Physics
Magnets
Magnets are objects that produce a magnetic field, attracting or repelling other magnets or magnetic materials. They have two poles, north and south, and can be used in various applications such as electric motors, generators, and compasses. The behavior of magnets is described by the laws of magnetism, which are fundamental to understanding electromagnetic phenomena.
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12 Key excerpts on "Magnets"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Library Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 4 Magnetic Field A magnetic field is a field of force produced by a magnetic object or particle, or by a changing electric field an d is detected by the force it exerts on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The complex mathematics underlying the magnetic field of an object is usually illustrated using magnetic field lines. These lines are strictly a mathematical concept and do not exist physically. Nonetheless, certain physical phenomena, such as the alignment of iron filings in a magneti c field, produces lines in a similar pattern to the imaginary magnetic field lines of the object. Magnets exert forces and torques on each other through the magnetic fields they create. Electric currents and moving electric charges produce magnetic fields. Even the magnetic field of a magnetic material can be modeled as being due to moving electric charges. Magnetic fields also exert forces on moving electric charges. The magnetic fields within and due to magnetic materials can be quite complicated and is d escribed using two separate fields which can be both called a magnetic field : a magnetic B field and a magnetic H field. Energy is needed to create a magnetic field. This energy can be reclaimed when the field is destroyed and, therefore, can be considered as being stored in the magnetic field. The value of this energy depends on the values of both B and H . An electric field is a field created by an electric charge and such fields are intimately related to magnetic fields; a changing magnetic field genera tes an electric field and a changing electric field produces a magnetic field. The full relationship between the electric and magnetic fields, and the currents and charges that create them, is described by the set of Maxwell's equations.
eBook - PDF- Paul Peter Urone, Roger Hinrichs(Authors)
- 2012(Publication Date)
- Openstax(Publisher)
Magnetism is used to explain atomic energy levels, cosmic rays, and charged particles trapped in the Van Allen belts. Once again, we will find all these disparate phenomena are linked by a small number of underlying physical principles. Figure 22.2 Engineering of technology like iPods would not be possible without a deep understanding magnetism. (credit: Jesse! S?, Flickr) 22.1 Magnets Figure 22.3 Magnets come in various shapes, sizes, and strengths. All have both a north pole and a south pole. There is never an isolated pole (a monopole). All Magnets attract iron, such as that in a refrigerator door. However, Magnets may attract or repel other Magnets. Experimentation shows that all Magnets have two poles. If freely suspended, one pole will point toward the north. The two poles 854 Chapter 22 | Magnetism This OpenStax book is available for free at http://cnx.org/content/col11406/1.9 are thus named the north magnetic pole and the south magnetic pole (or more properly, north-seeking and south-seeking poles, for the attractions in those directions). Universal Characteristics of Magnets and Magnetic Poles It is a universal characteristic of all Magnets that like poles repel and unlike poles attract. (Note the similarity with electrostatics: unlike charges attract and like charges repel.) Further experimentation shows that it is impossible to separate north and south poles in the manner that + and − charges can be separated. Figure 22.4 One end of a bar magnet is suspended from a thread that points toward north. The magnet’s two poles are labeled N and S for north- seeking and south-seeking poles, respectively. Misconception Alert: Earth’s Geographic North Pole Hides an S The Earth acts like a very large bar magnet with its south-seeking pole near the geographic North Pole.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 3 Magnetic Field A magnetic field is a field of force produced by a magnetic object or particle, or by a changing electrical field and is detected by the force it exerts on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The complex mathematics underlying the magnetic field of an object is usually illustrated using magnetic field lines. These lines are strictly a mathematical concept and do not exist physically. Nonetheless, certain physical phenomena, such as the alignment of iron filings in a magnetic field, produces lines in a similar pattern to the imaginary magnetic field lines of the object. Magnets exert forces and torques on each other through the magnetic fields they create. Electric currents and moving electrical charges produce magnetic fields. Even the ma-gnetic field of a magnetic material can be modeled as being due to moving electrical charges. Magnetic fields also exert forces on moving electrical charges. The magnetic fields within and due to magnetic materials can be quite complicated and is described using two separate fields which can be both called a magnetic field : a magnetic B field and a magnetic H field. Energy is needed to create a magnetic field. This energy can be reclaimed when the field is destroyed and, therefore, can be considered as being stored in the magnetic field. The value of this energy depends on the values of both B and H . An electric field is a field created by an electrical charge and such fields are intimately related to magnetic fields; a changing magnetic field generates an electric field and a changing electric field produces a magnetic field. The full relationship between the electric and magnetic fields, and the currents and charges that create them, is described by the set of Maxwell's equations.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler, Heath Jones, Matthew Collins, John Daicopoulos, Boris Blankleider(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
