Magnetism Objectives 1. Explain how a magnet exerts a force on an object. 2. Describe the properties of temporary and permanent magnets. 3. Explain why some materials are magnetic and others are not. 4. Model magnetic behavior using magnetic domains. 5. Describe the magnetic field produced by an electric current. 6. Explain how an electromagnet produces a magnetic field. 7. Describe how electromagnets are used. 8. Explain how an electric motor operates. 9. Define electromagnetic induction. 10. Describe how a generator produces an electric current.

Magnetism Objectives Distinguish between alternating and direct current. Explain how a transformer can change the voltage of an alternating current.

Magnetism & Its Uses magnetism: properties and interactions of magnets Magnets can exert forces on each other: -can attract (different polarity-N & S) -can repel (same polarity-S & S or N & N) The strength of the force depends on the distance between the magnets -increases as the magnets get closer together -magnetic force creates a magnetic field around the magnet

magnetic field: force exerted on other objects made of magnetic material or other magnets -represented by lines of force (magnetic field lines) -has direction, as shown by the arrows, which always are drawn from the north end to the south end. Notice that at the ends of the bar, the lines are close together -these regions are called magnetic poles

magnetic poles: regions of a magnet where the magnetic force exerted by the magnet is the strongest -all magnets have a north and south pole ~bar magnets has their poles at opposite ends ~horseshoe magnets have their poles on the same side ~a round magnet has its poles on opposite sides (top and bottom)

Magnets can interact with one another when they come into close contact with one another. -see p 226, Figure 4 -unlike poles attract and like poles repel Magnetic fields can be detected with a compass: -the needle (small bar magnet) will rotate until it lines up with the magnetic field lines -the needle always points from magnetic north to magnetic south

Earth also has magnetic poles: -acts like a huge bar magnet and is surrounded by a magnetic field that extends into space -north pole of a magnet points toward the geographic north pole, which is actually the magnetic south pole ~the magnetic south pole is about 1500 km from the geographic north pole and is slowly changing as time passes

-no one is sure what produces Earth’s magnetic field, but some believe that the circulation of molten iron and nickel in the outer core is the cause Not all materials are attracted to magnets, so what makes a material magnetic? -in most materials, the electron’s charge in the atoms cancel out (even though each electron has magnetic properties) -iron, cobalt and nickel have magnetic properties because the magnetic properties of the electrons do not cancel out. ~not all objects made from them have magnetic properties

Even if a material made from iron, cobalt or nickel is not permanently magnetic, you can sometimes temporarily magnetize it. -when the magnetic field produced by atoms comes in contact with other atoms, the groups of atoms can align their magnetic poles so that they all point in the same direction magnetic domain: groups of atoms with aligned magnetic poles -each domain contains an enormous number of atoms -since all atoms are aligned, the domain itself can behave like a magnet with north and south poles

How do we line up these domains to create a magnet? -only when the domains all line up in the same direction will it create a magnet -if the domains do not line up in the same direction, it will not create a magnet ~notice in the nail the domains do not line up (in Fig 7, p 229)

If you place a magnet against the same nail, the atoms in the domains orient themselves in the direction of the nearby magnetic field. -this forms a temporary magnet -as soon as the external magnetic field (from the magnet) is removed, the nail is no longer magnetized and the magnetic domains will unalign

Permanent magnets can be made by placing a magnetic material in a strong magnetic field, forcing a large number of magnetic domains to line up. -the magnetic fields all add together and create a strong magnetic field inside the material, which prevents the constant motion of the atoms from bumping the domains out of alignment. However, they can lose their magnetic properties if they are heated, which causes the atoms to move faster, knocking the domains out of alignment.

Magnetism Review 1 (Write Q and A) What does the force between 2 magnets depend on? How do magnetic poles interact with each other? Which way would a compass needle point if Earth’s magnetic poles switched places? How does a compass needle move when placed in a magnetic field? Why are certain materials magnetic and others are not? How does heating a bar magnet change its magnetic field?

Electricity and Magnetism When a compass was placed near an electric current, the current affected the direction of the compass needle. -it was determined that an electric current produces a magnetic field around the wire -the magnetic field around the wire changes direction as the direction of current changes.

