Objectives: Vocabulary: Materials: Students will: Safety: Magnet Electricity Electromagnet Charge Current Magnetic Field

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1 Electromagnets Author: Jane Earle, Lauren Downing, Kevin Dilley Date Created: July 2007 Subject: Physics Level: High School Standards: New York State- Physics ( Standard 1- Analysis, Inquiry and Design Standard 4- Moving electric charges produce magnetic fields. Standard 7- Interdisciplinary Problem Solving Schedule: Two Class Periods Objectives: Students will understand what an electromagnet is and how it works. They will be able to explain the relationship between electricity and magnetism. Students will: Experimentally discover how to strengthen electromagnets Establish a definition for magnetism that incorporates its relationship to electricity Present/share results Design and conduct their own experiements Vocabulary: Magnet Electricity Electromagnet Charge Current Magnetic Field Materials: For Group: Wire Batteries Bolts Dowels Straws Standard Compass Paperclips Electrical Tape Round Magnet Bar Magnet Battery Holder(s) For Class: Electromagnetic Lift Object to lift For Each Student: Activity Sheet 1 Station Sheets 1-3 Provided by Teacher Safety: Do not hold wires to battery terminals with fingers, they get hot.

2 Science Content for the Teacher: Bar Magnet A magnet always has two poles, the North Pole and the South Pole. By convention, magnetic field lines point outward at the North Pole and inward at the South Pole and form closed loops that circle the entire magnetic. Because opposite poles (North- South) attract while similar poles (North-North or South-South) repel, the magnetic field lines of a bar magnet can be visualized using another magnet. A compass is an excellent tool for this. The needle of a compass is itself a small permanent magnet and the North indicator (typically colored red or white) is a magnetic North Pole. The needle is free to rotate and will tend to align itself such that the North indicator points toward a magnetic South Pole. For example, in response to the Earth s magnetic field, the compass will point toward the geographic North Pole of the Earth because it is in fact a magnetic South Pole. - 2

3 Electric and Magnetic Sources The electric field of a positive point charge is radially outward. The magnetic field of a bar magnet: Electric sources are inherently monopole or point charge sources. Magnetic sources are inherently dipole sources you can t isolate North or South monopoles. The magnetic field produced by electric current in a solenoid coil is similar to that of a bar magnet: An iron core has the effect of multiplying greatly the magnetic field of a solenoid compared to the air core solenoid on the left. - 3

4 Electromagnet Electromagnets are usually in the form of iron core solenoids. The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands. The magnetic field (B) at the center of the solenoid is: B = µ n I, where µ = k µ 0 Where n is the number of coils in the wire, I is the current in the wire, µ 0 is the magnetic permeability constant (4π x 10-7 N/A 2 ), k is the relative permeability of the core (for iron ~200). The magnifying effect is proportional to k, so for a solenoid with an iron core you would expect a magnification of about 200 compared to a solenoid with an air core. - 4

5 Preparation: Assemble electromagnet set-ups (battery in holder, wires from holder attached to wire coiled around a bolt) Organize and lay out materials Classroom Procedure: Engage (Time: 10mins) Divide students into groups of 2 or 3. Hand out one battery, one holder, one wire coiled around a bolt and one compass to each group (This set up can be pre-assembled). Each group must move their compass around the set up to test for magnetism. During this time, the facilitator should circulate between groups challenging those who have been able to make the compass move to see if they can get it to point in another direction. All observations should be recorded. After about 10 minutes, groups should be asked to share their results. Any hypotheses about the relationship between electricity and magnetism should be recorded. Ask students what questions they have about the electromagnet set up and how it works. Divide students into small groups based on the questions they are interested in researching. Groups will design their own experiments in an attempt to begin to answer the questions they have posed. A variety of materials will be made available to each group. In order to pick up these materials, groups must establish an initial plan for their experiment, and have it approved by the facilitator, so that they do not overwhelm themselves with unnecessary materials. Materials can always be changed or added later in the experiment. Explore (Time: 30mins) During this time student groups will research various aspects of the phenomenon they have just observed. Possible examples include: Why are the coils important, What if we use more batteries or a different kind of battery, What is the best material to use in the core, is our hypothesis about the relationship between electricity and magnetism true apart from the electromagnet, how do the filed lines in the electromagnet set up differ from a bar magnet? Each group will design and conduct their own experiment. Data should be collected and organized in some form so that it can be later shared with the larger group. Groups interested in testing the strength of their electromagnet(s) should use the ability to pick up paper clips as their measure. Students should be reminded that they can measure in any way they d like, but they need to be precise and consistent. - 5

6 The facilitator should aid in the provision of materials as well as pose new questions to idle groups. Explain (Time: 20mins) Each group should share their experiment design and its results with the larger group. Students should be encouraged to ask questions about their classmates experiments. The facilitator should then bring out the electromagnetic lift and demonstrate how it works using a chair or other heavy object. Each group should then attempt to explain how the lift works and why it is so strong from the perspective of the research they did. - 6

7 Assessment: The following rubric can be used to assess students during each part of the activity. The term expectations here refers to the content, process and attitudinal goals for this activity. Evidence for understanding may be in the form of oral as well as written communication, both with the teacher as well as observed communication with other students. Specifics are listed in the table below. 1= exceeds expectations 2= meets expectations consistently 3= meets expectations occasionally 4= not meeting expectations Engage Explore Explain 1 Shows leadership in the discussion and offers creative ideas reflecting a good understanding of magnetism and electricity. Designs and conducts experiment creatively and scientifically while providing an explanation for what is observed. Works very well with Provides an in-depth explanation of findings, makes excellent use of vocabulary and examples. 2 Participates in the discussion and shows an understanding of electricity and magnetism. 3 Contributes to the discussion, but shows little understanding of electricity or magnetism. 4 Does not participate in discussion. Shows no understanding of electricity or magnetism. group. Designs and conducts experiment creatively and scientifically. Works cooperatively with group. Works cooperatively with group, but makes some mistakes with the procedure. Has trouble working with group. Does little to complete the procedure. Provides clear explanation of findings. Makes good use of vocabulary, provides some examples. Provides a limited explanation of findings. Uses a few vocabulary words or examples. Is not clear in explanation of findings. Does not make use of vocabulary or examples. - 7

8 Extension Activity: With the knowledge you ve gained from this activity, build an electromagnetic motor! Instructions are in the activity sheets part of this module. Safety: Do not hold wires to battery terminals with fingers, they get hot. Acknowledgments: