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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations. See the links below.
Click for the questions that go with this reading


This content is important and will be use when problem solving.s


Motional emf

When a wire moves across a magnetic field, a potential different difference is created between the ends of the wire. This potential, measured in volts is directly proportional to the velocity the wire moves, the length of the wire and magnetic field's strength. This video describes where it comes from.

This video can be found on YouTube at


Motional emf Equation

  • ε : The electro motive force. This is the potential difference induced by the moving wire. It is measured as the diffrence between the two ends of the wire. Measured in volts, [v.]
  • L : The length of the wire moving inthe magnetic field. If the wire is longer than the field is wide, then the length would only be the part of the wire that is in the field. Measured in meters, [m.]
  • ν : The speed of the wire as is moves across the magnetic field. Measured in meters per second, [m/s.]
  • B : The magnetic field's strength. Measured in Tesla's, [T.]


  • Question
  • Hint 1
  • Hint 2
  • Solution
  1. A metal bar is pushed along the bare copper wires as shown. What is the potential difference between the top and bottom copper wire if the metal bar is pushed at 3.57 m/s?
  2. If the metal bar has a resistance of 5 Ω, then how much force is used to push the bar?

Hint for part 1 of the question

The metal bar is moving across the magnetic field. This is a classic example of motional emf. (It is important to recognize that the bar is metal because most metals are conductors. If the "bar" was an insulator, like plastic, the charges would not be able to move the potential difference would be zero.)

The potential difference is measured from the voltage, V. The voltage is directly proportional to the bar's velocity, the length in the field and the magnetic field strength itself.

The copper bars limit the length of the bar in the magnetic field. Even though the steel metal bar is longer than the two copper wires, the problem states that the potential difference is measured on the copper wires. Therefore, the "length" of the metal bar to consider is the distance between the copper bars.

Hint for part 2 of the question

As the bar slides along the copper wires, there be a resistance to their motion. (This is from Lenz's Law.) The amount of resistance can be determined from the magnetic force, BILsin(θ). But the motional emf calculates the voltage. Recall, that the voltage is proportional to the current by the resistance, V=IR. Using this equation you can determine the current and then the force.

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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)