Field Lines Domains. Horseshoe Ring/Disc

Transcription

1 Magnets Info Lab (25 pts) 1. nickel, cobalt, iron 2.Iron, Nickel,and Cobalt have an unmatched electron spinning either UP or DOWN. The result is an atom with a net spin and thus a magnetic field. 3, Field Lines Domains N N S N S S Bar/Rod Horseshoe Ring/Disc 4. Box #1 : 1 bar and 1 horseshoe Box #2: bar and ring 5. a) Needle switches direction b) When you switch the current, you change the direction of the magnetic field. Table: Force increases with current and wraps. Pencil picks up 0 nails and poorly wrapped nail is very weak. 6. a) Metals/ Conductors work better than insulators b) strength increases with voltage c) increases with wraps d) Neatness increases magnet strength 7. iron core becomes magnetized adding the the overall the field strength. (FYI: 600 X increase) 8. Compare wire turns on primary and secondary. More turns means higher voltage. 9. a) turn ratio= 4:7 4 = V p = Vp Vs = is R 7 = Vs V V = is * 1000 ohms Vp = V i s =.220 amps b) turn ratio= 5:3 5 = V p = 120 Vs = is R 3 = V Vs 72.0 V = is * 1000 ohms Vs = V is =.072 amps 10. AC Alternating Current crerates a moving magnetic field that can induce a current in the secondary Any current creates a magnetic field around it. 2. A magnet moving near a wire will generate a current in the wire.

2 Magnets Info Lab (25 points) Name Per. Modeling Magnetism 1. What three elements are naturally magnetic?,, 2. Why do only certain atoms behave like magnets? Seeing is Believing: Magnetic Fields 3, Label on the figure: North and South magnetic poles, field lines, magnetic domains. Bar Magnet Draw and label a horsehoe or ring magnet (Label all parts) A C B D A B C D 4. What magnet(s) is (are) in the box? Explain how you know? Box # Magnetism from Electricity 5. a) What happens to the direction the compass needle moves when you switch the direction of current in the circuit? b) Write a general rule about switching current direction and the magnetic field. Record the number of screws you can pick up in each blank in the table below 1.5 volts 3.0 votls P e n c i l FEW wraps 6. Write a general rule for each of the variables below that describes how the variable affects the strength of the electromagnet: MORE wraps POORLY wrapped a) m a t e r i a l around which the wire is wrapped. b) v o l t a g e applied to the wire c) number of coil wraps d) wrapping n e a t n e s s

3 7. Speculate why the wrapped nail electromagnet has a stronger magnetic field (HINT: Nails are mostly iron) Magnetic Magic: Transformers 8. How can you tell if a transformer increases or decreases the voltage? 9. For the transformers pictured below, resistance = 1000 ohms in the secondary coil. Determine the unknown voltage and secondary current (is). SHOW ALL WORK!! (HINTS: The ratio of coils on the primary vs. the secondary is equal to the voltage ratio. Ohm s law always applies.) a) i s = amps Primary (4 turns) is Vp =? Secondary (7 turns) Vs = V 1000 ohms = volts Vp Primary (5 turns) Vp = 120.0V is Secondary (3turns) 1000 ohms Vs =? b) i s = amps Vs = volts 10.Transformers only work with what kind of current? (Circle One) AC DC Explain why this is so. 11. Overall: Write 2 general rules about the connection between electricity and magnetism: 1. 2.

