Chapter 20 - Electromagnetic Induction Powerpoint Presentation - Holt Physics

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Electromagnetic InductionChapter 20

Table of Contents

Section 1 Electricity from Magnetism

Section 2 Generators, Motors, and Mutual Inductance

Section 3 AC Circuits and Transformers

Section 4 Electromagnetic Waves

Section 1 Electricity from MagnetismChapter 20

Objectives

• Recognize that relative motion between a conductor and a magnetic field induces an emf in the conductor.

• Describe how the change in the number of magnetic field lines through a circuit loop affects the magnitude and direction of the induced electric current.

• Apply Lenz’s law and Faraday’s law of induction to solve problems involving induced emf and current.

Electromagnetic Induction

• Electromagnetic induction is the process of creating a current in a circuit by a changing magnetic field.

• A change in the magnetic flux through a conductor induces an electric current in the conductor.

• The separation of charges by the magnetic force induces an emf.

Chapter 20

Electromagnetic Induction in a Circuit Loop

Electromagnetic Induction, continued

• The angle between a magnetic field and a circuit affects induction.

• A change in the number of magnetic field lines induces a current.

Chapter 20

Ways of Inducing a Current in a Circuit

Characteristics of Induced Current

• Lenz’s Law

The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it.

• Note: the induced current does not oppose the applied field, but rather the change in the applied field.

Chapter 20

Lenz's Law for Determining the Direction of the Induced Current

Characteristics of Induced Current, continued

• The magnitude of the induced emf can be predicted by Faraday’s law of magnetic induction.

• Faraday’s Law of Magnetic Induction

average induced emf = –the number of loops in the circuit

the time rate of change in the magnetic flux

– Memf Nt

• The magnetic flux is given by M = ABcos

Sample Problem

Induced emf and Current

A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude of the induced current in the coil.

Sample Problem, continued

1. Define

Given:

∆t = 0.85 s A = 1.8 m2 = 0.0º

N = 25 turns R = 2.5 Ω

Bi = 0.00 T = 0.00 V•s/m2

Bf = 0.55 T = 0.55 V•s/m2

Unknown:

emf = ?

Induced emf and Current1. Define, continued

Diagram: Show the coil before and after the change in the magnetic field.

2. Plan

Choose an equation or situation. Use Faraday’s law of magnetic induction to find the induced emf in the coil.

cos– –M

ABemf N N

t tSubstitute the induced emf into the definition of resistance to determine the induced current in the coil.

2. Plan, continued

Rearrange the equation to isolate the unknown. In this example, only the magnetic field strength changes with time. The other components (the coil area and the angle between the magnetic field and the coil) remain constant.

– cos

Bemf NA

Induced emf and Current3. Calculate

Substitute the values into the equation and solve.

V•s0.55 – 0.00

m–(25)(1.8 m )(cos0.0º ) –29 V

(0.85 s)

–29 V–12 A

2.5 Ω

–29 V

–12 A

Induced emf and Current4. Evaluate

The induced emf, and therefore the induced current, is directed through the coil so that the magnetic field produced by the induced current opposes the change in the applied magnetic field. For the diagram shown on the previous page, the induced magnetic field is directed to the right and the current that produces it is directed from left to right through the resistor.

Section 2 Generators, Motors, and Mutual InductanceChapter 20

Objectives

• Describe how generators and motors operate.

• Explain the energy conversions that take place in generators and motors.

• Describe how mutual induction occurs in circuits.

Generators and Alternating Current

• A generator is a machine that converts mechanical energy into electrical energy.

• Generators use induction to convert mechanical energy into electrical energy.

• A generator produces a continuously changing emf.

Chapter 20

Induction of an emf in an AC Generator

Chapter 20

Function of a Generator

Generators and Alternating Current, continued

• Alternating current is an electric current that changes direction at regular intervals.

• Alternating current can be converted to direct current by using a device called a commutator to change the direction of the current.

Chapter 20

Comparing AC and DC Generators

Motors

• Motors are machines that convert electrical energy to mechanical energy.

• Motors use an arrangement similar to that of generators.

• Back emf is the emf induced in a motor’s coil that tends to reduce the current in the coil of a motor.

Chapter 20

DC Motors

Mutual Inductance

• The ability of one circuit to induce an emf in a nearby circuit in the presence of a changing current is called mutual inductance.

• In terms of changing primary current, Faraday’s law is given by the following equation, where M is the mutual inductance:

– –M I

emf N Mt t

Chapter 20

Mutual Inductance

Section 3 AC Circuits and TransformersChapter 20

Objectives

• Distinguish between rms values and maximum values of current and potential difference.

• Solve problems involving rms and maximum values of current and emf for ac circuits.

• Apply the transformer equation to solve problems involving step-up and step-down transformers.

Effective Current

• The root-mean-square (rms) current of a circuit is the value of alternating current that gives the same heating effect that the corresponding value of direct current does.