CHAPTER 21 Magnetic forces and magnetic fields LEARNING OBJECTIVES After reading this module, you should be able to: 21.1 define magnetic field 21.2 calculate the magnetic force on a moving charge in a magnetic field 21.3 analyse the motion of a charged particle in a magnetic field 21.4 describe how the masses of ions are determined using a mass spectrometer 21.5 calculate the magnetic force on a current in a magnetic field 21.6 calculate the torque on a current‐carrying coil 21.7 calculate magnetic fields produced by currents 21.8 apply Ampère’s law to calculate the magnetic field due to a steady current 21.9 describe magnetic materials. INTRODUCTION This beautiful display of light in the sky is known as the northern lights (aurora borealis). It occurs when charged particles, streaming from the sun, become trapped by the earth’s magnetic field. The particles collide with molecules in the upper atmosphere, and the result is the production of light. Magnetic forces and magnetic fields are the subjects of this chapter. Source: Suranga Weeratunga / 123RF.com 21.1 Magnetic fields LEARNING OBJECTIVE 21.1 Define magnetic field. FIGURE 21.1 The needle of a compass is a permanent magnet that has a north magnetic pole (N) at one end and a south magnetic pole (S) at the other. S N Permanent Magnets have long been used in navigational compasses. As figure 21.1 illus- trates, the compass needle is a permanent magnet supported so it can rotate freely in a plane. When the compass is placed on a horizontal surface, the needle rotates until one end points approximately to the north. The end of the needle that points north is labelled the north magnetic pole; the opposite end is the south magnetic pole. Magnets can exert forces on each other. Figure 21.2 shows that the magnetic forces between north and south poles have the property that like poles repel each other, and unlike poles attract. This behaviour is similar to that of like and unlike electric charges.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
LEARNING OBJECTIVES After reading this module, you should be able to... 21.1 Define magnetic field. 21.2 Calculate the magnetic force on a moving charge in a magnetic field. 21.3 Analyze the motion of a charged particle in a magnetic field. 21.4 Describe how the masses of ions are determined using a mass spectrometer. 21.5 Calculate the magnetic force on a current in a magnetic field. 21.6 Calculate the torque on a current- carrying coil. 21.7 Calculate magnetic fields produced by currents. 21.8 Apply Ampère’s law to calculate the magnetic field due to a steady current. 21.9 Describe magnetic materials. Suranga Weeratunga/123RF. com CHAPTER 21 Magnetic Forces and Magnetic Fields This beautiful display of light in the sky is known as the northern lights (aurora borealis). It occurs when charged particles, streaming from the sun, become trapped by the earth’s magnetic field. The particles collide with molecules in the upper atmosphere, and the result is the production of light. Magnetic forces and magnetic fields are the subjects of this chapter. 21.1 Magnetic Fields Permanent Magnets have long been used in navigational compasses. As Figure 21.1 illustrates, the compass needle is a permanent magnet supported so it can rotate freely in a plane. When the compass is placed on a horizontal surface, the needle rotates until one end points approximately to the north. The end of the needle that points north is labeled the north magnetic pole; the opposite end is the south magnetic pole. Magnets can exert forces on each other. Figure 21.2 shows that the magnetic forces between north and south poles have the property that like poles repel each other, and unlike poles attract. This behavior is similar to that of like and unlike electric charges. However, there is a significant difference between magnetic poles and electric charges. It is possible to separate positive from negative electric charges and produce isolated charges of either kind.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
The more physically correct description of magnetism involves atomic sized loops of current distributed throughout the magnet. Non-uniform magnetic field causes like poles to repel and opposites to attract The force between two small Magnets is quite complicated and depends on the strength and orientation of both Magnets and the distance and direction of the Magnets relative to each other. The force is particularly sensitive to rotations of the Magnets due to magnetic torque. The force on each magnet depends on its magnetic moment and the magnetic field B of the other. To understand the force between Magnets and to generalize it to other cases, it is useful to examine the magnetic charge model given above (with the caveats given above as well). In this model, the H-field of the first magnet pushes and pulls on the magnetic charges near both poles of the second magnet. If the H-field due to the first magnet is the same at both poles of the second magnet then there is no net force on that magnet since the force is opposite for opposite poles. The magnetic field is not the same, though; the magnetic field is significantly stronger near the poles of a magnet. In this nonuniform magnetic field, each pole sees a different field and is subject to a different force. This difference in the two forces moves the magnet in the direction of increasing magnetic field and may also cause a net torque. This is a specific example of a general rule that Magnets are attracted (or repulsed depending on the orientation of the magnet) into regions of higher magnetic field. Any non-uniform magnetic field whether caused by permanent Magnets or by electric currents will exert a force on a small magnet in this way.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
