A basic electromagnet. click for copyright

Magnets are not the only object that can produce a magnetic field. Electric charge moving in a wire also produces a magnetic field. If we wrap a current-carrying wire in a cylindrical coil, the magnetic field produced resembles that produced by a bar magnet. The coil has a much stronger magnetic field than that of a straight piece of wire. If we put an iron (or nickel, cobalt, etc.) rod through the center of the coil, the rod becomes magnetized, creating an electromagnet (see Figure 1). This electromagnet has a stronger magnetic field than that of a current-carrying coil alone. The strength of the magnetic field can be increased either by increasing the current or by increasing the number of wraps in the coil of the electromagnet.

In a motor, the interaction between two magnets causes a rotating motion. A basic direct current (DC) motor has six parts: the field magnet, armature, commutator, brushes, axle and DC power supply (battery). The armature is an electromagnet. An axle is inserted through the armature at its middle, and the axle is perpendicular to the armature at its midpoint. When there is a current in the electromagnet, it produces a magnetic field. Since we have learned that opposite poles of a magnet attract and like poles repel, the magnetic interactions of repulsion and attraction between the electromagnet and the permanent magnet cause the armature to rotate on the axle.

The commutator is a pair of metal plates attached to the axle that provide the electrical current for the electromagnet. The plates do not completely surround the axle, and this gap between the plates is vital to motor design. Brushes, two pieces of metal or carbon, connect the battery to the electromagnet via the commutator. One brush is connected to the negative terminal on the battery while the other is connected to the positive terminal. As the armature turns on the axle, the direction of current in the electromagnet changes because each half of the commutator contacts alternating brushes. Whenever the current direction in the electromagnet changes, the magnetic field orientation changes. This allows the armature to keep spinning in the same direction on the axle. (Without the commutator and brushes, the armature would rotate at most 180° until the north pole of the electromagnet and south pole of the permanent magnet were together.) The kinetic energy of the rotating axle can be used to do work. For example, the axle of a fan motor is connected to the fan blades.

The electric and magnetic force are really two faces of one phenomenon - electromagnetism. Electromagnetism is responsible for the phenomena of electricity, magnetism and electromagnetic radiation. On one hand, moving charges produce a magnetic field. On the other hand, an electric current can be produced in a circuit by a changing magnetic field. This effect is known as electromagnetic induction and was discovered in 1831 by the experimental chemist/physicist Michael Faraday.

Consider a coil of wire connected to a galvanometer (a device for measuring electric current). If a bar magnet is inserted into the coil, the galvanometer needle deflects only while the magnet is moving. If we hold the magnet still inside the coil there is no longer a current. While we pull the magnet out there is again a current induced in the coil, but in the opposite direction. The electrons in the wire experience a force only when they are exposed to a changing external magnetic field. When the electrons experience this force they move, producing a current.

Discussion Question: Solicit, integrate and summarize answers from students. Do not worry about giving correct answers at this time. Tell students the answers will be discussed later in the lesson.

• How do you think a motor works?
• What does a battery do?
• How do you think that electricity and magnetism are connected?
• What kind of engineers do you think would work on motors and design cars?

Question/Answer: Ask the students and discuss as a class:

• What is a hybrid car? (Answer: Hybrid gasoline-electric cars have both a gasoline engine and an electric motor.)
• Why is a hybrid car better than a regular car? (Answers: There are several reasons why hybrid gasoline-electric cars are more efficient than gasoline-only cars. The engine in a hybrid car is smaller and more efficient than the engine in a conventional gasoline-only car. Special technologies in hybrid car engines decrease the air pollutants produced compared to conventional car engines. In a hybrid car, the electric motor is used to brake the car, generate electricity, and recharge the batteries. Mechanical engineers design some hybrid cars to be ultra-low weight.)

Flashcards: Each student on a team creates a flashcard with a question on one side and the answer on the other. The questions should be about electromagnetism and how engineers design electromagnets and motors. If the team cannot agree on the answers, they should consult the teacher. Pass the flashcards to the next team. Each member of the team reads a flashcard, and everyone attempts to answer it. If they are right, they can pass on the card to the next team. If they feel they have another correct answer, they should write their answer on the back of the flashcard as an alternative. Once all teams have done all the flashcards, clarify any questions.

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One and Done: Ask the students to think of an item that works using a motor, and raise their hands (or indicate thumbs up) when they have an example. (Possible answers: washing machine, dishwasher, can opener, garbage disposal, computer printer, vacuum cleaner, electric toothbrush, compact disc [CD] player, digital video disc [DVD] player, VCR tape player, computer, electric razor, an electric toy [radio-controlled vehicles, moving dolls], etc.). Call on students at random to state their answer (the item with a motor). Students put their hands down once they've contributed an answer. No repeat answers permitted. Remind them that all these items have motors that use magnets to work!