What Factors Affect the Strength of an Electromagnet?
Making an electromagnet from a battery, nail, and wire is a classic science demonstration. But instead of just demonstrating this for your students, let them explore it themselves! In this lesson they will discover how different variables affect the strength of an electromagnet.
- Build a working electromagnet from supplied materials
- Ask a scientific question about electromagnets that can be answered with an in-class experiment
- Plan and conduct an experiment to determine how a certain variable affects the strength of an electromagnet
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- MS-PS2-3. Ask questions about data to determine the factors that affect the strength of electric and magnetic forces.
|Science & Engineering Practices||Disciplinary Core Ideas||Crosscutting Concepts|
|Science & Engineering Practices||Planning and Carrying Out Investigations.
Plan and conduct an investigation individually and collaboratively, and in the design: identify independent and dependent variables and controls, what tools are needed to do the gathering, how measurements will be recorded, and how many data are needed to support a claim.
||Disciplinary Core Ideas||PS2.B: Types of Interactions.
Electric and magnetic (electromagnetic) forces can be attractive or repulsive, and their sizes depend on the magnitudes of the charges, currents, or magnetic strengths involved and on the distances between the interacting objects.
||Crosscutting Concepts||Cause and Effect.
Cause and effect relationships may be used to predict phenomena in natural or designed systems.
Materials used to create an electromagnet include: a straw, a wooden pencil, metal nails of various sizes, alligator clips, paper clips, sandpaper, a spool of copper wire, tape, a D cell battery, a battery holder and scissors. These items can be used test the strength of the electromagnet based on how many paperclips it can lift.
For each group of students:
- D battery
- Battery holder
- Alligator clips (2)
- Small piece of fine-grit sandpaper, roughly 3×3 cm
- Tape (any type is OK)
- Magnet wire
- Assorted nails
- Other cylindrical items to use as magnet cores, like wooden pencils and plastic straws
- Metal paper clips (one box can be shared among several groups). Note that plastic-coated metal paper clips are OK, but do not use clips that are entirely plastic.
For classroom demonstration (optional):
Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
Watch this video for an introduction to magnetism and electromagnetism:
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.
|Number of times you wrap the wire around the core (referred to as "number of turns in coil")||More turns make the magnet stronger.|
|Amount of current flowing through wire||More current makes the magnet stronger. The amount of current can be increased by using multiple batteries or thicker wire (see variations—you will need to purchase additional materials to test this).|
|Core material||Ferromagnetic materials (iron, steel) make the magnet much stronger than non-ferromagnetic materials like paper, wood, or air.|
|Width of the nail||A wider nail will make the magnet stronger.|
|Length of the nail||Making the nail longer will not make the magnet stronger, unless you also add more turns to the coil.|
|Spacing of turns in coil||Tightly-spaced coils will make the magnet stronger|
|Surface coating of nail||Nails have different surface coatings (shiny, dull, smooth, rough, etc.), but this generally does not affect the strength of the magnet.|
|Location of coil on nail (in the middle vs. toward the end)||Assuming the coil does not take up the entire length of the nail, the magnet will be stronger if you place the coil near one end, and use that end to lift the paper clips.|
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.
Figure 1. Example data.