic materials create tiny magnetic fields in each atom. The magnetic fields of adjacent atoms can align to reinforce each other, forming small regions, or domains, with intense magnetic fields.When a ferromagnetic material is in an unmagnetized state, the orientations of the domains are random.

The magnetic fields largely balance each other, leaving the material with little or no overall magnetization. However, when the material is placed in an external magnetic field, the domains become aligned with the external field. This causes the material to become magnetized.

Induced MagnetizationOne way to make an object made of ferromagnetic material temporarily magnetic is to hold it close to a permanent magnet. This is called induced magnetization. For example, if you hang an iron nail by a string and bring a magnet close to the nail, the nail will rotate toward the magnet even before they touch. The nail is not a magnet with distinct poles, yet a magnetic attraction exists between it and the magnet. When the magnet is moved away, the domains in the nail return to random orientations and the nail loses most of its magnetization.The nail will be much more strongly magnetized if it is rubbed with a pole of the magnet. The magnetic fields of many of the domains in the nail will align along the direction of motion of the magnet. This magnetization is strong enough that the nail will remain somewhat magnetized when the magnet is removed.DemagnetizationA magnetic material can become demagnetized when the aligned domains return to random orientations. Dropping or heating a magnet can cause the domains to become unaligned.

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