Lesson Plan 2 --- Magnetic Force

By Fengfeng Zhou

Objectives:

  1. Students will be trained on making a scientific inquiry and construct logical verifications.
  2. Students will understand that a magnetic field always exerts a force on any current.
  3. Students will be able to explain the appropriate procedures and variables in a scientific experiment.
  4. Students will be able to draw conclusions from their inquiry, use the third right-hand rule and the formula F = BIL to solve relevant problems.
  5. Students will understand the nature of the magnetic force and be able to determine the force exerted on an electric charge in a magnetic field.
  6. Students will understand the principle and operation of an electric motor.

Materials:

A power supply with maximum voltage output less than 12 V, a sensor to measure magnetic induction, a “trapeze” as shown in Figure 1, 4 alligator connectors, 4 leads, a strong horseshoe magnet, a ruler, a steel tape, 1 meter of thin wire, 2 large paper clips, a small piece of strawboard, 5 disk magnets, a small file or a piece of sandpaper

Figure 1. A “trapeze”

Activities:

  1. Introduce the identity magnetic induction, B, and units Tesla and Gauss.
  2. Demonstrate that magnetic field will exert a force on electric current.
  3. Guide students to do an experiment. Ask students to connect the trapeze and power supply, put the horizontal metal (copper or aluminum) bar hung in the trapeze between the two poles of the magnet, then turn on the power and set the current I to be 1 ~ 2 amps. Due to the magnetic force exerted on the metal bar, the bar will be pushed away from its original position. Ask students to measure the distance the bar moved and compute the swing angle between the current and original position of the bar. With the angle known and the weight of the bar given to students, they should be able to calculate the magnetic force F exerted on the bar. The procedures and free body diagram are given to students as shown in Figure 2. Now ask students to measure the magnetic induction B of the magnetic field using the sensor, compute the product of B, I, and the length of the bar, L. Ask them to compare the product with F. Their values should be close to each other. If the difference is significant, ask them which value is more dependable, also ask them to identify sources of error.

Figure 2. Illustration of Step 3

  1. Tell students the formula to calculate magnetic force F in terms of B, I, and L.
  2. Project a graph to illustrate the relationship of directions of B, I, and F and introduce the third right-hand rule.
  3. Tell students the magnetic force exerted on current is the resultant force exerted on all charged particles forming the current. Derive the formula to calculate the force F exerted on any charge q by a magnetic field B: F = qvB where v is the speed the charge moves at.
  4. Ask students to give examples of use of magnetic force. Present several slides to show applications of magnetic force which students may not know. Tell student that exerting a force on any current is the most important property of a magnetic field and the most important application of this property is the invention of electric motors.
  5. Demonstrate how an electric motor works (see Figure 3).

Figure 3. A model electric motor

  1. Guide students to construct an electric motor. Procedures are described as follows:
  2. Wrap #26 or #28 wire about 1 m long around a bottle with a diameter of 4 cm. The coils will serve as the armature. Leave 2 cm of straight wire at each end of the coils. The straight parts will serve as the axis of the armature.
  3. Use thin strips of tape to keep the coils in a loop.
  4. Bend the paper clips as shown in the following sketch and insert the other ends of the clips into a piece of cardboard as the support of the armature.
  5. Use sandpaper to remove the coating on the top side only of the armature axis.
  6. Place the armature on the clips and put several disk magnets beneath the armature.
  7. Connect the power supply to the clips. Turn on the supply and set the voltage at 1.5 V.
  8. Give the armature a small spin and it should start to spin.
  9. Explain why only the coating on one side of the armature axis should be removed.
  10. Ask students to reverse the direction of current and watch the result. Ask them to list 3 ways the experiment could be modified to make the armature turns faster.