Transcript Chapter 6
Chapter 6 Magnets and Magnetism Objectives After studying this chapter, you will be able to: • Describe various types of magnets • Describe the nature of magnetic fields and forces • Explain the theories of magnetism Objectives (cont’d.) • List the various uses for magnetism and methods of controlling magnetic forces • Explain the relationship between magnetism and electricity Magnets • Materials that attract other metals • Three classes: natural, artificial and electromagnets • Laws of magnetic attraction and repulsion – Like magnetic poles repel each other – Unlike magnetic poles attract each other – The closer together the magnets, the greater the attraction or repulsion Magnetic Fields and Forces • Magnetic lines of force – Causes iron filings to form a pattern when placed on paper over a magnet • Magnetic field is the space in which a magnetic force exists Magnetic Theories • Molecular theory of magnetism – A magnetic substance is composed of molecular magnets • Electron theory of magnetism – Magnetic field is produced by more electrons spinning in one direction than the other direction Magnetic Materials • Permanent magnets retain their magnetic properties for many years • Temporary magnets lose magnetism once removed from magnetizing influence • Magnetic Shields – Magnetism passes through some materials more readily than others – Surrounding sensitive instruments with iron provides a magnetic path around them Electromagnetism • An electric current produces a magnetic field • Left-hand rule for a single conductor – Can determine direction of lines of force by grasping conductor with left hand with thumb in the direction of the current flow • Fingers will point in the direction of lines of force Electromagnetism (cont’d.) • Magnetic forces – If two magnetic forces are within reach of each other, their fields will react according to laws of attraction and repulsion Electromagnetism (cont’d.) • Left-hand rule for a coil – Determine magnetic polarity of a coil by grasping coil in left hand with fingers pointing in direction of current flow • Extended thumb will point in direction of lines of force Electromagnetism (cont’d.) • Electromagnets – Consist of a coil – Magnetic force can be turned on and off with the current – Field strength can vary – Inserting iron core into coil will increase field strength – Solenoid is electromagnet with movable core Electromagnetism (cont’d.) • Application of electromagnets – Used for lifting large amounts of iron and steel – Magnetic clutches to connect and disconnect the load from the driving source – Solenoids used to open and close valves – Other examples: relays, circuit breakers and door chimes Magnetic Circuits and Measurements • Electromagnet strength depends on its ability to conduct magnetism • Amount of magnetic flux is measured in maxwells (Mx) • Magnetomotive force (mmf) – Influences amount of flux produced in an electromagnet; measured in gilberts (Gb) Magnetic Circuits and Measurements (cont’d.) • Reluctance – Opposition to the magnetic flux • Permeability – Ability of a material to conduct lines of force • Electromagnet strength can be varied by changing the mmf – Increasing or decreasing coil current Magnetic Circuits and Measurements (cont’d.) • Other factors to consider when designing magnets – Retentivity – Residual magnetism – Magnetic saturation – These depend on type, size and length of the core Magnetic Circuits and Measurements (cont’d.) • Flux depends on mmf and reluctance and can be calculated using the following equation Solenoids • Type of electromagnet with moveable core called armature – Made by winding magnet wire on hollow fiber or plastic form – When coil is energized, the current’s magnetic field pulls core into the coil • Commonly used in doorbells (door chimes) Summary • Magnets can be classified into natural, artificial or electromagnets • Laws of magnetic attraction and repulsion govern behavior • Magnetic field is the space in which magnetic force exists • There are two main theories of magnetism: molecular and electron Summary (cont’d.) • An electric current produces a magnetic field; this fact is used in electromagnets • Applications of electromagnets include lifting large metal objects, clutches, and opening and closing valves • A solenoid is a type of electromagnet that has a movable core in its coil • Iron in the core increases the field strength