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5 Seeing is Believing: Magnetic Fields I know what you are thinking. Magnetic fields? Electric fields? Big talk but I can t see them! You probably understand that you can only detect fields John Travolta by the effect they have on objects nearby. has nothing For example, you detect the gravitational field around the Earth when you fall down. Didn t you know that in the language of translates into, Gee, I observe the presence of a gravitational field! We can use iron filings [File-ings] and a compass needle to detect the presence of magnetic fields. Iron is one of those metals that is attracted to magnets. Look closely and you ll notice the filings are long and narrow like arrows. Each one will act like a tiny compass needle and align itself with the direction of the magnetic field lines. DO NOT PLACE IRON FILINGS DIRECTLY ON A MAGNET OR A BOX YOU WILL BE PUT ON MP IF YOU DO ( M AGNET PATROL: YOU GET TO CLEAN THE MAGNETS) DON T DROP, OR STRIKE A MAGNET. ANY BLOW CAN WEAKEN A MAGNET Place a magnet on a white sheet of paper and place a plastic sheet over the magnet and gently sprinkle iron filing on the sheet to reveal the shape of the magnetic field around the magnet. Avoid dropping the BIG clumps. Use the compass needle to determine which pole is north and which is south. Remember, a needle is north seeking. Remove the plastic sheet, bend it like a taco shell and gently transfer the filings back into the beaker. Try other magnets. Try the mystery box. Can you figure out what type of magnet(s) is(are) inside? REMEMBER: Use a plastic sheet over the box. Wash your hands if necessary when you re done...yuck!! puny, insignificant human being Earth molten iron and nickel core Unmagnetized Domain Magnetized (Permanent

6 Modeling Magnetism Like electricity, the source of magnetism is the charged particles in the atom i.e. electrons and protons. Well, actually it s the electrons but we wouldn t want the protons to feel left out. Ever since that day when electrons were talking negatively (as they always do) about the protons and the protons were, like, so positive (as they always are) that, like, the electrons and the protons were, like, Large current (i) Small Voltage (V) Primary Secondary Small current (i) Large Voltage (V) totally talking behind their backs and that was, like, so not right and since then, the protons get their feelings hurt easily, but I am getting off the subject here... V = i R Iron Core use the most confusing names possible for them (they are so good at that). Electrons can spin UP or DOWN. V = i R You already know that electrons are charged negatively but did you also know that they spin? Well, they do and I m not talking cotton and wool here. They actually spin! Electrons can spin in two directions (duh) and scientists Many turns = higher voltage In most atoms, each electron that is spinning UP has a partner electron that is spinning DOWN. The result is ZERO spin, since UP and Down cancel. But in some special Vp = 110 Volts atoms, there is an unmatched Standard home electron and the atom has a net voltage spin (UP or DOWN). Those special atoms behave like tiny Fewer turns = lower voltage magnets. You probably want to know which atoms get to be so special. Okay, I ll tell you but keep it to yourself; we don t want the other atoms thinking they are NOT special. elements. Primary (connected to wall socket) iron core Secondary (connected to the appliance) Appliance (phone, answering machine, boombox, etc.) Iron, Cobalt, and Nickel are the three naturally occurring magnetic We can make magnets out of other materials too, but it takes some fancy chemistry to leave an atom with a net spin if it doesn t naturally have one. The Earth's core is made up mainly of iron and nickel so compass needles are reacting to the Earth s magnetic field arising from the molten metal at it s core. Primary (50 turns) is Secondary (100 turns) As a chunk of metal cools, like iron for example, the atoms get locked into 500 ohms place. Neighboring atoms influence each other to align their individual magnetic fields. Each of those neighborhoods is called a DOMAIN. Vp =100 V Vs =? You probably already know how neighborhoods in towns can be very different and the same is true for metals. The orientation of the LOCAL magnetic field can vary from domain to domain. However, if you bring that chunk of easily magnetized material (iron, nickel, or cobalt) near a strong magnetic field, the domains align themselves and VOILA!! you ve got a magnet. The difference between non-magnetic iron and a permanent magnet is the orientation of the domains.