• rms Current

maxmax0.707

Effective Current, continued

• The rms current and rms emf in an ac circuit are important measures of the characteristics of an ac circuit.

• Resistance influences current in an ac circuit.

Chapter 20

rms Current

Sample Problem

rms Current and emf

A generator with a maximum output emf of 205 V is connected to a 115 Ω resistor. Calculate the rms potential difference. Find the rms current through the resistor. Find the maximum ac current in the circuit.

1. DefineGiven:

∆Vrms = 205 VR = 115 ΩUnknown:

∆Vrms = ? Irms = ? Imax = ?

rms Current and emf2. Plan

Choose an equation or situation. Use the equation for the rms potential difference to find ∆Vrms.

∆Vrms = 0.707 ∆Vmax

Rearrange the definition for resistance to calculate Irms.

rmsrms

Use the equation for rms current to find Irms.

Irms = 0.707 Imax

rms Current and emf2. Plan, continued

Rearrange the equation to isolate the unknown. Rearrange the equation relating rms current to maximum current so that maximum current is calculated.

max 0.707rmsI

rms Current and emf3. Calculate

Substitute the values into the equation and solve.

(0.707)(205 V) 145 V

145 V1.26 A

115 Ω1.26 A

1.78 A0.707

4. Evaluate The rms values for emf and current are a little more than two-thirds the maximum values, as expected.

Transformers

• A transformer is a device that increases or decreases the emf of alternating current.

• The relationship between the input and output emf is given by the transformer equation.

induced emf in secondary =

number of turns in secondaryapplied emf in primary

number of turns in primary

Chapter 20

Transformers

Transformers, continued

• The transformer equation assumes that no power is lost between the primary and secondary coils. However, real transformers are not perfectly efficient.

• Real transformers typically have efficiencies ranging from 90% to 99%.

• The ignition coil in a gasoline engine is a transformer.

Chapter 20

A Step-Up Transformer in an Auto Ignition System

Section 4 Electromagnetic WavesChapter 20

Objectives

• Describe what electromagnetic waves are and how they are produced.

• Recognize that electricity and magnetism are two aspects of a single electromagnetic force.

• Explain how electromagnetic waves transfer energy.

• Describe various applications of electromagnetic waves.

Propagation of Electromagnetic Waves

• Electromagnetic waves travel at the speed of light and are associated with oscillating, perpendicular electric and magnetic fields.

• Electromagnetic waves are transverse waves; that is, the direction of travel is perpendicular to the the direction of oscillating electric and magnetic fields.

• Electric and magnetic forces are aspects of a single force called the electromagnetic force.

Chapter 20

Electromagnetic Waves

Propagation of Electromagnetic Waves, continued• All electromagnetic waves are produced by

accelerating charges.

• Electromagnetic waves transfer energy. The energy of electromagnetic waves is stored in the waves’ oscillating electric and magnetic fields.

• Electromagnetic radiation is the transfer of energy associated with an electric and magnetic field. Electromagnetic radiation varies periodically and travels at the speed of light.

Chapter 20

The Sun at Different Wavelengths of Radiation

Propagation of Electromagnetic Waves, continued• High-energy electromagnetic waves behave like

particles.

• An electromagnetic wave’s frequency makes the wave behave more like a particle. This notion is called the wave-particle duality.

• A photon is a unit or quantum of light. Photons can be thought of as particles of electromagnetic radiation that have zero mass and carry one quantum of energy.

The Electromagnetic Spectrum

• The electromagnetic spectrum ranges from very long radio waves to very short-wavelength gamma waves.

• The electromagnetic spectrum has a wide variety of applications and characteristics that cover a broad range of wavelengths and frequencies.

The Electromagnetic Spectrum, continued

• Radio Waves– longest wavelengths– communications, tv

• Microwaves– 30 cm to 1 mm– radar, cell phones

• Infrared– 1 mm to 700 nm– heat, photography

• Visible light– 700 nm (red) to 400 nm

(violet)

• Ultraviolet– 400 nm to 60 nm– disinfection,

spectroscopy• X rays

– 60 nm to 10–4 nm– medicine, astronomy,

security screening• Gamma Rays

– less than 0.1 nm– cancer treatment,

astronomy

Chapter 20

The Electromagnetic Spectrum

Multiple Choice

1. Which of the following equations correctly describes Faraday’s law of induction?

Standardized Test PrepChapter 20

( tan ) –

( cos )

( cos ) –

( cos )

ABemf N

emf Nt

ABemf N

emf Mt

Multiple Choice, continued

1. Which of the following equations correctly describes Faraday’s law of induction?

( tan ) –

( cos )

ABemf N

emf Nt

ABemf M

ABemf N

2. For the coil shown at right, what must be done to induce a clockwise current?

F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil.

G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil.

