A2L Item 235

• Description: Identify the correct statement(s) regarding a conducting bar moving through a uniform magnetic field.
• Goal: Hone understanding of Faraday's Law
• Source: Hone understanding of Faraday's Law
• Keywords: Forces, Electromagnetism, Electric Field, Magnetic Field

---

The question for students:

A long conducting bar moves with a constant velocity in a uniform magnetic field. If the bar and the velocity of the bar are perpendicular to the magnetic field as shown. Which of the following statements are true?

1. At steady state there is an E field in the bar
2. At steady state there is a current in the bar
3. At steady state there is a mag. force on bar

1. A only
2. B only
3. C only
4. A and B only
5. A and C only
6. B and C only
7. A, B, and C

---

Commentary for teachers:

Answer

(1) This question is often given as an example of Faraday’s law. Relating Emf to flux change is difficult for some students to perceive when there is no circuit. Creating an imaginary circuit helps, but many students continue to get the direction of the field incorrect even though the magnitude of the potential difference is now understood. It is useful to view the process using the Lorentz force. This helps students understand which charges migrate to which end and, therefore, what the direction of the electric field in the bar is.

---