Induced motional emf conceptual question

[EngineerHead]

A magnetic field points into the page. A conductive bar is moved to the right (perpendicular to the magnetic field). Why do the electrons move downwards toward the bottom of the bar? The book doesn't offer a good explanation of this... using right hand rule I would say that the electrons are moving to the right and therefore there is a force pointing upward?

 

[Doc Al]

using right hand rule I would say that the electrons are moving to the right and therefore there is a force pointing upward?

The right hand rule will give you the force on a positive charge. You need to reverse it, since the electrons are negative.

 

[technician]

When charges move through a magnetic field they experience a force.
The force is at right angles to the magnetic field and at right angles to the velocity of the charge.
This relationship is given in Fleming's LEFT hand rule. I hope you recognise this and realize in this rule the fingers of the LEFT hand have the following meaning:
1) The First finger points in the direction of the magnetic Field ( The 'F' in 'F'ield)
2) the seCond finger is the direction of the Current (this is the direction + charges would move in). This is a major issue ! conventional 'current' is the direction + charges would flow. When you are dealing with a flow of electrons (- charge) the second finger must point in the opposite direction of electron flow. This is the main aspect of this work to sort out !
3) The thuMb points in the direction of the force on the charges. ( M = Motion !)
In your example the magnetic field is INTO the plane of the paper...point the first finger of your left hand into the paper. The electrons in the wire are moving to the right so you must point your second finger to the LEFT (it takes some getting used to !)
Which way is your thumb pointing? ( I hope that it is DOWN (towards you if you are sitting like I am !)).
Electrons feel a force towards the bottom of the wire, they will move in this direction until a voltage is built up, with the bottom end of the wire -ve and the top end +ve opposing the further movement of electrons. A voltage is developed between the top end and the bottom end of the moving wire.
This effect is called the Hall effect and the voltage is called the Hall voltage.
If you need to know more look up Hall effect.

 

[technician]

I should add and qualify that the Hall effect is usually associated with a current flowing from one end of a PLATE (rather than a wire) to the other. The electrons are deflected down as in your example and a voltage appears between the top edge of the plate and the bottom edge.

 

[asteriatic]

The movement of electrons in a conductive bar is governed by the principles of electromagnetism. When a conductive bar is moved through a magnetic field, it experiences a force known as the Lorentz force. This force is perpendicular to both the direction of motion and the direction of the magnetic field.

In this scenario, the conductive bar is moving to the right, perpendicular to the magnetic field which is pointing into the page. This means that the Lorentz force will act upwards on the bar, causing the electrons to move downwards towards the bottom of the bar.

The reason for this is that the electrons are negatively charged particles and therefore experience a force in the opposite direction to the conventional current flow. Using the right-hand rule, we can see that the electrons will experience a force pointing upwards, in the opposite direction to the conventional current flow.

In summary, the movement of electrons downwards towards the bottom of the bar is a result of the Lorentz force acting upwards on the bar due to its motion through the magnetic field. This is a fundamental principle of electromagnetism and is consistent with the right-hand rule.