Magnetic Force And Fields

1. Magnetic Force And Fields

2. Magnets • A magnet is a material that can attract other materials with magnetic properties. • These substances are called ferromagnetic. Nickel, cobalt and iron and any alloy containing these are behave in the same way. • You can induce them to become magnetized by placing them in a magnetic field.

3. Magnets • A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. • An everyday example is a refrigerator magnet used to hold notes on a refrigerator door.

4. Magnets • An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. • Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.

5. Magnets • Magnets have areas of concentrated magnetic force on the poles. • The N-pole is called the north seeking pole and the S-pole is called the south seeking pole. • They will seeking the North & South pole of the Earth.

7. Law of Magnetic Poles • Opposite poles attract and similar poles repel. • N-pole will attract an S-pole • S-pole will attract an N-pole • N-pole will repel an N-pole • S-pole will repel an S-pole

8. Magnetic Field Lines • Magnetic Field of Force: the space around a magnet in which magnetic forces are exerted. • We can find the magnetic field by using iron filings or small compasses around a magnet. • Magnetic fields are represented by a series of lines around a magnet, showing the path the N-pole of a small test compass would take if it were allowed to move freely in the direction of the magnetic force.

9. The characteristics of Magnetic Field Lines • The spacing of the lines indicates the relative strength of the force. The closer together the lines are, the greater the force. • Outside a magnet, the lines are concentrated at the poles. They are closest within the magnet itself. • By convention, the lines proceed from S to N inside a magnet and from N to S outside a magnet, forming closed loops. (A plotting compass indicates these directions.) • The lines do not cross one another. • Note that the magnetic field around a bar magnet is three-dimensional in nature; it does not exist just in the horizontal plane.

10. Practice Problems: page 474 #’s 1-6 Section 13.1 Questions: page 474-475, #’s 1-4

11. Magnetic Materials

12. Domain Theory of Magnetism • The atoms of ferromagnetic substances can be thought of as tiny magnets with N-poles and S-poles. • These atomic magnets, or dipoles, interact with the nearest neighbouring dipoles and a group of them line up with their magnetic axes in the same direction to form a magnetic domain. • An unmagnetized piece of iron contains millions of these domains, but they are pointing in random directions so that the piece of iron, as a whole, is not magnetized.

13. Domain Theory of Magnetism • When an unmagnetized piece of iron is placed in a magnetic field (that is, near another magnet), the dipoles act like small compasses and rotate until they are aligned with the field. • The piece of iron will then contain a large number of dipoles pointing north, causing one end to become an N-pole, and the other end to become an S-pole.

14. Effects of the Domain Theory Magnetic Induction • A permanent magnet brought near an iron nail will cause the nail to become a temporary magnet. • The field of the permanent magnet causes the dipoles in the iron nail to align momentarily. This process of magnetizing an object from a distance is called magnetic induction. • If magnetic induction is applied to a steel nail rather than an iron nail, the dipoles of the steel tend to retain their alignment for a longer time due to the carbon atoms in the steel. This causes the steel to act more like a permanent magnet.

15. Effects of the Domain Theory Demagnetization • When a piece of iron becomes demagnetized, its aligned dipoles return to random directions. • Dropping or heating an induced magnet will cause this to occur. Some materials, such as pure iron, revert to random alignment as soon as they are removed from the magnetizing field. • Substances that can become instantly demagnetized are called soft ferromagnetic materials. • Iron can be alloyed with certain materials, such as aluminum and silicon, that have the effect of keeping the dipoles aligned even when the magnetizing field is removed. These alloys are used to make permanent magnets and are referred to as hardferromagnetic materials.

16. Effects of the Domain Theory Reverse Magnetization • If a bar magnet is placed in a strong enough magnetic field of opposite polarity, its domains can turn and point in the opposite direction. • In that case, the N-pole of the magnet is at the end marked “S.” • The magnet is reverse-magnetized. • Small compass needles easily become reverse-magnetized.

17. Effects of the Domain Theory Breaking a bar magnet • Breaking a bar magnet produces two pieces of iron whose dipole alignment is identical to the original piece. • Both pieces will also be magnets, with N-poles and S-poles at opposite ends. • Continued breaking will produce the same results, since the domains within the magnet remain aligned even when the magnet is broken.

18. Effects of the Domain Theory Magnetic Saturation • In most magnets many (but not all) of the dipoles are aligned in the same direction. • The strength of a bar magnet can be increased only up to a certain point. • The peak will occur when the maximum number of dipoles are aligned. The material is then said to have reached its magnetic saturation.

19. Effects of the Domain Theory Induced Magnetism by Earth • If a piece of iron is held in Earth’s magnetic field and its atoms are agitated, either by heating or by mechanical vibration (that is, by hitting the iron with a hammer), its dipoles will align. • This is most easily accomplished by holding the piece of iron pointing north and at the local angle of inclination, while tapping it with a hammer. • Steel columns and beams used in building construction are usually found to be magnetized. Steel hulls of ships and railroad tracks are also magnetized by Earth’s magnetic field.

20. Effects of the Domain Theory Keepers for Bar Magnets • A bar magnet will become demagnetized over time as the poles at its ends start to reverse the polarity of the atomic dipoles inside; this occurs because of random thermal motion of the atoms of the bar magnet. • Bar magnets can be stored in pairs with their opposite poles adjacent and with small pieces of soft iron (called “keepers”) across the ends, so that demagnetization does not occur. • The keepers themselves become strong induced magnets and form closed loops of magnetic dipoles that prevent the poles from demagnetizing.

21. Practice Problems: page 478 #’s 1-6 Section 13.1 Questions: page 478 #’s 1-5