Explaining the Decrease in Current in a Rotating Motor

[blooperkin]

Homework Statement

The armature of a simple motor consists of a square coil and carries a current of 0.55 A just before it starts to move. The coil is positioned perpendicular to a magnetic field. Explain briefly why the current falls below 0.55A once the coil of the motor is rotating.

Homework Equations

Lenz' Law
Faraday's Law

The Attempt at a Solution

My answer: When the coil rotates, there is a rate of change of cutting of magnetic flux linking the coil, so by Faraday's Law, an induced emf and thus current is induced. By Lenz' Law, the current acts in a direction to oppose the change that is producing it, and thus flows in a direction opposite to the initial current, resulting in a net loss of current flowing in the original direction.

However, I wasn't awarded the mark as my teacher said the induced emf acts in an opposite direction, not the induced current. However, I'm confused as I've always thought current can flow in different directions. Is it because a current is a scalar quantity and emf is a vector quantity?

 

[rude man]

Homework Statement

The armature of a simple motor consists of a square coil and carries a current of 0.55 A just before it starts to move. The coil is positioned perpendicular to a magnetic field. Explain briefly why the current falls below 0.55A once the coil of the motor is rotating.

Homework Equations

Lenz' Law
Faraday's Law

The Attempt at a Solution

My answer: When the coil rotates, there is a rate of change of cutting of magnetic flux linking the coil, so by Faraday's Law, an induced emf and thus current is induced. By Lenz' Law, the current acts in a direction to oppose the change that is producing it, and thus flows in a direction opposite to the initial current, resulting in a net loss of current flowing in the original direction.

However, I wasn't awarded the mark as my teacher said the induced emf acts in an opposite direction, not the induced current. However, I'm confused as I've always thought current can flow in different directions. Is it because a current is a scalar quantity and emf is a vector quantity?

No, emf is not a vector although it does have direction. Think a dry cell battery.
This kind of emf is called 'back emf' since it subtracts from the externally-applied emf. A motor with no load, therefore spinning fast, will have a large back emf; a heavily loaded one, spinning more slowly, a much smaller one.
The net applied emf is the externally-applied emf minus the back emf to generate a resultant lower emf, and therefore current.

 

[Delta2]

Both current and emf are scalars. It is current density J and electric field E that are vector quantities (look at wikipedia about current density if you are not sure what i am talking about). Both you and your teacher are correct. Probably the teacher insisted in his/her opinion because it is the induced EMF that causes the induced current and not the other way around.

Probably also your expression is not strictly correct, the current does not act in a direction, the current flows in a direction such as to oppose the "cause" that has created it. The cause that has created it is the laplace force from the external magnetic field to the coil with the initial current and this force causes the movement of the coil which causes the induced EMF which causes the induced current. So the induced current will have to flow in such a direction as to oppose the initial cause that is the laplace force, and the way to do it is to flow in an opposite direction to the initial current as to produce an opposite laplace force.

 

[rude man]

The cause that has created it is the laplace force from the external magnetic field to the coil with the initial current and this force causes the movement of the coil which causes the induced EMF which causes the induced current.

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[Delta2]

?

Laplace force, you know the force that produces the torque in all electric motors.

http://en.wikipedia.org/wiki/Lorentz_force#Force_on_a_current-carrying_wire

In short it is the macroscopic Lorentz force.