30.9
Loi de Lenz
La direction dans laquelle la fem induite entraîne le courant autour d'une boucle de fil peut être trouvée grâce au signe négatif. Cependant, il est g…
Lenz's law states that the direction of induced current opposes the cause producing it. The cause can be either a time-varying magnetic field, a non-stationary conductor, or both the causes together.
If a bar magnet is moved toward a coil, then the magnetic flux passing through the coil changes. This induces a current that creates a magnetic field having a direction opposite to the original magnetic field.
If the stretched thumb of the right hand is in the opposite direction to the original magnetic field, then the curled fingers give the direction of the induced current.
On the other hand, if the polarity of the bar magnet is reversed, while it moves towards the coil, the induced current creates a magnetic field in the opposite direction to the original magnetic field.
Similarly, if the motion of the conductor changes the magnetic flux through the coil, the current is induced. This current is in a direction such that the magnetic force on the conductor opposes the motion of the conductor.
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Q1: What does Lenz's law state about induced current direction?
Lenz's law states that the direction of induced current always opposes the cause producing it. This cause can be a time-varying magnetic field, a moving conductor, or both. The induced current creates a magnetic field that resists changes in the original magnetic flux, consistent with the maxwell equation of electromagnetism that governs electromagnetic phenomena.
Q2: How do you determine induced current direction using the right-hand rule?
Point your right hand's stretched thumb opposite to the original magnetic field direction. Your curled fingers then indicate the direction of the induced current flowing through the coil. This method provides a quick way to visualize how the induced current opposes the changing magnetic flux without detailed calculations.
Q3: What happens to induced current when a bar magnet moves toward a coil?
When a bar magnet moves toward a coil, magnetic flux through the coil increases. The induced current creates a magnetic field opposing the magnet's field, effectively resisting the magnet's approach. If the magnet's polarity reverses while moving toward the coil, the induced current reverses direction to continue opposing the original magnetic field.
Q4: How does Lenz's law relate to the conservation of energy?
When pushing a magnet into a coil, work must be done against the induced magnetic field opposing the motion. This energy converts into electrical current. Without this opposition, the magnet would accelerate freely, violating energy conservation by creating electrical potential without an energy source.
Q5: What magnetic force does induced current exert on a moving conductor?
The induced current creates a magnetic force that opposes the conductor's motion. When magnetic flux through a coil changes due to conductor movement, the resulting induced current generates a force resisting that motion. This opposition ensures the system resists changes in magnetic flux, consistent with Lenz's law principles.
Q6: Can induced current be generated if the magnet stays stationary?
Yes, induced current can be created by moving the coil toward or away from a stationary magnet. The relative motion between magnet and coil changes the magnetic flux, inducing current. This induced current exerts a magnetic force on the coil opposing its motion, demonstrating that Lenz's law applies regardless of which component moves.
Q7: Why is Lenz's law easier to use than mathematical equations for finding current direction?
Lenz's law provides an intuitive approach by focusing on opposition to flux changes, while mathematical methods rely on the negative sign in equations. Using Lenz's law, you simply determine whether flux is increasing or decreasing, then identify the current direction that opposes this change. This conceptual method is faster than solving equations for most practical problems.