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Faraday's Law of Induction

LEARNING COMPETENCIES

  • Identify the factors that affect the magnitude of the induced emf and the magnitude and direction of the induced current

  • Relate Faraday's experiments and Maxwell's evaluation to a given experiment

BIG IDEA

Electricity is related to magnetism

History Links

 

In 1820, Hans Christian Oersted disovered that electricity can create magnetic field. It was an accidental discovery that paved the way for electromagnetism.

"Oersted (right) and his assistant (left) hold a wire over a compass needle in a north-south direction. The compass needle is then deflected."

Retrieved from"The Worlds of David Darling, Encyclopedia of Science 5 November 2015.

How does the deflection of needle in the compass become a conclusive proof of the relationship between electricity and magnetism? ____________________________________________________________________________________________________________________________________________________

SUGGESTED ANSWER

A compass needle is a magnet and it usually points North because of Earth's magnetic field. When the needle was deflected, in the presence of electric current, it goes to show that there was a magnetic field induced.

MISCONCEPTION ALERT

Students might misinterpret the meaning of "induced" so the teacher must explain that this term could also mean "triggered" or "produced".

FORMATIVE ASSESSMENT

Essential Question

Can a Magnetic Field Produce Current?

An English scientist in the name of Michael Faraday proved that the opposite is true. He connected a galvanometer to a coil of wire. When he moved the magnet back and forth, the galvanometer needle moved which indicated the presence of current.

This video shows how Faraday conducted his experiment on EM induction which will be perfomed later by the students. Tutor Vista (2010) Electromagnetic Induction Retrieved from https://www.youtube.com/watch?v=KGTZPTnZBFE

 What is Maxwell's  Evaluation?

Essential Question

      According to Maxwell's Third Equation, 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

 

      The significance of Maxwell's equation is that it confirms what Faraday had discovered.That is according to him, the curl of an electric field is equal to a time-varying magnetic field.

      Now, it is important that the learners get to know the physical interpretation of the curl of electric field (symbol ∇x E) in the equation to fully grasp what it meant. The teacher might need to recall in calculus the meaning of curl as the "rotation" of the vector field in consideration. Inspecting further, if there is a rotation of electric field around a loop, a changing magnetic field is produced as given by the other side of the equation -(∂ B)/∂t which means that if there is an electric field, an accompanying magnetic field is induced and vice versa. The negative sign tells us the direction of the induced magnetic field as opposite to the electric field that created it. This will be further discussed in Lenz's law.

Math Integration:

At this point the teacher might need to recall Stokes' Theorem as shown below if the students ask how the formula was derived.

 

 

 

 

 

It is important that they can relate this equation to Maxwell's equation and its physical meaning. For now, the teacher can provide the interpretation of the above equation as: 

" integrating (averaging) of a field around a loop is exactly equivalent to integrating the curl of the field within the loop."

The teacher can show the derivation later after the discussion of induced emf and Lenz's law for it to make sense.

LET'S INVESTIGATE

The goal of this activity is for the students to discover that the relative speed of the magnet, the number of coils and the strength of the magnet affect the  magnitude of the induced emf

Materials

galvanometer

solenoid of varying number of coils

different bar magnets

Guided Inquiry:

This type of inquiry allows the teacher to assist the students in the process of discovering the factors. Along the way, the teacher can pose questions such as: "How does the speed of the magnet affect the deflection of the needle?" 

"Does increasing the number of coils affect the induced current?

21ST CENTURY SKILL

Critical Thinking and Problem Solving

Faraday's Experiment

galvanometer

bar magnets

solenoid with varying number of coils

Independent Inquiry: The teacher can make use of this strategy if he/she thinks that the learners are ready for this type of inquiry. The teacher can just pose a prompt such as: "using the materials mentioned, investigate the factors that would affect the amount of induced current". After the prompt is given, the students start planning and brainstorming on the possible procedure to arrive at the expected result.

Procedure

The teacher can make use of  different strategies depending on the types of learner.

ALTERNATIVE ACTIVITY

 

   In case that materials won't be available, the students can still explore this phenomenon using an electromagnet. It is easy to make an electromagnet using a  nail, magnetic wire and a battery.

  Wrap around the magnetic wire into the nail for about 20 loops, then connect this to a battery. Place iron fillings near the created electromagnet and observe what happens to the filings

  They can explore the amount of magnetic field produced by increasing the number of coils or increasing the amount of voltage.

 

GUIDED INQUIRY:

The teacher can guide the students into inquiry of the factors affecting the amount of induced magnetic field by comparing the amount of filings attracted by the electromagnet.

This video shows how to make an electromagnet using simple materials. Tinker Crate (2014) Retrieved from https://www.youtube.com/watch?v=_odHVX4mUAQ

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