The Atom's Family

The Atom's Family

Electricity and Magnetism Two sides of the same coin NBSP Physical Science Institute Thursday July 25, 2002 7/25/02 Prof. Lynn Cominsky 1 Standard Connections Electricity and magnetism are related effects that have many useful applications in everyday life. How are they related? How are they used everyday?

7/25/02 Prof. Lynn Cominsky 2 Key concepts: Fields Fields are a mathematical representation of the way that forces are transmitted between objects The mathematical representation is a vector which is graphically indicated as an arrow The length of the arrow indicates the strength of the field, and it points in the direction of the force Regions with stronger forces have more field lines

Stronger field 7/25/02 Weaker field Prof. Lynn Cominsky 3 Key concepts: Electric Fields The field of a positive charge is outward The field of a negative charge is inward Field lines start and end on charges Positive charge 7/25/02

Negative charge Prof. Lynn Cominsky 4 Key concepts: Electric dipole An electric dipole is a pair of charges: one positive and one negative The field pattern resembles that of a bar magnet, which is a magnetic dipole What is the direction of the field between the + and - charges

(inside the dipole)? 7/25/02 Prof. Lynn Cominsky + 5 Key concepts: Magnetic Fields The field of a North pole is outward The field of a South pole is inward Magnetic poles are always found in pairs, so field lines are always loops What is the direction

of the field between the N and S poles (inside the magnet)? 7/25/02 Prof. Lynn Cominsky 6 Equipment for first activity Magnets of various types Iron filings, etc. Paper envelopes 7/25/02

Prof. Lynn Cominsky 7 A few things to try: Put a magnet inside the envelope to keep it clean Sprinkle the iron filings on top of the envelope Take the compass and move it along the direction of the lines traced by the filings What do you notice about the compass needle? Put two bar magnets inside the envelope How do the field lines change when the 2 magnets are end to end? Side by side? A few things on your own

7/25/02 Prof. Lynn Cominsky 8 First Activity: See the magnetic field Are all the fields patterns dipoles? Can you find a magnet with more than two poles? What do the field lines look like for refrigerator magnets? 7/25/02 Prof. Lynn Cominsky

9 Key concepts Both magnetic and electric fields can be visualized by using field lines Electric and magnetic dipole fields have similar configurations Electric charges can be isolated; magnetic poles cannot Field lines show the direction of force 7/25/02 Prof. Lynn Cominsky 10

Vocabulary for ELL Electric field: a way to picture the effects that electric charges have on one another Magnetic field: a way to picture the effects that magnetic poles have on other magnets Dipole: the combined field of two charges of opposite signs or a pair of magnetic poles 7/25/02 Prof. Lynn Cominsky

11 ELD Activities What other types of fields do you know? What do they have in common with electric and magnetic fields? 7/25/02 Prof. Lynn Cominsky 12 Break some things to think about What is the electric field at the midpoint of

two unlike charges (a dipole)? ? 7/25/02 Prof. Lynn Cominsky 13 Break some things to think about What is the electric field at the midpoint of two like charges? ?

7/25/02 Prof. Lynn Cominsky 14 Standard Connections Students know electric currents produce magnetic fields and know how to build a simple electromagnet. How do we see electric currents produce a magnetic field? What does this field look like? 7/25/02

Prof. Lynn Cominsky 15 Key concepts Electric charges have a static electric field When electric charges move, they create a current Any current can create a magnetic field (A changing magnetic field can also create an electric field) 7/25/02 Prof. Lynn Cominsky

16 Second Activity: Build a circuit with a switch Put a compass underneath one of the wires What happens when you close the switch? Why? + 7/25/02 switch Prof. Lynn Cominsky

17 A few things to try: Move the compass to different places along the wire Does the same thing happen to the compass at each place? Flip the battery around How does the compass direction compare to the direction found with the battery in its original orientation? Some things on your own! 7/25/02

Prof. Lynn Cominsky 18 Key Concepts The magnetic field encircles a current carrying wire, with the field lines curling around like the fingers of your right hand, if the current sticks up like your right thumb current

Magnetic field This is known as the right hand rule 7/25/02 Prof. Lynn Cominsky 19 Key Concepts If you reverse the direction of the current, the

direction of the magnetic field reverses also Which direction is the current flowing in the green circle in the center of these field lines? 7/25/02 Prof. Lynn Cominsky 20 Key Concepts A good way to increase the strength of the magnetic field is to coil the wire into a loop Then all the field lines point in the same direction

in the center of the loop How would you verify this using your circuit and compass? What are other ways to make the magnetic field stronger? 7/25/02 Prof. Lynn Cominsky 21 Key Concepts More coiled loops stronger magnetic field If you put an iron nail through the center of the

loops, you can make an electromagnet Can you draw the magnetic field lines in this diagram? What do you think happens when the switch is opened again? 7/25/02 Prof. Lynn Cominsky 22 A few more things to try:

How can you test the strength of an electromagnet? What happens to the strength of the electromagnet when you turn off the current? Can you turn the electromagnet off completely? 7/25/02 Prof. Lynn Cominsky 23 Vocabulary for ELL Coil a loop of wire Electromagnet a magnet made using electricity (current)

7/25/02 Prof. Lynn Cominsky 24 ELD Activities Make a list of things around your house that are found in coils What properties do these things have in common? 7/25/02 Prof. Lynn Cominsky

25 Publishers Materials Take some time to look through the state- adopted texts to find activities relating to electromagnets that could be used in your classroom. 7/25/02 Prof. Lynn Cominsky 26 Lunch break brain teaser An electric current is flowing through

two parallel wires in the same direction. Do the wires tend to a) repel each other b) Attract each other c) Exert no force on each other d) Twist at right angles to each other e) Spin 7/25/02 Prof. Lynn Cominsky 27 Standard Connections Students know the role of electromagnets in the construction of electric motors, electric generators, and simple devices, such as doorbells and earphones.

Students know electrical energy can be converted to motion. How do motors and generators work? How do motors convert electrical energy into motion? 7/25/02 Prof. Lynn Cominsky 28 Key concepts Start with an electromagnet

If you put an electromagnet on a pivot and turned on the current, it would swing to align itself with the permanent magnetic field of a horseshoe magnet So, what do you have to do to keep the electromagnet spinning? 7/25/02 Prof. Lynn Cominsky 29

Key concepts Parts of a DC motor Armature or rotor Commutator Brushes Axle Field magnet DC power supply of some sort 7/25/02 Prof. Lynn Cominsky 30 Key concepts: Motor in action

The axle of the electromagnet is connected to commutators (green) Brushes (red) touch the commutators as axle spins around Poles flip as armature North pole always stays passes through horizontal position above the horizontal and is repelled from field magnet 7/25/02

Prof. Lynn Cominsky 31 Third Activity: Spin a DC motor Get a small DC motor and connect it to a battery + 7/25/02 Prof. Lynn Cominsky 32 A few questions:

How can you tell which way it is spinning? How can you reverse its spin direction? How can you slow down the motor? How can you use the motor to move something? 7/25/02 Prof. Lynn Cominsky 33 Vocabulary for ELL Armature or rotor - similar to the nail in the electromagnet, the armature forms its core while mounted on a spinning axle

Commutator attached to the spinning axle and to each end of the coil. Commutators spin, connecting to the brushes on each side, thereby flipping the pole of the electromagnet for each 180 degree rotation. 7/25/02 Prof. Lynn Cominsky 34 Vocabulary for ELL Brushes connect the battery to the

commutators. Usually made of springy metal. Axle the part that the electromagnet spins around 7/25/02 Prof. Lynn Cominsky 35 ELD Activities Make a list of things in each room of your house that have motors How many motors does each device have?

Room Kitchen Device Number of motors Refrigerator 3 (with icemaker) What are some other things that have axles? 7/25/02

Prof. Lynn Cominsky 36 Publishers Materials Take some time to look through the state- adopted texts to find activities relating to motors that could be used in your classroom. 7/25/02 Prof. Lynn Cominsky 37

Break some things to think about How strong are permanent magnets? How strong are electromagnets? Why cant permanent magnets be used in all applications? What limits the strength of an electromagnet? 7/25/02 Prof. Lynn Cominsky 38 Publishers Materials Take some time to look through the state-

adopted texts to find activities relating to motors and generators that could be used in your classroom. 7/25/02 Prof. Lynn Cominsky 39 Lesson Study Activities Identify a key concept from todays lecture for further development Review the publishers materials about this key concept Discuss the best way to present this key

concept in your classroom 7/25/02 Prof. Lynn Cominsky 40 Resources (continued) http://solar.physics.montana.edu/YPOP/Spotlight/Magneti

c/loi.html http://www.colorado.edu/physics/2000/waves_particles/wa vpart3.html http://www.howstuffworks.com/electromagnet.htm http://purcell.phy.nau.edu/SeatExpts/resource/rhr/rhr.htm http://www.howstuffworks.com/motor.htm http://www.ed.uri.edu/SMART96/ELEMSC/ SMARTmachines/wheel.html 7/25/02 Prof. Lynn Cominsky 41

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