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The Arizona STEM Acceleration Project

Introduction to Electricity - Magnetism (Induction)

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Introduction to Electricity - Magnetism

(Induction)

9 - 12 Physics STEM Lesson

Maria Masouraki

March 2024

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Notes for Teachers

Gather materials from the Physics Lab for teacher demonstration: Setup provided in the pictures. Enhance student understanding by allowing groups of 4-6 people to experiment independently.

List of Materials

Battery DC, AC Source
Wires
Magnetic Compass
Bar magnets
Horseshoe Magnets
Magnetic copper wire for winding
Plastic tubes for wrapping
Simulation of bar magnets moving close to coils

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Physics Standards

Explanation

Arizona K9-K12 Physics Standards Highlighed in Blue
Essential HS.P2U1.5 Construct an explanation for a field’s strength and influence on an object (electric, gravitational, magnetic).	Crosscutting Concepts & Background Information for Educators Crosscutting Concepts: Patterns; Cause and Effect; Scale, Proportion and Quantity; Systems and System Models; Energy and Matter; Structure and Function; Stability and Change4 Background Information: Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects. Forces at a distance are explained by fields permeating space that can transfer energy through space. Magnets or changing electric fields cause magnetic fields; electric charges or changing magnetic fields cause electric fields. 4 (p. 118) Some cases of action at a distance are not explained in terms of radiation from a source to a receiver. A magnet, for example, can attract or repel another magnet and both play equal parts. Similarly, the attraction and repulsion between electric charges is reciprocal. The idea of a field is useful for thinking about such situations. A field is the region of the object’s influence around it, the strength of the field decreasing with distance from the object. Another object entering this field experiences an effect – attraction or repulsion. Gravity, electric and magnetic interactions can be described in terms of fields. 2(p. 21)
Physical Science Plus (+) Standards HS+P are supporting standards designed to be used with the essential standards for students taking a high school physics (P) course.
Plus HS+Phy.P2U1.1 Plan and carry out investigations to design, build, and refine a device that works within given constraints to demonstrate that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.

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By the end of this week you will be able to:

State that a magnetic field exists around a current carrying wire.
Identify circumstances in which a voltage will be induced in a conductor.
State the factors which affect the size of the induced voltage, i.e. field strength, number of turns on a coil, relative movement.
Predict directions of induced fields based on relative motion on a transformer.

Weekly Learning Objectives

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Weekly Agenda

Timeline: Introduction to electromagnetic induction (3x45 minutes)

How electric field creates magnetic field and vice versa

Use of the right hand rule

Introduction to magnetic flux as the number of field lines crossing a loop

Discussion of Lenz Law: how coils tend to object to any changes of magnetic flux by creating their own current

Experiments: 45 Minutes Lab Time

Please allow students to have experiments before the discussion to develop the right questions themselves for the purposes of theory and explanations.

Lab Experiment Suggestions:

1.Oersterd Experiment

2. Magnetic levitation

3. Moving a bar magnet in and out of a solenoid (simulation for online instruction) or Lab setup

4. Construction of a simple step up or step down transformer

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Intro/Driving Question/Opening

Why do magnetic needles move around current carrying wires?

Why are current carrying wires lifted by the magnetic fields?

How do Maglev Trains work?

How do transformers carry electricity from the power plants to our households with minimum loses?

What are step up and step down transformers?

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Hands-on Activity Instructions

Suggested group structure: 4-6 people
Step-by-step instructions : Gather materials and follow teacher demonstration to conduct depicted experiments; Switch student roles to familiarize your group with all aspects of the experiments. Provide pre-lab (prediction questions for credit) and post lab (questions for experimental confirmation/understanding) for credit.
Allow some room for errors for students that experiment for the first time as they may run out of time. Adjust number of experiments according to your student level of understanding.(Honors to AP)
Images may be helpful to show how things are setup: See example setups here or follow slides:

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The relationship between electricity and magnetism evidence: The Oersterd experiment

Moving electrons create a magnetic field B by setting up a compass needle through a wire carrying an electric current I. The B field which is created by the current goes in circles around the wire. A magnetic Force shifts the needle!

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The right hand rule:

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The relationship between electricity and magnetism evidence:

Magnetic Levitation

Current carrying wires lifted by strong magnetic B fields experience an uplifting magnetic force:

Fmag=BIl

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The relationship between electricity and magnetism evidence:

There is no physical battery present in this setup, yet when the metal rod moves either light or left the light bulb turns on.