7 Magnetic Magic:Transformers Transformers are nothing short of the magic of magnetism. It s not really magic but it may appear to be. Transformers allow you to get current flowing in a circuit without a power source. Faraday and Henry discovered that if you get a magnet moving near a wire, you generate a current in the wire. it also works if the magnet is stationary but it s field is changing. How can you get a magnetic field to change? Take a look at the transformer on the table. It has two coils of wire embedded in an iron casing. There are connections for both the coils BUT THE COILS ARE NOT CONNECTED TO EACH OTHER. The electrons that flow in the primary coil will not flow in the secondary coil. When you power the primary, the current in the coil generates a magnetic field. (Remember, Oersted discovered this!) That magnetic field induces a current in the secondary coil (Faraday and Henry s discovery!). VOILA!! Current without a power source!! BUT WAIT, the magnet has to move in order to induce a current, it can t just sit there near the secondary coil! Well, it is moving..in a way. It is switching! the current running through the primary is Alternating Current (AC) and it switches direction in the primary 120 times every second. That means the magnetic field is switching at the same rate. The iron core helps to amplify the magnetic field of the primary. Now notice the thickness of the wire and the number of turns in the coils on the wall cube style transformer. The primary is thinner and has many turns. The secondary is thicker and has less turns. Fewer turns means lower voltage so the secondary has the smaller voltage. This is called a step-down transformer since the voltage is reduced. Step up transformers do just the opposite. For example, say the primary of a step up transformer has 50 turns connected to 100V power source and the secondary has 100 turns and is connected to a 500 ohm resistor. We find the secondary voltage (Vs) by using the ratio of turns. Primary turns = Primary Voltage (Vp) 50 = > Vs = 200 Volts Secondary turns Secondary voltage (Vs) 100 Vs We can use Ohm s Law (V=iR) to find the current in the secondary (is). V = IR 200 volts = I * 500 ohms ----> I = 2/5 amp =.4 amps Transformers are used everywhere to control the current and voltage in circuits without using any additional power supplies. Why do you want to do that? Well, it s cheaper to ship electricity from the generators at high voltage and low current (less loss to heat dissipation) but the electrical devices in your home use higher current and lower voltage. There are at least five transformers at different locations between your home and the generators at a power plant.

8 Magnetism from Electricity 1. Create a circuit with 4 batteries and light bulb as shown. DON'T CONNECT IT YET 2. Place the compass under one of the wires. MAKE SURE THE NEEDLE IS PARALLEL TO THE WIRE. 3. BRIEFLY complete the circuit and notice the needle. 4. Switch the leads on the battery and do it again 4 not connected You can make use of this phenomenon by creating a simple electromagnet. electromagnets (wire wrapped nails and pencil) on the table. Notice the 1. Connect the wrapped pencil electromagnet to 1 battery and bring it near the compass needle. 2. Repeat step two for 2, 3, and 4 batteries. to battery pack 3. With 4 batteries connected, try to pick up screws by dipping the pencil into the beaker of screws. What do you notice? 4. Connect the wrapped nail electromagnet that has the FEWEST wraps on it and connect it to 1 battery. Try and pick up screws from the beaker. Record your results 5. Repeat step 4 for 2, 3, and 4 batteries 6. Repeat steps 4 and 5 for the remaining electromagnets. What is Going On: Compass needles react to the presence of a magnetic field. Normally, it points North reacting to the Earth s magnetic field, but it will respond to any magnetic field close by. Hans Oersted ( ) first noticed how any conductor carrying a current sets up a magnetic field around it. It was accidental really. Oersted had a several pieces of gear lying about on his lab bench during a lecture to his students. When he completed a circuit near a compass, the needle moved (he did not expect it to.) I guess it pays to be a slob. REMEMBER!! You got current electricity by moving a magnet near a wire (electromagnetic induction). Both Joseph Henry ( ) and Michael Faraday ( ) discovered this. Now in this lab, you ve done just the opposite; you got a magnetic field from current electricity. So, electricity and magnetism are really two sides of the same coin. 1. Any current creates a magnetic field around it. 2. A magnet moving near a wire will generate a current in the wire. Nearly everything we rely for survival and comfort comes from these two phenomena. The electric motor makes use of phenomenon 1, and the electric generator makes use of phenomenon 2. Generators provide the electricity we use and motors use that electricity to operate just about anything you can imagine.

9 Modeling Magnetism Modeling Magnetism Magnetic Magic: Transformers Magnetic Magic: Transformers Seeing is Believing: Magnetic Fields

10 Seeing is Believing: Magnetic Fields Magnetism From Electricity Magnetism From Electricity

11 Believe it or not, this copper wire is insulated with clear plastic. You must clip onto the sanded ends of the copper wire. DO NOT clip in the middle or you will not get any results. Clip Here Do NOT clip anywhere along here