ELECTROMAGNETISM
Magnetic Induction, Inductance, AC and LC Circuits
Lenz's Law
LEARNING COMPETENCIES
Describe the direction of the induced electric field, magnetic field and current on a conducting/nonconducting loop using Lenz's law
How to determine the direction of induced emf?
ESSENTIAL QUESTION
BIG IDEA
The direction of the induced emf is in opposition of the original magnetic flux that created it.
Revisiting Faraday's Law
In Faraday's equation, the presence of the negative sign tells us that direction of the induced emf is opposite the change in magnetic flux that created it.
This is in consonance with the law of conservation of energy. If the direction of original magnetic field that created the current is in the same direction of the induced magnetic field created by this current, then these two magnetic fields would add up creating more magnetic field, which will mean more current. If this process continue, then it would violate the law of conservation of energy.
This can further be supported by Newton's Third Law of Motion. If the direction of the induced current creates a magnetic field, that is equal in magnitude but opposite in direction to the original magnetic field, then that's where resistance to the magnetic field in the area occurs.
Key Concept:
We have to keep in mind that the resulting magnetic field inside a loop of wire is a result of two contributing factors. On the one hand, is the original magnetic field of the magnet thrusting inside the coil which produces the induced current.On the other hand, the induced current produces its own magnetic field which is called the induced magnetic field.
MISCONCEPTION ALERT
Teacher must make sure that the learners understand the two sources of magnetic field; the original magnetic field and the induced magnetic field. Therefore, the presence of the induced current and induced electric field must also be emphasized.
21ST CENTURY SKILL
Critical Thinking and Problem-Solving
FORMATIVE ASSESSMENT
Observe the following experiment. Explain how the direction of the induced current and magnetic field is determined as shown.
This activity will allow the students to process what Lenz's law is. If materials are available, such as solenoid, magnet and galvanometer, this can be converted into an inquiry activity.
Use the following guide questions to answer:
What happens to the magnetic flux as the magnet moves toward the coil?______________________________________________________________________________________________
Why did the solenoid set-up a North-South Orientation with respect to the incoming magnet?______________________________________________________________________________________________
Use the following guide questions to answer:
What happens to the magnetic flux as the magnet moves away from the coil?____________________________________________________________________________________________
Why did the solenoid set-up a South-North Orientation with respect to the outgoing magnet?____________________________________________________________________________________________
SUGGESTED ANSWER
Magnetic flux increases as the magnet moves through the coil. Therefore the coil would set up a magnetic field that would oppose the incoming flux. Hence, the North pole of the incoming magnet would be repelled by the coil's magnetic field.
The same is true when the magnet moves away. Since the magnetic flux is decreasing as it moves away, the coil would set up field to attract the outgoing field.
The Right HandRule
For a current-carrying wire:
the thumb will point to the direction of the conventional current the curl of the fingers point to the direction of magnetic field
The Right Hand Rule for Straight Wire
Wikimedia Foundation Inc. (2015 November 30) Right Hand Rule Retrieved from https://en.wikipedia.org/wiki/Right-hand_rule
For a solenoid when a magnet is moving:
the thumb will point to the direction of the North Pole of the magnet created in the solenoid and the curl of fingers point to the direction of induced current
