objectives

1. objectives • Understand the motion of charges relative to each other produces a magnetic force. • For given situations, predict whether magnets will repel or attract each other. • Describe the magnetic field around a permanent magnet. • Describe the orientation of Earth’s magnetic field. • Understand the relative motion between a conductor and a magnetic field may produce a potential difference in the conductor.

2. Magnetism • Magnetism is a FIELD FORCE of attraction or repulsion in and around a material • Acts over a DISTANCE without CONTACT • Magnets produce FIELDS around them that influence some types of metal • IRON, NICKEL, and COBALT • Closely related to ELECTRICITY

3. What is a magnet? • A magnet is any piece of material that has the property of attracting iron (or steel). • Magnetism may be naturally present in a material or the material may be artificially magnetized by various methods. • Magnets may be permanent or temporary. • Materials which can be magnetized are called ferromagnetic materials.

4. MAGNETIC PROPERTIES • Attracts iron containing objects. • It has two ends called poles: north pole and south pole. North pole points to North of Earth and south pole points to South of Earth. • No matter how many times a magnet is broken, each piece always has a north pole and a south pole. • Like poles repel each other, unlike poles attract each other.

5. What is a magnetic field and how is it created? • The cause of magnetism is from a property of the atoms. • Atoms have a positively charged center called the nucleus. A nucleus contains one or more protons and neutrons and is orbited by one or more negatively charged particles called electrons. • The electrons spin as they travel around the nucleus (which contain protons and neutrons) much like the earth spins as it orbits the sun. • As the electrons spin and orbit the nucleus, they produce a magnetic field. Moving electrons creates magnetic fields.

6. Not all atoms have magnetic fields • All the electrons produce a magnetic field as they spin and orbit the nucleus; however, in some atoms, two electrons spinning and orbiting in opposite directions pair up and the net magnetic field of the atom is zero. • Materials with one or more unpaired electrons are magnetic. Materials with a small attraction to a magnet are called paramagnetic materials, and those with a strong attraction are called ferromagnetic materials. Iron, cobalt, and nickel are examples of ferromagnetic materials. • Not all the fields are aligned, but when canceling spins are accounted for, a net magnetic field remains.

7. MAGNETIC DOMAIN • A magnetic domain is a region in which the magnetic fields of atoms are grouped together and aligned. • You can think of magnetic domains as miniature magnets within a material. • In an unmagnetized object, all the magnetic domains are pointing in different directions. When the metal became magnetized, all like magnetic poles lined up and pointed in the same direction.

8. When we rubbed the magnet over the surface of the metal, some of the magnetic domains aligned and the metal became partially magnetized. The more we rub the magnet over the metal, the stronger the magnetic domains became aligned and the metal became a stronger magnet. You can turn a paper clip into a magnet this way.

9. THE TWO ENDS OF A MAGNET • One end of any bar magnet will always want to point north if it is freely suspended. This is called the north-seeking pole of the magnet, or simply the north pole. The opposite end is called the south pole. • The needle of a compass is itself a magnet, and thus the north pole of the magnet always points north. • However, when you bring the compass near a strong bar magnet, the needle of the compass (north pole) points in the direction of the south pole of the bar magnet. • We can conclude that the north end of a compass is attracted to the south end of a magnet. • ..\..\labs\phet labs\magnets-and-electromagnets_en.jar

10. Since the north seeking pole of the compass needle is always attracted to the north, then the earth must be like a huge magnet with a magnetic pole at each end. • This can be a little confusing since it would seem that what we call the North Pole of the Earth is actually itsmagnetically south pole. • This is exactly the case but magnetic north is slightly different from the north axis of rotation of the earth. Scientists believe that the movement of the Earth's liquid iron core and other things are responsible for the magnetic field around the earth.

11. Magnetic fields • The region where magnetic force exists around a magnet is called its ___________________. • Magnetic field allows magnets interact without touching. Magnetic force is a non-contact force. • A magnetic field exerts a force on any moving charge and can be measured and detected by this effect. magnetic field

12. Magnetic Field Lines Magnetic fields can be represented using FIELD LINES drawn around magnetic objects Magnetic fields lines travel from NORTH poles to SOUTH polesOUTSIDE magnets Field lines DO NOT CROSS one another

13. Magnetic flux lines (field lines) • Magnetic flux lines never intersect and are closed – north to south • The direction is defined as the direction the N-pole of a compass would point in the field. • Magnetic field is strongest where the lines are closest.

