Presentation on theme: "Magnetism Objectives 1. Explain how a magnet exerts a force on an object. 2. Describe the properties of temporary and permanent magnets. 3. Explain."— Presentation transcript:
1 Magnetism Objectives1. Explain how a magnet exerts a force on an object.2. Describe the properties of temporary and permanentmagnets.3. Explain why some materials are magnetic and othersare not.4. Model magnetic behavior using magnetic domains.5. Describe the magnetic field produced by an electriccurrent.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.
2 Magnetism ObjectivesDistinguish between alternating and direct current.Explain how a transformer can change the voltage of an alternating current.
3 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 themagnet
4 magnetic field: force exerted on other objects made of magnetic material or other magnets -represented by lines offorce (magnetic field lines)-has direction, as shown bythe arrows, which always aredrawn from the north end to thesouth end.Notice that at the ends of the bar, the lines are close together-these regions are called magnetic poles
5 magnetic poles: regions of a magnet where the magnetic force exerted by the magnet is the strongest -all magnets have a north andsouth pole~bar magnets has their polesat opposite ends~horseshoe magnets havetheir poles on the same side~a round magnet has its poles onopposite sides (top and bottom)
6 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 polesrepelMagnetic fields can be detected witha compass:-the needle (small bar magnet) willrotate until it lines up with the magneticfield lines-the needle always points from magnetic north tomagnetic south
7 Earth also has magnetic poles: -acts like a huge bar magnetand is surrounded by amagnetic field that extendsinto space-north pole of a magnet pointstoward the geographic northpole, which is actually themagnetic south pole~the magnetic south pole is about 1500 km fromthe geographic north pole and is slowlychanging as time passes
8 -no one is sure what produces Earth’s magnetic field, but some believe that the circulation of molten ironand nickel in the outer core is the causeNot all materials are attracted to magnets, so what makes a material magnetic?-in most materials, the electron’s charge in the atomscancel out (even though each electron has magneticproperties)-iron, cobalt and nickel have magnetic propertiesbecause the magnetic properties of the electrons donot cancel out.~not all objects made from them have magneticproperties
9 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 incontact with other atoms, the groups of atoms canalign their magnetic poles so that they all point in thesame directionmagnetic domain: groups of atoms with aligned magnetic poles-each domain contains an enormous number of atoms-since all atoms are aligned, the domain itself canbehave like a magnet with north and south poles
10 How do we line up these domains to create a magnet? -only when the domains all line upin the same direction will it createa magnet-if the domains do not line up inthe same direction, it will notcreate a magnet~notice in the nail the domains donot line up (in Fig 7, p 229)
11 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 externalmagnetic field (from the magnet)is removed, the nail is no longermagnetized and the magneticdomains will unalign
12 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 astrong magnetic field inside the material, whichprevents the constant motion of the atoms frombumping 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.
13 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?
14 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 producesa magnetic field around the wire-the magnetic fieldaround the wirechanges direction asthe direction of currentchanges.
15 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 decreaseselectromagnet: temporary magnet made by wrapping a wire coil carrying a current around an iron core-a magnetic field is only present when current isflowing through the wire coil-adding more turns to the coil or increasing the currentincreases the strength of the magnetic field
16 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 andelectric motors
17 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
18 GalvonometersHow 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.
19 Electric Motorselectric 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 padsconnected to the battery~commutators: conductingmetal ring that is split androtates with the coil
20 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 theelectromagnets magnetic fieldbecomes stronger, increasingthe force between the coil andmagnet
21 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?
22 Producing Electric Current electromagnetic induction: production of an electric current by1. moving a wire through a magnetic field2. moving a magnet through a wire loopMost 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 fielda. wire coil is wrapped around an iron core and placedbetween the poles of a permanent magnet
23 generator:b. the coil is rotated by an external source ofmechanical energyc. as the coil turns within the magnetic field of thepermanent magnet, an electric current flowsthrough the coild. the direction of currentin the generator’s coilchanges twice for eachrevolution
24 What are uses of electric generators? 1. alternator in your car-provides electrical energy to operate lights andother accessories-spark plugs in your car use this electrical energy toignite the fuel in the cylinders2. generating electricity-uses a fixed coil and permanent magnet thatrotates, producing electric current in the fixed coiland mechanical energy is used to rotate themagnet, which is connected to a turbine
25 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 isdifferent than the current from an electric generator.~batteries produce direct current, current that flowsin one direction~generators produce alternating current, whichreverses the flow of current in a regular pattern
26 In the US, generators produce AC current at a frequency of 60 cycles/sec (60Hz) -changes direction 120 times each secondA 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 thatconverts the alternating current to direct current andreduces the voltage of the alternating current
27 Sometimes we need to increase or decrease the voltage of an alternating current: -transformer: device that increases or decreases thevoltage of alternating current~step-up transformer: increases voltage by puttingmore turns of wire on the secondary coil~step-down transformer: decreases voltage byplacing fewer turns of wire on the secondary coil
28 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?