A copper bar is moved to the right while its axis is...

A copper bar is moved to the right while its axis is maintained in a direction perpendicular to a magnetic field, as shown in Figure $\mathrm{P} 20.27$. If the top of the bar becomes positive relative to the bottom, what is the direction of the magnetic field?

Key Concepts

Lorentz Force

The Lorentz force describes the force experienced by a charged particle moving in a magnetic field. It is given by the cross product of the velocity and the magnetic field vectors, meaning that the force is perpendicular to both the velocity of the particle and the magnetic field. This concept is fundamental in understanding how moving charges in a conductor experience a force that leads to charge separation and an induced electromotive force.

Right-Hand Rule

The right-hand rule is a mnemonic used to determine the direction of the vector resulting from a cross product, such as the magnetic part of the Lorentz force. By pointing the fingers of the right hand in the direction of the velocity vector and curling them toward the magnetic field, the thumb points in the direction of the force on a positive charge. This rule helps predict the polarity developed across a moving conductor in a magnetic field.

Motional EMF

Motional electromotive force (emf) arises when a conductor moves through a magnetic field, causing a separation of charges along the conductor due to the Lorentz force. This induced emf is directly related to the velocity of the conductor, the magnetic field strength, and the length of the conductor. It is a key principle behind devices such as generators and explains how motion through a magnetic field can create a measurable voltage.

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