What Factors Affect the Strength of an Electromagnet?

Background Information for Teachers

Watch this video for an introduction to magnetism and electromagnetism:

A brief introductory video to magnets and electromagnets. Continue reading for more details.

https://www.youtube.com/watch?v=V-Gus-qIT74

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The second half of the video demonstrates how electrical current creates a magnetic field. Electrical current flows through a wire connected between the two terminals of a battery. However, the magnetic field around a single, straight piece of wire is fairly weak. You can make the magnetic field much stronger by wrapping the wire into a coil, also called a solenoid. As long as each turn of the coil goes in the same direction, the magnetic fields around each turn will add up, resulting in a stronger field. This is important; if two turns go in opposite directions, their fields will cancel out instead of adding up!

You can make the field even stronger by wrapping the wire around a ferromagnetic core, like a nail. A solenoid with a ferromagnetic core is commonly called an electromagnet. Unlike a permanent magnet, an electromagnet can be turned on and off using electrical current.

Many variables affect the strength of this electromagnet, and there are some variables that do not affect the strength. While the underlying physics are more advanced (see Additional Background section), middle school students can still do an experiment to identify the relationships between these variables and the strength of the electromagnet. Table 1 provides a summary of some variables and how they do or do not affect the strength of the magnet, but this list is not exhaustive.

Figure 1 shows example data collected for the number of paper clips the magnet could lift vs. the number of turns in the coil (averages for three trials). Based on these data, adding more turns of wire makes the magnet stronger because it can lift more paper clips.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 1. Example data.

Additional Background Links

• Electricity, Magnetism, & Electromagnetism Tutorial, Science Buddies
• Magnetism and Electromagnetism Tutorial (video), Science Buddies
• Wire Stripping Tutorial: How to Remove Insulation from Enamel-coated Magnet Wire, Science Buddies on YouTube
• What are the uses of electromagnets?, Universe Today
• Bar Magnet and Solenoid, HyperPhysics
• Solenoid, HyperPhysics

Prep Work (5 minutes)

If you purchased the Strength of an Electromagnet kit to use as a demonstration, set it up as shown in Figure 2.

1. Wrap 50 turns of wire around one of the iron bolts. Make sure the turns are neatly parallel to each other and all go in the same direction. Cut the wire, leaving about 10 cm of extra wire on each end. Use a small piece of tape to hold the wire in place and prevent it from unraveling.
2. Use a small piece of fine-grit sandpaper to remove the insulation from the last 2 cm on each end of the wire. Fold the paper in half, pinch the wire with it and pull the wire through, as shown in this video.
3. Use the alligator clips to connect the ends of the wire to the battery terminals. This allows electrical current to run through the wire. In other words, it turns the electromagnet on.
4. Dip the end of the bolt into a pile of paper clips, then lift it up. It should lift some of the paper clips.
5. Disconnect one of the alligator clips for now. Leaving them connected when the electromagnet is not in use will drain the battery and can cause the wire and bolt to get hot. Your demonstration is now ready for the class.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. Strength of an Electromagnet demonstration kit.

If you did not purchase the Strength of an Electromagnet kit, follow the same steps using a D battery, battery holder, and a nail (Figure 3).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Smaller electromagnet with a D battery and a nail. This electromagnet will not be as strong as the one in Figure 2.

Engage (5 minutes)

1. Show your students this video:
   

   https://www.youtube.com/watch?v=XBWy9gzGGd4

   What did we see in the video? What questions come up when you see this? Was anything surprising?

   We saw a large magnet attached to a crane lifting some scrap metal and then dropping it. Students might ask how the crane is able to drop the metal instead of the metal remaining stuck to the magnet.

2. Explain that the magnet in the video is an electromagnet. An electromagnet is a magnet that can be turned on and off using electrical current. This allows you to pick things up and then turn the magnet off to drop them, like the crane in the video.
3. Demonstrate the electromagnet you made in the Prep Work section. Show the students how you can connect the alligator clips, use the magnet to lift some paper clips, and then disconnect an alligator clip to drop the paper clips. Make sure all students have a good view of the electromagnet.

   How did I turn the magnet on? How did I turn it off?

   You turned the magnet on by connecting both alligator clips to the battery. This allows electrical current to flow. You turned the magnet off by disconnecting at least one of the alligator clips, which stops current from flowing. (Note: you can connect this to open and closed circuits if you have previously covered those topics in your classroom.)

   The magnet in the video was obviously much stronger than this magnet. It lifted huge chunks of metal and this one only lifts paper clips. How do you think they made the magnet in the video so strong? How could we make our magnets stronger?

   Possible answers include using a bigger piece of metal for the core, using more electricity, using more wire, etc. Students might have some answers that do not actually influence the strength of the magnet, but that is OK! They will do some experiments to find out what affects the strength of an electromagnet. For now, make a list of all the possible variables on the board.