H. Move either pole of the magnet down into the coil.

J. Move either pole of the magnet up and out of the coil.

2. For the coil shown at right, what must be done to induce a clockwise current?

F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil.

G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil.

H. Move either pole of the magnet down into the coil.

J. Move either pole of the magnet up and out of the coil.

3. Which of the following would not increase the emf produced by a generator?

A. rotating the generator coil faster

B. increasing the strength of the generator magnets

C. increasing the number of turns of wire in the coil

D. reducing the cross-sectional area of the coil

3. Which of the following would not increase the emf produced by a generator?

A. rotating the generator coil faster

B. increasing the strength of the generator magnets

C. increasing the number of turns of wire in the coil

D. reducing the cross-sectional area of the coil

4. By what factor do you multiply the maximum emf to calculate the rms emf for an alternating current?

5. Which of the following correctly describes the composition of an electromagnetic wave?A. a transverse electric wave and a magnetic transverse wave that are parallel and are moving in the same directionB. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving in the same directionC. a transverse electric wave and a magnetic transverse wave that are parallel and are moving at right angles to each otherD. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving at right angles to each other

6. A coil is moved out of a magnetic field in order to induce an emf. The wire of the coil is then rewound so that the area of the coil is increased by 1.5 times. Extra wire is used in the coil so that the number of turns is doubled. If the time in which the coil is removed from the field is reduced by half and the magnetic field strength remains unchanged, how many times greater is the new induced emf than the original induced emf ?F. 1.5 timesG. 2 timesH. 3 timesJ. 6 times

7. From left to right, what are the types of the two transformers?A. Both are step-down transformers.B. Both are step-up transformers.C. One is a step-down transformer; and one is a step-up transformer.D. One is a step-up transformer; and one is a step-down transformer.

Use the passage below to answer questions 7–8.A pair of transformers is connected in series, as shown in the figure below.

7. From left to right, what are the types of the two transformers?A. Both are step-down transformers.B. Both are step-up transformers.C. One is a step-down transformer; and one is a step-up transformer.D. One is a step-up transformer; and one is a step-down transformer.

8. What is the output potential difference from the secondary coil of the transformer on the right?

F. 400 V

G. 12 000 V

H. 160 000 V

J. 360 000 V

8. What is the output potential difference from the secondary coil of the transformer on the right?

F. 400 V

G. 12 000 V

H. 160 000 V

J. 360 000 V

9. What are the particles that can be used to describe electromagnetic radiation called?

A. electrons

B. magnetons

C. photons

D. protons

9. What are the particles that can be used to describe electromagnetic radiation called?

A. electrons

B. magnetons

C. photons

D. protons

10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit?

F. 300 W

G. 600 W

H. 1200 W

J. 2400 W

10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit?

F. 300 W

G. 600 W

H. 1200 W

J. 2400 W

Short Response

11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current?

Short Response, continued

11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current?

Answer:

8.48 A

12. What is the purpose of a commutator in an ac generator?

Answer:

It converts ac to a changing current in one direction only.

13. How does the energy of one photon of an electromagnetic wave relate to the wave’s frequency?

Answer:

The energy is directly proportional to the wave’s frequency (E = hf ).

14. A transformer has 150 turns of wire on the primary coil and 75 000 turns on the secondary coil. If the input potential difference across the primary is 120 V, what is the output potential difference across the secondary?

Answer:

6.0 104 V

Extended Response

15. Why is alternating current used for power transmission instead of direct current? Be sure to include power dissipation and electrical safety considerations in your answer.

Extended Response, continued

15. Answer:For electric power to be transferred over long distances without a large amount of power dissipation, the electric power must have a high potential difference and low current. However, to be safely used in homes, the potential difference must be lower than that used for long-distance power transmission. Because of induction, the potential difference and current of electricity can be transformed to higher or lower values, but the current must change continuously (alternate) for this to happen.

Base your answers to questions 16–18 on the information below.

A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.

16. Why must the current enter the coil just as someone comes up to the table?

Answer: The change in current in the coil will produce a changing magnetic field, which will induce a current in the ring. The induced current produces a magnetic field that interacts with the magnetic field from the coil, causing the ring to rise from the table.

17. Using Lenz’s law, explain why the ring flies upward when there is an increasing current in the coil?

Answer: According to Lenz’s law, the magnetic field induced in the ring must oppose the magnetic field that induces the current in the ring. The opposing fields cause the ring, which can move freely, to rise upward from the coil under the table’s surface.

18. Suppose the change in the magnetic field is 0.10 T/s. If the radius of the ring is 2.4 cm and the ring is assumed to consist of one turn of wire, what is the emf induced in the ring?

Answer: 1.8 10–4 V

Chapter 20

Ways of Inducing a Current in a Circuit

Chapter 20 - Electromagnetic Induction Powerpoint Presentation - Holt Physics

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