The faster the motion, the more intense the brightness.

What creates current?

Electromagnetic induction!

emf = Bℓv

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What is Electromagnetic Induction?

When a wire is moved in a magnetic field:
A voltage is created – or induced.
For this reason we call this electromagnetic induction.

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Faraday’s apparatus: a magnetic field can produce a current

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How does it work?

B field change at the top coil induces an emf and a current in the bottom coil.
When the switch is opened and closed, the galvanometer registers currents in opposite directions.
No current flows through the galvanometer when the switch remains closed or open.

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Movement of a magnet relative to a coil produces emfs, i.e. induced voltages

Think of it as a battery!

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B lines around coils and magnets

The field line density represents the magnetic field strength B in Teslas. Lines starting from North, ending in South outside of the magnet, always closed and never crossing each other!

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Lenz Law: electromagnetic induction opposes any change in flux.

(a) When a bar magnet is thrust into the coil, B field increases in the coil. The current I induced in the coil creates another field, in the opposite direction of the bar magnet’s to oppose the increase. When getting close: The coil repels the bar magnet North by creating its’ own North. When moving away:The coil attracts the magnet by creating its’ own South!!!

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Induced Voltage: alternatively called electromotive force, EMF:

When a current I passes through a coil of wire, there is a B magnetic field around the wire.
Changing direction of the current changes the direction of the field.
When we move a wire in a B field, voltage EMF is induced.
When we move a magnet in a coil of wire,a voltage EMF is induced.
To summarize: Changing the number of magnetic field lines passing through one or many loops leads to induced EMF’s!

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Induced Voltages generate electricity!

Induced EMF depends on:

B (in Teslas) magnetic field strength (the stronger the greater the induced voltage)
speed v ( in m/s), (the faster the greater the induced voltage).
number N of turns in the coil (the more loops, the greater the induced voltage)

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What causes Electromagnetic Induction ?

Change in Magnetic Flux…

Magnetic Flux Φ:The number of B lines crossing the surface A at right angles

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How does Flux Φ Change? ΔΦ=?

Φ=ΒΑcosθ

Change the B field strength: ΔΦ=(ΔΒ)Αcosθ
Change the area A of the loop where B lines cross: ΔΦ=Β(ΔΑ)cosθ
Change the relative angle θ between the loops and the B lines (rotate!) ΔΦ=BΑcos(Δθ)

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EMF defined as a negative Change of Flux rate

EMF is defined as ε=-ΔΦ/Δt in Volts

The minus in the formula refers to Lenz Law, i.e. induced fields tend to bring back flux to original values in the coil.

Notice how the faster the changes in flux the greater the induced voltage!

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Lab Practice example : Build a transformer

Wrap 20 loops of wire around paper tube No1 and 10 loops of wire around tube No2. Use a battery and a tubular magnet to change the magnetic flux in its’ loops Observe how the second wire responds by connecting to a galvanometer or polymeter (miliΩ) region. Can you explain what you observed?

Figure 8.3 Step down transformer example.

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Generators producing electricity by induced EMF

Coil rotation in a B field produces an EMF.
Picture: the basic construction of a generator, where work done to turn the coil is converted to electric energy.
The generator is similar in construction to a motor. The reverse is true about energy transformations.

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Practice HW problem…

What is the polarity of the induced emf when the metal rod moves to the right? Does the current run CW or CCW?
What is the polarity of the induced emf when the metal rod moves to the left? Does the current run CW or CCW?
What is the value of the induced current as a function of resistance?
Does the mathematical definition of Emf help you predict?
Do you think you can follow free electrons inside the rod and electrical forces to explain what happens instead? Hint : Fel=qvB

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Questions?

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Assessment

Choose homework problems from a traditional textbook that match their math level.

Provide worksheets with fill-in-the blanks with the right glossary as classwork or multiple choice questions.

Provide lab reports, turn it for credit for K12 , dual enrollment level students or project-based teaching Physics.

End of Week 10 minute assessment: Mix and Match questions from their week long classwork or labs in the form of a quiz (see lecture slides for examples).

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Differentiation

See suggestions throughout the presentation and teacher footnotes.

Each slide is dedicated to a specific level of students.

A few are for teacher demonstration only: Adjust accordingly.

Remediation

Extension/Enrichment

This presentation has gathered the most representative examples best suitable for 45 minute sessions.

Please consult with AP Physics website for further practice problems, homework, test banks and labs and University of Colorado Boulder Physics for simulations.