Physics 9 Lesson 32: Conditions for the occurrence of induced currents prepared by teachers of Cmm.edu.vn school hope to be useful documents to help students master the lesson knowledge and achieve good results in the lessons. exams and tests in class.

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## Summary of the theory of Physics 9 Lesson 32

### Variation of the number of magnetic field lines through the cross-section of the coil

When one pole of a magnet is brought near or away from the end of a coil, the number of magnetic field lines through the cross section of the coil increases or decreases (variable).

### Conditions for the appearance of induced current

The condition for induced current in a closed conductor is that the number of magnetic field lines through the cross-section S of the coil varies.

In general, an induced current occurs when the following conditions are satisfied:

– When a closed circuit or a part of a closed circuit travels in the magnetic field and cuts the magnetic induction lines.

– When a closed circuit does not travel in a magnetic field, but the magnetic field passes through the circuit, it is a time-varying magnetic field.

### Actual contact

Our country’s 500 kV high voltage power line also acts as a strong electromagnet. If the house is close to this line, we may encounter many uncertainties due to the phenomenon of electromagnetic induction: being electrocuted when touching the corrugated iron roof, metal windows, televisions, phones are quickly damaged …

Fluorescent bulbs are self-illuminating when placed directly under high voltage power lines.

### How to detect the presence of induced current

– use ammeter, galvanometer to identify.

– use a test magnet for identification.

– Can use light bulbs for identification.

## Solve exercises in Physics textbook 9 Lesson 32

### Lesson C1 (page 87 Physics Textbook 9)

Observe how the magnetic field lines (Figure 32.1 Textbook) through the cross-section s of the coil change (increase or decrease) in the following cases:

– Bring the magnet back to the coil in the direction perpendicular to the cross-section S of the coil

– Place the magnet stationary in the coil.

– Take the magnet away from the coil in a direction perpendicular to the coil’s cross-section

– Let the magnet lie still, let the coil move around the magnet.

The answer:

– Bring the magnets back to the coil in the direction perpendicular to the cross-section S of the coil, the number of magnetic lines of force through the section S increases

– Place the magnet at rest in the coil, the number of magnetic lines of force through the cross-section S remains constant.

– Take the magnet away from the coil in the direction perpendicular to the section S of the coil, the number of magnetic lines of force through the cross section S decreases.

– For the magnet to rest, for the coil to move back and forth near the magnet, the number of magnetic field lines through the cross-section S increases.

### Lesson C2 (page 88 Physics Textbook 9)

Compare the results of the test in question 1 with the investigation of the number of magnetic field lines through the section S of the coil, choose the appropriate word to fill in the boxes in Table 1 (Textbook).

The answer:

Do test | Is there an induced current? | Does the number of magnetic field lines through S change? |

Put the magnet back to the coil | Have | Yes and increase |

Let the magnet rest | Are not | No change |

Move the magnet away from the coil | Have | Yes and down |

### Lesson C3 (page 88 Physics Textbook 9)

From table 1 (SGK) infer under what conditions, induced currents appear in closed coils?

The answer:

The induced current in the closed coil occurs when the number of magnetic field lines through the cross-section S of the coil increases or decreases

### Lesson C4 (page 88 Physics Textbook 9)

Applying Comment 2 in the lesson to explain why in the test shown in Figure 31.3 of the textbook, when the electromagnet is closed or disconnected, induced current appears in the closed coil.

The answer:

+ When the circuit is closed, the amperage increases from zero to yes, the magnetic field of the electromagnet increases, the number of magnetic field lines showing the magnetic field increases, the number of magnetic field lines through the S cross section of the coil also increases. , so an induced current appears.

+ When the current in the coil is stable, the magnetic field generated by it will remain constant, that is, the number of magnetic lines of force sent through the S-section of the LED coil will also remain unchanged, so there will be no current in this coil. touch.

+ When the circuit is cut off, the amperage decreases to zero, the magnetic field of the electromagnet decreases, so the number of magnetic field lines showing the magnetic field through the LED coil will decrease → induced current appears to light up the lamp in short time at that time.

### Lesson C5 (page 89 Physics Textbook 9)

Use the conclusion in the lesson to explain why when turning the knob of the dynamo, the bicycle lights come on.

The answer:

Turn the knob of the dynamo, the magnet rotates. When one pole of the magnet is about to wind again, the number of magnetic lines of force through the cross-section s of the coil increases, then an induced current appears. When that pole of the magnet is far from the coil, the number of magnetic lines of force through the cross section s of the coil decreases, at that time induced current also appears.

### Lesson C6 (page 89 Physics Textbook 9)

Please explain why when the magnet rotates as shown in Figure 31.4 of the textbook, induced current appears in the closed coil.

The answer:

When the magnet is rotated, one pole of the magnet (assuming the north pole is about to coil again, the number of lines of force through the coil’s cross section S increases, then an induced current appears. Then the north pole of the magnet away from the coil, the number of magnetic lines of force through the coil’s cross-section S decreases, then induced current also appears

## Physics Quiz 9 Lesson 32 with answers

Lesson 1: When one pole of the magnet is about to or far away from the end of the coil,

A. the number of magnetic field lines through the cross-section S of the coil remains constant

B. the number of magnetic field lines through the cross-section S of the coil is always increasing

C. the number of magnetic field lines through the cross-section S of the coil increases or decreases (variable)

D. the number of magnetic field lines through the cross-section S of the coil is always decreasing

The answer

When bringing a pole of a magnet closer to or away from the end of a coil, the number of magnetic lines of force through the cross section of the coil increases or decreases (variable).

Answer:

Lesson 2: Under what conditions does an induced current appear in a closed coil?

A. When the number of magnetic field lines through the coil cross section is extremely large.

B. When the number of magnetic field lines through the coil cross section is kept unchanged.

C. When there is no magnetic field line through the coil cross section.

D. When the number of magnetic field lines through the coil cross section changes.

The answer

The condition for the appearance of induced current in a closed conductor is that the number of magnetic field lines through the cross-section S of that coil varies.

Answer: EASY

Lesson 3: In which of the following cases, induced current appears in a closed coil?

A. The number of magnetic field lines through the cross-section S of the closed coil is large.

B. The number of magnetic field lines through the cross-sectional area S of the closed coil is kept unchanged.

C. The number of magnetic field lines through the cross-section S of the closed coil varies.

D. The magnetic field through the cross-section S of the closed coil is strong.

The answer

The condition for the appearance of induced current in a closed conductor is that the number of magnetic field lines through the cross-section S of that coil varies.

Answer:

Lesson 4: In the picture below, how does the bar magnet move without creating an induced current in the coil?

A. travel from the outside to the inside of the conduit.

B. Rotation around the axis AB.

C. Rotation around the CD axis.

D. Rotation around the PQ axis.

The answer

We have:

+ The condition for the appearance of induced current in a closed conductor is that the number of magnetic field lines through the cross-section S of that coil varies.

+ When one pole of the magnet is near or far away from the end of a coil, the number of magnetic field lines through the cross section of the coil increases or decreases (variable).

=> The case of rotating the magnet around the PQ axis will not change the number of magnetic lines in the coil => does not generate induced current in the coil.

Answer: EASY

Lesson 5: In the following figure, when the wire frame is rotated around the PQ axis, does the induced current appear in the wire frame?

A. Yes

B. No

C. The induced current is increasing

D. Appears then turns off immediately

The answer

When the wire frame is rotated around the PQ axis, there is no induced current in the wire frame because the number of magnetic field lines through the wire frame does not change.

Answer: REMOVE

Lesson 6: Find suitable words for the blanks: The induced current only appears in the closed coil during the time when there is….through the cross-section of the coil.

A. Variation of amperage

B. Variation of time

C. Variation of induced current

D. Variation of the number of magnetic field lines

The answer

We have: The condition for the appearance of induced current in a closed conductor is that the number of magnetic field lines through the cross-section S of that coil varies.

Answer: EASY

Lesson 7: Why when the magnet rotates in front of a closed coil as tested in the following figure, induced current appears in the coil?

A. Since the amperage in the coil changes

B. Because the voltage in the coil changes

C. Since the induced current in the coil varies

D. Since the number of magnetic field lines through the cross section S of the coil changes

The answer

When the magnet is rotated in front of a closed coil as tested in Figure 32.1, an induced current appears in the coil because when the magnet rotates, the number of magnetic lines of force through the section S of the coil changes.

Answer: EASY

Lesson 8: Using the following tools, we can test for continuous induced current:

A. A magnet and a sealed conduit

B. A magnet, an ammeter and a voltmeter

C. A sealed conduit, a magnet and a unit that make the coil or magnet rotate continuously

D. A sealed conduit, an ammeter and a unit that causes the coil or magnet to rotate continuously

The answer

In order to have a continuous induced current => There should be a change in the number of magnetic field lines through the coil cross section continuously

=> The tools needed to be able to do the test for continuous induced current are: a magnet, a sealed conduit and a unit that makes the coil or magnet rotate continuously.

Answer:

Lesson 9: A student said that: “The condition for induced current to appear in a closed coil is the relative travel between the magnet and the coil”. Is this statement true or false? Why?

A. Yes, because there is always a change in the number of magnetic field lines through the cross-section of the coil

B. Wrong because there is a case that traveling between the magnet and the coil does not make the number of magnetic field lines through the coil cross section change.

C. True because traveling between the magnet and the coil does not produce a change in the number of magnetic field lines through the coil cross section.

D. Wrong because there is always no change in the number of magnetic field lines through the cross-section of the coil

The answer

The student’s statement is wrong because: because there is a case where traveling between the magnet and the coil does not make the number of magnetic field lines through the coil cross section change.

Answer: REMOVE

Lesson 10: Which of the following cases has a different number of magnetic lines of force through the section S of the coil than the other cases?

A. Bring the male to arrange the coil in the direction perpendicular to the cross-section of the coil S

B. Place the magnet at rest in the coil

C. Let the magnet stand still, let the coil move around near the magnet

D. Bring the magnet and coil together

The answer

A, C, D – the number of magnetic field lines through the section S of the coil increases

B – the number of magnetic field lines through the cross-section S of the coil remains constant

Answer: REMOVE

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