As the strength of the current increases, the strength of the magnetic field increases. -as you move farther away from the wire, the magnetic field decreases electromagnet: temporary magnet made by wrapping a wire coil carrying a current around an iron core -a magnetic field is only present when current is flowing through the wire coil -adding more turns to the coil or increasing the current increases the strength of the magnetic field

The magnetic fields around the different parts of the wire loop combine to form the field inside the loop. Electromagnets convert electrical energy into mechanical energy to do work -stereo speakers and electric motors

Electromagnets and Sound How does musical information stored on a CD become sound you can hear? -sound is produced by a loudspeaker that contains an electromagnet connected to a flexible speaker cone that is usually made from paper, plastic, or metal. -the mechanical energy vibrates the speaker cone to produce sound

Galvonometers How does a change in the amount of gasoline in a tank or the water temperature in the engine make a needle move in a gauge on the dashboard? -these gauges are galvanometers, which are devices that use an electromagnet to measure electric current. -changing the current in the electromagnet causes the needle to rotate to different positions on the scale.

Electric Motors electric motor: device that changes electrical energy into mechanical energy -contains an electromagnet that is free to rotate between the poles of a permanent, fixed magnet. -a coil is connected to a source of electric current, such as a battery -also contains ~brushes: conducting pads connected to the battery ~commutators: conducting metal ring that is split and rotates with the coil

A magnetic field is produced in the coil when a current flows through the electromagnet -changing the direction of this current causes the coil to rotate -the rotation speed of the motor can be controlled by varying the amount of current ~turns faster when more current flows because the electromagnets magnetic field becomes stronger, increasing the force between the coil and magnet

Magnetism Review 2 (Q and A) 1. What does the force between 2 magnets depend on? 2. How do magnetic poles interact with each other? 3. How does a magnetic field change when the current changes direction? 4. What is an electromagnet? 5. How does a stereo speaker use an electromagnet to produce sound? 6. How can you change the strength of a magnetic field produced by an electromagnet? 7. List 3 devices that contain electric motors. 8. What does an electric motor do?

Producing Electric Current electromagnetic induction: production of an electric current by 1. moving a wire through a magnetic field 2. moving a magnet through a wire loop Most of the electrical energy you use comes from a generator, a device the produces electric current by using electromagnetic induction to rotate a coil of wire in a magnetic field a. wire coil is wrapped around an iron core and placed between the poles of a permanent magnet

generator: b. the coil is rotated by an external source of mechanical energy c. as the coil turns within the magnetic field of the permanent magnet, an electric current flows through the coil d. the direction of current in the generator’s coil changes twice for each revolution

What are uses of electric generators? 1. alternator in your car -provides electrical energy to operate lights and other accessories -spark plugs in your car use this electrical energy to ignite the fuel in the cylinders 2. generating electricity -uses a fixed coil and permanent magnet that rotates, producing electric current in the fixed coil and mechanical energy is used to rotate the magnet, which is connected to a turbine

turbine: large wheel that rotates when pushed by water, wind or steam Because power outages sometimes occur, some electrical devices use batteries as a backup source of electrical energy. -however, the current produced by a battery is different than the current from an electric generator. ~batteries produce direct current, current that flows in one direction ~generators produce alternating current, which reverses the flow of current in a regular pattern

In the US, generators produce AC current at a frequency of 60 cycles/sec (60Hz) -changes direction 120 times each second A device that uses batteries as backup usually require direct current, however when plugged up to an outlet, it would be receiving alternating current -there are electronic components in the device that converts the alternating current to direct current and reduces the voltage of the alternating current

Sometimes we need to increase or decrease the voltage of an alternating current: -transformer: device that increases or decreases the voltage of alternating current ~step-up transformer: increases voltage by putting more turns of wire on the secondary coil ~step-down transformer: decreases voltage by placing fewer turns of wire on the secondary coil

Magnetism Review 3 (Q and A) 1. What is the difference between direct current and alternating current? 2. What is a generator’s purpose? 3. What is a problem with using batteries as backup power for electronic devices? 4.Tell the difference between the two types of transformers. 5. How is a generator different from an electric motor?

Final Review p 250-251 # 1-14, 18-20 P 252 # 1-6