14. Conventions for the direction of a magnetic field

15. Magnetic field strength • The number of magnetic lines of flux per unit area passing through a plane perpendicular to the direction of the lines is called the magnetic field strength, B, or flux density. • Magnetic field strength is a vector quantity • Weber, or Wb, is a derived SI unit for measuring the number of lines of flux • Tesla, T, is the derived SI unit of flux density or magnetic field strength 1 T = 1 Wb/1 m2

16. example • In the diagram, a steel paper clip is attached to a string, which is attached to a table. The clip remains suspended beneath a magnet. As the magnet is lifted the paper clip begins to fall as a result of • an increase in the potential energy of the clip • an increase in the gravitational field strength near the magnet • a decrease in the magnetic properties of the clip • a decrease in the magnetic field strength near the clip

17. example • The diagram shows the magnetic field that results when a piece of iron is placed between unlike magnetic poles. At which point is the magnetic field strength greatest? • A • B • C • D

18. example • The diagram below shows a bar magnet. Which arrow best represents the direction of the needle of a compass placed at point A? • ↑ • ↓ • → • ←

19. example • Which diagram best represents the magnetic field between two magnetic north poles? B A C D

20. example • The diagram below represents the magnetic field near point P. If a compass is placed at point P in the same plane as the magnetic field, which arrow represents the direction the north end of the compass needle will point? S A B C D N

21. ELECTROMAGNETS • Moving electrons produce magnetic field. • An electric current are moving charges. An electric current can cause a magnetic field around it just like a magnet causes a magnetic field. • When the compass is put near the electrical wire with current flowing through it, the compass needle pointed in the direction of the current's magnetic field.

22. Moving CHARGESmakes aMAGNETIC FIELD Magnetic field around a current carrying wire

23. Magnetic field of a current carrying wire • Two current carrying wires exert a force on each other through their magnetic fields. • The magnetic field produced by one wire exerts a force on the charges in the other wire. opposite direction of current, wires repel each other Same direction of current, wires attracts each other

24. A conductive wire with a current flowing through it creates a magnetic field. However, the magnetic field of one wire is small and does not have much strength, if we take a wire and coil it several times to form a long coiled piece of electrical wire, we would have a magnetic field much bigger and stronger when we turn on the current. • An iron bar placed through the center of the coiled wire would become a temporary magnet, called an electromagnet, as long as the electric current is flowing through the wire. • ..\..\labs\phet labs\magnets-and-electromagnets_en.jar

25. Electromagnetic induction • ..\..\labs\phet labs\generator_en.jar • http://www.vjc.moe.edu.sg/academics/dept/physics_dept/applet/fara/faraday_demo.htm • Just as an electrical current induces a magnetic field, When a magnet is moved in and out of coils of wire or when an electrical wire cuts across magnetic lines of force, a magnetic force acts on the electrons in the conductor causing a difference of the amount of negative charge at each end of the conductor and producing an induced potential difference. As a result, current is generated. This process is called electromagnetic induction. • It does not matter if the magnet is moved or if the coils of wire are moved. The important thing is that there is motion within the magnetic field, and that the magnetic lines of force are cut.

26. Generator • The electromagnetic induction is the principle by which electric generators can make electricity. Through the use of magnets, a generator can convert mechanical energy to electrical energy. • Inside a generator is a magnet, some electrical wire, and a source of mechanical energy. The mechanical energy moves the wire into the magnetic field of the magnet so that the wire cuts through the magnetic lines of force. As a result, electric current is produced. • ..\..\RealPlayer Downloads\How generator works by Khurram Tanvir.flv • ..\..\RealPlayer Downloads\Magnetism- Motors and Generators.flv

27. example • An accelerating particle that does not generate electromagnetic waves could be • a proton • a neutron • an electron • an alpha particle

28. Example • Which procedure will produce the greatest induced potential difference in the conductor? • holding the conductor stationary between the poles • moving the conductor out of the screen • moving the conductor toward the right side of the screen • moving the conductor toward the N-pole

29. Electromagnetic radiation • Electromagnetic radiation is the changing electric and magnetic fields that radiate outward into the surrounding space in the form of waves. It is also called electromagnetic waves. • Electromagnetic radiation is produced by oscillating or accelerating electric charges. • http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html