Explore (40 minutes)

1. As a class, narrow the list down to variables that are feasible to test in the classroom given the materials you have available (e.g. if you only have one type of magnet wire, they will not be able to test different wire diameters). Students can follow along with this part on their worksheets.
2. Break students into groups of 3–4 and assign one variable to each group. If you have more groups than variables, it is OK if some groups have the same variable.
3. Show the class how to build a basic electromagnet (Figure 4). This will ensure that all students have a single "working" electromagnet initially so they can see how it works, and explore making changes from there.
   1. Wrap wire around a nail about 25 times. Start the wire at the flat end of the nail and make sure the turns are wound tightly against each other. Cut the wire, leaving about 10 cm of wire on both ends. Use a small piece of tape to hold the wire in place and prevent it from unraveling.
   2. Use sandpaper to remove the insulation from about 2 cm at both ends of the wire. Fold the paper in half, pinch the wire with it, and pull the wire through. Important: students might have trouble with this step. Their magnets will not work if they do not remove the insulation, as electricity will not be able to flow through the wire. Remember to refer to this video for instructions on how to remove the insulation.
   3. Press the D battery into the battery holder.
   4. Use the alligator clips to connect the bare wire ends to the clips on the battery holder.
   5. Try to pick up a paper clip with the end of the nail.
   6. Important: disconnect one alligator clip when the magnet is not in use. The electromagnet and battery can get hot if they are left connected. This will also prevent the battery from draining.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 4. Electromagnet made with a nail and D battery.
   
   
4. Now, let each group come up with a plan for how they will figure out whether the variable they have been assigned impacts the strength of the electromagnet. They will have to consider:
   1. What is their independent variable (what are they changing)? This will vary by group.
   2. What is their dependent variable (what are they measuring)? This should be the number of paper clips the magnet can lift (this is what you can measure with the materials you have available).
   3. What are their control variables (what do they need to keep constant)? This will be all the other variables except the independent variable they have been assigned.
   4. What values of the independent variable will they test? These values should be reasonable given the materials and time you have available (try to keep the number of turns to a maximum of 100).
   5. How many trials will they do for each value of the independent variable? Repetition is important for this experiment. Make sure students compare averages over at least three trials to make conclusions.
   6. For example, the group investigating the number of wire turns in the coil might decide to test 10, 20, 30, and 40 turns. They could build four separate magnets, each with a different number of wire turns, but otherwise identical (e.g. same size nail, same coil spacing), and do three trials with each magnet. It is important to perform each trial in the same manner (i.e., the motion to touch the end of the nail to the paper clips should be the same each time).
5. Let the groups carry out their experiments and collect their data.
6. Each group should analyze their data and determine what, if any, relationship there is between their dependent and independent variables.

Reflect (10 minutes)

Ask each group to report their findings to the class:

• What variable did they investigate?
• Did this variable affect the strength of the electromagnet? In other words, what, if any, was the relationship between the dependent and independent variables?
• Can you think of an explanation for why this variable does (or does not) affect the strength of the electromagnet?
• If your results are inconclusive (you cannot tell whether there is a relationship between the independent and dependent variables), can you suggest further experiments to clarify this? (For example, doing more trials or trying different values for the independent variable.)

Now circle back to the video you saw at the beginning of class.

Collectively, what variables did we discover that can affect the strength of an electromagnet? How do we need to change those variables to make the electromagnet stronger? Why do you think the magnet in the video was so strong?

Your students might have discovered that a wider nail, more coils of wire, and tightly wound coils could all make the electromagnet stronger. The magnet in the video had a much larger metal core than our tiny nails. It probably had a lot of tightly wound coils (although these were not visible in the video). Big electromagnets can have hundreds or even thousands of turns of wire. It also probably had a much more powerful source of electricity than a single battery, which can also make the magnet stronger (see Variations section).

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Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Electrician

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Career Profile

Did you like working with wires and electricity? Did you notice that sometimes your electromagnets got hot? Electricians install electrical wiring in homes and buildings. They have to make sure the wiring is installed properly and safely to avoid fire hazards. Read more

Electrical & Electronics Engineer

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Career Profile

Do you want to learn more about the science behind how electromagnets work? Do you want to design and build more advanced electrical circuits? Electrical engineers design circuits that use electromagnets, like the one in the crane we saw at the beginning of class, and other things ranging from medical equipment to computers and speakers. Read more

Lesson Plan Variations

• Purchase additional battery holders and let students explore how combining multiple batteries in series or parallel affects the strength of the electromagnet.
• Purchase wire with different diameters and let students explore how wire diameter affects the strength of the magnet.
• Electromagnets are a critical part of speakers. Your students can build their own speakers using some simple materials, as shown in the Build a Paper Speaker activity.