The more physically correct description of magnetism involves atomic sized loops of current distributed throughout the magnet. Non-uniform magnetic field causes like poles to repel and opposites to attract The force between two small Magnets is quite complicated and depends on the strength and orientation of both Magnets and the distance and direction of the Magnets relative to each other. The force is particularly sensitive to rotations of the Magnets due to magnetic torque. The force on each magnet depends on its magnetic moment and the magnetic field B of the other. To understand the force between Magnets and to generalize it to other cases, it is useful to examine the magnetic charge model given above (with the caveats given above as well). In this model, the H-field of the first magnet pushes and pulls on the magnetic charges near both poles of the second magnet. If the H-field due to the first magnet is the same at both poles of the second magnet then there is no net force on that magnet since the force is opposite for opposite poles. The magnetic field is not the same, though; the magnetic field is significantly stronger near the poles of a magnet. In this nonuniform magnetic field, each pole sees a different field and is subject to a different force. This difference in the two forces moves the magnet in the direction of increasing magnetic field and may also cause a net torque. This is a specific example of a general rule that Magnets are attracted (or repulsed depending on the orientation of the magnet) into regions of higher magnetic field. Any non-uniform magnetic field whether caused by permanent Magnets or by electric currents will exert a force on a small magnet in this way. Mathematically, the force on a small magnet having a magnetic moment m due to a magnetic field B is:
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
(b) Bars 1 and 3 will either be attracted to or repelled from each other, while bars 2 and 3 will always be attracted to each other. (c) Bars 1 and 3 will always be repelled from each other, while bars 2 and 3 will either be attracted to or repelled from each other. (d) Bars 1 and 3 will always be attracted to each other, while bars 2 and 3 will either be attracted to or repelled from each other. Guideway Rail Arm Guideway (b) (a) Levitation electromagnet S N S N N S N Figure 21.41 (a) The Transrapid maglev (a German train) has achieved speeds of 110 m/s (250 mph). The levitation electroMagnets are drawn up toward the rail in the guideway, levitating the train. (b) The magnetic propulsion system. CONCEPT SUMMARY 21.1 Magnetic Fields A magnet has a north pole and a south pole. The north pole is the end that points toward the north magnetic pole of the earth when the magnet is freely suspended. Like magnetic poles repel each other, and unlike poles attract each other. A magnetic field exists in the space around a magnet. The magnetic field is a vector whose direc- tion at any point is the direction indicated by the north pole of a small compass needle placed at that point. As an aid in visualizing the magnetic field, magnetic field lines are drawn in the vicinity of a magnet. The lines appear to originate from the north pole and end on the south pole. The magnetic field at any point in space is tangent to the magnetic field line at the point. Furthermore, the strength of the magnetic field is proportional to the number of lines per unit area that passes through a surface oriented perpendicular to the lines. 21.2 The Force That a Magnetic Field Exerts on a Moving Charge The direction of the magnetic force acting on a charge moving with a velocity v B in a magnetic field B B is perpendicular to both v B and B B . For a positive charge the direction can be determined with the aid of Right-Hand Rule No.
eBook - PDF- Richard L. Myers(Author)
- 2005(Publication Date)
- Greenwood(Publisher)
ElectroMagnets consist of a piece of iron inside a coil of wire through which current runs. When the current is on, the magnetic domains in the iron core of the electromagnet align with the magnetic field produced, creating a strong magnetic force. The magnetic field in space can be deter- mined using a compass. This can be read- ily seen when a compass is brought near a wire conducting a current. If the compass is moved to various points around the wire, a field map can be made. The field map shows the direction and strength of the magnetic force at various points in space. The direc- tion of the magnetic field at a point in space is defined as the direction the north pole of a compass points at that location. A mag- net always has two poles: a north pole, N, and south pole, S. Magnetic field lines exit at the north pole and enter at the south pole. Where the magnetic lines of force are close together, the magnetic field is stronger. The magnetic fields surrounding two types of Magnets are illustrated in Figure 14.2. Another aspect related to the magnetic field that will be important in discussing electromagnetic induction is magnetic flux. Magnetic flux is the number of field lines that pass through an area in space. A magnetic field with strength B whose field lines passes through an area, A, at right angles has a mag- netic flux of BA (Figure 14.3). As the angle between field lines and the plane of the area changes, the magnetic flux will decrease. In this case, the magnetic flux is given by the equation BA cos , where (/> is the angle Magnetism and Electromagnetism 259 between the normal to the plane of the opening and magnetic field lines. When the area and field lines are parallel (4> is 90°) the magnetic flux is zero. The units for mag- neticfluxare teslas-meters 2 (T-m 2 ), and 1 T-m 2 is called a weber and abbreviated Wb. The unit is named after the German physicist Wilhelm Weber (1804-1891).
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
Like the electric field, the magnetic field has both a magnitude and a direction. We postpone a dis- cussion of the magnitude until Section 21.2, concentrating our attention here on the Chapter | 21 LEARNING OBJECTIVES After reading this module, you should be able to... 21.1 | Define magnetic field. 21.2 | Calculate the magnetic force on a moving charge in a magnetic field. 21.3 | Analyze the motion of a charged particle in a magnetic field. 21.4 | Describe how the masses of ions are determined using a mass spectrometer. 21.5 | Calculate the magnetic force on a current in a magnetic field. 21.6 | Calculate the torque on a current- carrying coil. 21.7 | Calculate magnetic fields produced by currents. 21.8 | Apply Ampère’s law to calculate the magnetic field due to a steady current. 21.9 | Describe magnetic materials. 518 Suranga Weeratunga/123RF. com S N Figure 21.1 The needle of a compass is a permanent magnet that has a north magnetic pole (N) at one end and a south magnetic pole (S) at the other. Like poles repel (a) Unlike poles attract (b) Figure 21.2 Bar Magnets have a north magnetic pole at one end and a south magnetic pole at the other end. (a) Like poles repel each other, and (b) unlike poles attract. 21.1 | Magnetic Fields 519 direction. The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle placed at that point. In Fig- ure 21.3 the compass needle is symbolized by an arrow, with the head of the arrow representing the north pole. The drawing shows how compasses can be used to map out the magnetic field in the space around a bar magnet. Since like poles repel and unlike poles attract, the needle of each compass becomes aligned relative to the magnet in the manner shown in the picture. The compass needles provide a visual picture of the magnetic field that the bar magnet creates. To help visualize the electric field, we introduced electric field lines in Section 18.7.
- Stephen Herman(Author)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
1 Objectives After studying this unit, you should be able to: • Discuss the properties of permanent Magnets. • Discuss the difference between the axis poles and the magnetic poles of the earth. • Discuss the operation of electroMagnets. • Determine the polarity of an electromagnet when the direction of the current is known. • Discuss the different systems used to measure magnetism. • Define terms used to describe magnetism and magnetic quantities. Magnetism is one of the most important phenomena in the study of electricity. It is the force used to produce most of the electrical power in the world. The force of magnetism has been known for over 2000 years. It was first discovered by the Greeks when they noticed that a certain type of stone was attracted to iron. This stone was first found in Magnesia in Asia Minor and was named magnetite. In the Dark Ages, the strange pow -ers of the magnet were believed to be caused by evil spirits or the devil. THE EARTH IS A MAGNET The first compass was invented when it was noticed that a piece of magnetite, a type of stone that is attracted to iron, placed on a piece of wood floating in water always aligned itself north and south (Figure 1-1) . Because they are always able to align themselves north and south, nat -ural Magnets became known as “leading stones” or lodestones . The reason that the lodestone aligned itself north and south is because the earth itself contains magnetic poles. Figure 1-2 illustrates the position of the true North and South poles, or the axis, of the earth and the posi -tion of the magnetic poles. Notice that what is considered as magnetic north is not located at the true North Pole of the earth. This is the rea -son that navigators must distinguish between true north and magnetic north. The angular difference between the two is known as the angle of UNIT 1 Magnetism Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Magnets can exert forces on each other. Figure 21.2 shows that the magnetic forces between north and south poles have the property that like poles repel each other, and unlike poles attract. This behavior is similar to that of like and unlike electric charges. How- ever, there is a significant difference between magnetic poles and electric charges. It is possible to separate positive from negative electric charges and produce isolated charges of either kind. In contrast, no one has Solar Dynamics Observatory, NASA Earth 654 CHAPTER 21 Magnetic Forces and Magnetic Fields S N FIGURE 21.1 The needle of a compass is a permanent magnet that has a north magnetic pole (N) at one end and a south magnetic pole (S) at the other. Like poles repel (a) Unlike poles attract (b) FIGURE 21.2 Bar Magnets have a north magnetic pole at one end and a south magnetic pole at the other end. (a) Like poles repel each other, and (b) unlike poles attract. Magnetic field lines (a) S N (c) S N FIGURE 21.4 (a) The magnetic field lines and (b) the pattern of iron filings (black, curved regions) in the vicinity of a bar magnet. (c) The magnetic field lines in the gap of a horseshoe magnet. (b) S N Cordelia Molloy/Science Source found a magnetic monopole (an isolated north or south pole). Any attempt to separate north and south poles by cutting a bar magnet in half fails, because each piece becomes a smaller magnet with its own north and south poles. Surrounding a magnet, there is a magnetic field. The magnetic field is analogous to the electric field that exists in the space around electric charges. Like the electric field, the magnetic field has both a magnitude and a direction. We postpone a discussion of the magnitude until Section 21.2, concentrating our attention here on the direction. The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle placed at that point.
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