The theme of this lesson is that moving charge creates a magnetic field, which is the first half of performance standard HS-PS2-5: Plan and conduct an investigation to provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current. After a short demonstration that shows how an electric current creates a magnetic field, as shown on Power Supply and Compass Demo, students use a computer simulation to quantify the relationship between the number of turns on a current-carrying coil of wire and magnetic field strength.
CCSS Math Practice 4: Model with mathematics is applied here as is NGSS Science Practice 1: Asking questions (for science) and defining problems (for engineering), Science Practice 3: Planning and carrying out investigations, Science Practice 4: Analyzing and interpreting data and Science Practice 5: Using mathematics and computational thinking.
Supplies used for this lesson include computers with internet access for student groups to use, a collection of magnets and compasses and a 6 volt power supply. This lesson takes two periods to complete and can be done either over the course of two days or over a double lab period.
The goal of these first few minutes of class is for students to observe and think about the behavior of a compass and how it interacts with an external magnetic field. As students come in, I hand out a compass and magnet. On the white board are the questions "Why does a compass point North?" and "How can you get a compass to point in a different direction?" along with the Lesson Objectives. I want students to think about and construct an explanation to those questions as this provides a foundation for the demonstration I am about to do. I call on random students to provide their explanations to the questions. After a few students give their answers, I repeat back a summary of what they said. In essence, "a compass is a tiny magnet that lines up with external magnetic fields"
Having students “play” with the compass and magnet reminds them of something they may have learned in elementary or middle school and sets the stage for the demonstration. It also gives them the physical experience to connect to the coming lesson and vocabulary.
The goal of the lesson is for students to quantify the magnetic induction from current that goes through a coil of wire. Before they do this, I demonstrate that a current in a wire creates a magnetic field. I set the context of the demonstration by talking about the discovery that electricity and magnetism are part of the same force. The year is 1820 and the electric battery had recently been created and refined in such a way that people could conduct experiments on electricity that involved more than a build up of charge on a conductor. Scientists could create easily electric currents and circuits. The story is that during a lecture, Hans Christian Ørsted was talking about the nature of electricity and how there was no known connection to magnetism, they were two separate, unrelated forces. As if to prove the point, he though to run an electric current through a wire to show there was no effect on a nearby magnetic compass. To his surprise, when he turned on the current, the compass needle deflected. It seems that he accidentally discovered that an electric current created a magnetic field!
While I tell the story, I demonstrate this for the class using a wire, power supply and compass needles projected onto the front screen using my document camera. This demonstration shows that there is a relationship between electricity and magnetism.
I ask the question, "What causes the magnetic field to form?" Students do a turn and talk to discuss their ideas. After 30 seconds, I call on random students to supply their ideas. Eventually, someone says "Moving charge creates a magnetic field". I instruct the students to write that in their notebooks and tell them that is what is known as electromagnetic induction.
Next, students quantify the magnetic induction from a current that goes through a wire based on different voltages and configurations of wire.
With the demo from the previous section, students see for themselves that a current creates a magnetic field. Now they have two activities to explore this concept further. Half the class works on the first activity while the other half works on the second activity. Each group has 30 minutes to complete its activity. After 30 minutes, students switch activities to work on the other. This can be completed over two 50 minute class periods or during a double lab period.
First activity: Electromagnetic Field Strength
Half the class works in groups of 2 on the Electromagnet PhET Lab handout. They collect a computer and perform two experiments to quantify the strength of the magnetic field based on different variables (number of coils and distance from coil). On the PhET simulator titled Generator, they use the magnetic induction tab. Their goal is determine how they can create an electromagnet that produces a magnetic field strength of 45,000 gauss. They have to plan, conduct and analyze an experiment that shows the mathematical relationship between the magnetic field strength and the number of coils. They graph their results and extrapolate the graph to make a prediction.
Second Activity: Magnetic Field Drawing
The other half of the class receives the Magnetism Reading and Drawing handout. This has students learn the right-hand rule and apply that rule to several different wire configurations. They also are to work in groups of 2 to complete this activity. The goal is for students to understand that for a current carrying wire, a loop creates a stronger magnetic field than a straight wire and that coil is the best configuration for a wire to create the strongest magnetic field for the given voltage.
It is important that students see and feel the physics we are teaching them. So while students work through the 2 electromagnet activities, I call up one group at a time to experience a strong electromagnet interact with a magnet. At the front of the room I have set up a coil of insulated copper wire with approximately 200 turns. I connect a 6 volt power supply that is able to push 2 amperes through the coil. (NOTE: This is a lot of current and the coil is in essence shorting the power supply, so I run the power supply for only 10-15 seconds at a time to prevent overheating.) I hand the students a strong neodinium magnet which they place near or in the coils of wire. The magnet interacts strongly with the magnetic field produced by the current in the coil.
The smiles on the student's faces and the comments they make as they feel the electromagnet pull their permanent magnet down makes me believe that these demonstrations have appeal and lasting impact. This connection between experience and understanding is important if I expect students to remember electromagnetic induction, a concept which is central to understanding how generators and motors work.
Each group takes about a minute to experience the electromagnet. After I am done with each group, I scan the room for red cups (click here to learn more about using cups as a strategy to quickly gauge student progress). If there are none, I call up another group, otherwise I go help the students who need it.
Once the experiments are complete and students have their analysis, I call on a few random groups to share their results. They share that the more coils, the stronger the magnetic field. Also, the greater the voltage, the more current that flows and so the magnetic field is stronger. This is seen with Magnetic Drawing Exemplar. For the experiment, students should have determined a linear relationship between the number of coils and the magnetic field strength, as seen on Electromagnet Graph Exemplar. Also, I display the PhET Activity Answers that students should have calculated for the experimental values.
To finish the lesson, I connect a hand-crank electric motor to a power supply and run a current through the motor. This makes the hand-crank spin in a circle. I explain that the current flows through the motor which induces a magnetic field. There are magnets attached to the hand-crack which interact with that magnetic field and causes it to spin. Here is the beauty of an electric motor - it is reversible! I disconnect the power supply from the hand-crank electric motor and connect a light bulb. I then spin the handle which causes the light bulb to light up. This is the idea of the century! With this simple device, we can turn electrical energy into kinetic energy or kinetic energy into electrical energy. We spend the rest of the unit exploring this idea.