"Electromagnet Electromagnetic Relay" Electricity and Magnetism PPT

"Electromagnet Electromagnetic Relay" Electricity and Magnetism PPT

Part One: Knowledge Management

1. electromagnet

Composition: It is a solenoid with an iron core, which is composed of two parts: a coil and an iron core. There is an iron core in the electromagnet to enhance the magnetism of the electromagnet.

Magnetic strength: (1) Relationship with the number of turns of the coil: For electromagnets with the same structure, when the current is constant, the more turns of the coil, the stronger the magnetism.

(2) Relationship with the size of the current: When the number of coil turns is constant, the greater the current, the stronger the magnetism.

Compared with permanent magnets: the presence or absence of magnetism is controlled by the on and off of the current; the strength of the magnetism is controlled by changing the number of turns of the coil and the size of the current; the polarity at both ends is controlled by changing the direction of the current in the coil.

Application: Electromagnets are mainly used in electromagnetic cranes, electric bells, telephones, automatic control systems, etc.

2. Electromagnetic relay

Composition: Composed of electromagnet, spring, armature and contacts.

Working principle: As shown in the figure, it consists of a low-voltage control circuit and a high-voltage working circuit. The low-voltage control circuit is composed of low-voltage power supply, electromagnet and switch; the high-voltage working circuit is composed of high-voltage power supply, contacts and electrical appliances (motor M in the figure). The operator directly contacts the switch of the low-voltage control circuit. Since the voltage is low and the current is weak, it is safer.

Function: Use the on and off of low voltage and weak current circuits to control the on and off of high voltage and strong current circuits to ensure the safety of operators; realize remote operation; realize remote control and automatic control, such as automatic temperature alarms.

Note: An electromagnetic relay is essentially a switch that uses electromagnets to control a working circuit.

Electromagnet Electromagnetic Relay PPT, Part 2: Classification and Research

One type: Electromagnet and its applications

[2018•Shandong Heze]A student used two identical large iron nails to be wound into an electromagnet for an experiment. As shown in the picture, the following statement is correct (　C　)

A. To make the electromagnet stronger, move the rheostat slide to the right

B． The coil of electromagnet B has many turns and the current through B is small.

C. The more pins an electromagnet can attract, the stronger its magnetism.

D. If the upper parts of two electromagnets are brought close to each other, they will attract each other

[Analysis] The slider of the varistor slides to the right, the resistance of the varistor connected to the circuit increases, and the current in the circuit decreases, so the magnetism of the electromagnet weakens, so A is wrong; the coils of the two electromagnets are connected in series, so through the two coils The currents are equal, and the size of the current has nothing to do with the number of turns of the coil, so B is wrong; in the experiment, the magnetic strength of the electromagnet cannot be directly seen. It can be judged by the number of pins it attracts. The more pins it attracts, the more electromagnetism it attracts. The stronger the magnetism of iron, so C is correct; according to the flow direction of the current and Ampere's rule, it can be seen that the upper end of A is the N pole and the lower end is the S pole, and the upper end of B is the N pole and the lower end is the S pole, so the two electromagnets are brought close to each other. , they will exclude each other, so D is wrong. So choose C.

[Method Summary] Factors that affect the magnetic strength of the electromagnet: the size of the current and the number of turns of the coil. The greater the current and the greater the number of turns in the coil, the stronger the electromagnet's magnetism.

Type 2: Electromagnetic relay

[2018•Harbin, Heilongjiang]The picture shows the circuit diagram of the "automatic water level alarm". The container is filled with salt water. L1 and L2 are marked "2.5 V 0.3 A" and "2.5 V 0.5 A" respectively. Light bulb, which of the following statements is incorrect (D)

A. When the liquid level rises to contact metal block B, the electromagnet works, indicating that salt water is a conductor.

B． As the liquid level rises, the liquid pressure on metal block A increases.

C. L1 works for 1 minute and the current does 45 J of work

D. If an electric bell is connected to point M in the circuit, the electric bell will ring to indicate that the water level has reached the warning line.

[Analysis] There is salt water in the container. When the liquid level rises to contact the metal block B, the electromagnet works, indicating that there is current passing through the electromagnet. The salt water is equivalent to a closed switch, so it is a conductor, so statement A is correct; the density of salt water is not Change, from the formula p=ρgh, we know that when the liquid level rises and the depth of the salt water increases, the liquid pressure on the metal block A becomes larger, so the statement B is correct; L1 works for 1 minute and the current does work W=UIt=2.5 V×0.3 A× 60 s = 45 J, so statement C is correct; as shown in the figure, if the electric bell is connected to point M, the bell is connected in series with two light bulbs, so the bell can sound regardless of whether the water level reaches the warning line, so statement D is wrong. So choose D.

Electromagnet electromagnetic relay PPT, the third part: evaluation in class

1. 2018•Qinghai

A. Reduce the number of turns in the coil

B． Use copper core instead of iron core

C. Move slide P toward end a

D. Move slide P toward end b

2. [2018•Jiangxi] When there is no power outage, the fire emergency lights are extinguished; during a power outage, the two lights marked with "36 V" will glow normally. The circuit shown in the figure that meets the requirements is (C)

[Analysis] When there is no power outage in picture A (that is, the 220 V circuit is connected), the electromagnet is magnetic and attracts the armature, causing a light bulb below to glow, so A is wrong; the power supply voltage of the circuit on the right side of picture B is 36 V. The rated voltage of both lamps is 36 V. If both lamps are to emit light normally when the external circuit is powered off, the two lamps should be connected in parallel. However, in the picture, the two lamps are connected in series and cannot emit light normally, so B is wrong; When there is no power outage in Figure C, the electromagnet is magnetic and attracts the armature, and the two bulbs above do not light up (i.e. go out); when there is a power outage, the armature is pulled up by the spring, and the two parallel bulbs above can operate normally at 36 V. light, so C is correct; when there is no power outage in picture D, the electromagnet is magnetic and attracts the armature, and the two bulbs below both light up. However, when there is a power outage, the armature is pulled up by the spring, and both bulbs do not light up, so D is wrong.

3. [2018•Meishan, Sichuan]As shown in the figure, when the switch is closed and the bar magnet is stationary, when the sliding varistor slide P is slid from left to right, the spring will (B)

A. shorten

B． elongation

C. stand still

D. Stretch first and then shorten

[Analysis] As the slide slides from left to right, the resistance of the sliding rheostat connected to the circuit becomes smaller, so the current in the circuit becomes larger, and the magnetism of the energized solenoid is enhanced; as shown in the figure, the current flows from the solenoid Inflow from below. According to Ampere's rule, it can be seen that the upper end of the solenoid is the N pole and the lower end is the S pole. The opposite magnetic poles of the solenoid and the bar magnet have different magnetic poles. Different magnetic poles attract each other. Because the solenoid is energized The magnetism is enhanced, so the solenoid's attraction to the magnet increases and the length of the spring becomes longer.

Electromagnet electromagnetic relay PPT, part 4 content: layered operation

1. [2017•Hunan Chenzhou]As shown in the figure is the circuit diagram of the electromagnet. After the switch S is closed, the slide P of the sliding rheostat is sliding to the right (D)

A. The right end of the electromagnet is the S pole, which increases the magnetism

B． The right end of the electromagnet is the S pole, and the magnetism weakens

C. The right end of the electromagnet is the N pole, which enhances the magnetism

D. The right end of the electromagnet is the N pole, and the magnetism is weakened

2. [2018• Zhangjiajie, Hunan Province The slider of the varistor, the following analysis is correct (B)

A. When the slide moves to the left, the magnetism of the electromagnet weakens and the brightness of the indicator light becomes dim.

B． When the slide moves to the left, the magnetism of the electromagnet increases and the indicator light becomes brighter.

C. When the slider moves to the right, the GMR resistance becomes smaller and the indicator light becomes brighter.

D. When the slider moves to the right, the GMR resistance becomes larger and the indicator light becomes brighter.

3. [2019•Chengde, Hebei Province]The picture below is a schematic diagram of the working principle of photographing a motor vehicle running a red light. The light control switch will automatically close when it receives the light from the red light, and the pressure switch will close when it receives the pressure from the motor vehicle. The camera system can automatically take pictures of illegal vehicles when the circuit is connected. Then the correct one of the following statements is (C)

A. As long as the light control switch receives red light, the camera system will automatically shoot

B． If the light control switch and the pressure switch are connected in parallel, it can also play the same role.

C. The camera system will automatically shoot only when the light control switch and pressure switch are both closed.

D. As long as a motor vehicle passes through an intersection with a pressure switch buried, the camera system will automatically take pictures

[Analysis] Analysis of the meaning of the question shows that the camera system will automatically shoot only when the light control switch and the pressure switch are both closed. Therefore, the two options A and B are wrong; the analysis shows that the light control switch and the pressure switch are mutually exclusive. They are restrained and affect each other, so the two switches can only be connected in series and not in parallel, so option D is also wrong. Choose C.

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