The first chapter sound phenomenon

Acoustic phenomenon

1, the appearance of sound

Everything that makes a sound vibrates, and when the vibration stops, the sound stops.

Sound is produced by the vibration of objects, but not all the sounds produced by vibration can be heard by human ears.

2. Transmission between voices

The transmission of sound needs medium, and vacuum cannot transmit sound.

(1) Sound propagates through various media, such as gas, liquid and solid, and is called medium. Even if the astronauts on the moon talk face to face, they need to rely on radio, because there is no air on the moon and vacuum can't spread sound.

(2) Sound travels at different speeds in different media. Generally speaking, solid >: liquid > air.

The speed of sound in the air is about 340 meters per second.

Step 3 echo

In the process of sound propagation, the sound that people hear again when they encounter obstacles is called echo.

The distinguishing condition between echo and original sound: the echo reaches people's ears more than 0. 1 second later than the original sound. Therefore, the sound must be reflected by obstacles with a distance exceeding 17m, so that people can hear the echo.

Below 0. 1 sec, the reflected sound can only strengthen the original sound.

Echo can be used to measure the depth of the sea or to detect how far an object is from an obstacle.

4. Music

The sound made by an object when it vibrates regularly is called music.

Three elements of musical sound: tone, loudness and timbre.

The sound level is called tone, which is determined by the vibration frequency of the speaker. The higher the frequency, the higher the pitch.

The loudness of sound is related to the amplitude of speaker vibration and the distance from the sound source to the human ear.

The sound quality produced by different vocalists is called timbre, which is used to distinguish different sounds.

5. Noise and its sources

From a physical point of view, noise refers to the sound made by the speaker when it vibrates irregularly. From the environmental point of view, all sounds that hinder people's normal rest, study and work, as well as those that interfere with people's desire to listen, belong to noise.

6, the division of sound level

People use decibels to divide sound levels. 30 dB -40 dB is an ideal quiet environment. If it exceeds 50dB, it will affect sleep; If it exceeds 70dB, it will interfere with the call and affect the work efficiency. If you live in a noise environment above 90dB for a long time, it will affect your hearing.

7. Ways to reduce noise

It can be weakened at the sound source (noise elimination), during propagation (sound absorption) and at the human ear (sound insulation).

Chapter II Light Phenomenon

1, light source: An object that can emit light by itself is called a light source.

2. Light propagates in a straight line in a uniform medium.

The atmosphere is unbalanced. When light hits the ground from outside the atmosphere, it will bend (a mirage, when you see the sun in the morning, the sun is still below the horizon, and the stars are flashing, etc. ).

3. the speed of light

The propagation speed of light in different substances is generally different, and it is the fastest in a vacuum.

The propagation speed of light in vacuum is V = 3× 108 m/s, which is close to this speed in air, 3/4V in water and 2/3V in glass.

4. Application of linear propagation of light.

Many optical phenomena can be explained: laser collimation, shadow formation, lunar eclipse, solar eclipse, pinhole imaging and so on.

5. light

Light: a straight line indicating the direction of light propagation, that is, draw a straight line along the direction of light propagation, and draw an arrow on the straight line to indicate the direction of light propagation (light is imaginary, but actually does not exist).

6. Light reflection

When light shoots from one medium to the interface of another medium, part of the light returns to the original medium, which changes the direction of light propagation. This phenomenon is called light reflection.

7, the law of light reflection

The reflected light is on the same plane as the incident light and the normal line; Reflected light and incident light are separated on both sides of the normal; The reflection angle is equal to the incident angle.

It can be summarized as: "Three lines * * * plane, two lines are separated, and the two angles are equal."

reason

The reflected light is determined by the incident light, and the word "anti" should be used as the head in the narrative.

Conditions of reflection: the junction of two media; Place of occurrence: incident point; Result: Return to the original media.

The reflection angle increases with the increase of incident angle and decreases with the decrease of incident angle. When the incident angle is zero, the reflection angle also becomes zero.

8, two kinds of reflection phenomenon

Specular reflection: parallel rays are reflected by the interface and emitted in parallel in a certain direction, and the reflected rays can only be received in a certain direction (the reflecting surface is a smooth plane).

Diffuse reflection: the parallel light is reflected in different directions after being reflected by the interface, that is, the reflected light can be received in different directions (the reflecting surface is a rough plane or curved surface).

Note: Both specular reflection and diffuse reflection follow the law of light reflection.

9. In the reflection of light, the light path is reversible.

10, the influence of plane mirror on light.

(1) imaging (2) changing the propagation direction of light.

1 1, the characteristics of plane mirror imaging

(1) is an isometric virtual image. (2) The connecting line between the image and the object is perpendicular to the mirror, and the distance between the image and the object and the mirror is equal.

The image and object formed by the plane mirror are symmetrical figures with the mirror surface as the axis, that is, the plane mirror is the vertical line connecting the object and the image.

12, the difference between real image and virtual image

The real image is the gathering of actual light, which can be received by the screen and seen by the eyes.

The virtual image is not formed by the convergence of actual light, but by the intersection of opposite extension lines of actual light, which can only be seen by the eyes and not received by the screen.

13, the application of plane mirror

(1) reflection in water (2) flat mirror imaging (3) periscope

Chapter III Lenses and Their Applications

1, light refraction

When light inclines from one medium to another, the propagation direction generally changes, which is called light refraction.

The refraction of Ricoh light, like the reflection of light, occurs at the junction of two media, except that the reflected light returns to the original medium and the refracted light enters the other medium. Because the propagation speed of light in two different substances is different, the propagation direction changes at the junction of two media, which is the refraction of light.

Note: At the junction of two media, refraction must occur simultaneously.

In refraction, the speed of light must change, while in reflection, the speed of light remains the same.

2, the law of refraction of light

When light is obliquely incident from the air into water or other media, the refracted light is on the same plane as the incident light and the normal line, and the refracted light and the incident light are separated on both sides of the normal line; The refraction angle is smaller than the incident angle; When the incident angle increases, the refraction angle also increases; When the light is perpendicular to the surface of the medium, the propagation direction is unchanged and the refraction of the light path is reversible.

The principle of refraction is divided into three points: (1) Three lines * * * plane (2) Two lines are separated (3) The relationship between the two angles is divided into three situations: ① When the incident light is incident on the vertical interface, the refraction angle is equal to the incident angle and equal to 0; (2) When light is obliquely incident from air to water and other media, the refraction angle is smaller than the incident angle; (3) When light enters the air obliquely from a medium such as water, the refraction angle is greater than the incident angle.

3. The optical path is also reversible in terms of light refraction.

4, lens and classification

Lens: Made of transparent material (usually glass), at least one surface is a part of a spherical surface, and the thickness of the lens is much smaller than its spherical radius.

Classification: convex lens: thin edge and thick center.

Concave lens: thick edge and thin center.

5. Main optical axis, optical center, focal point and focal length

Principal optical axis: a straight line passing through two spherical centers.

Optical center: there is a special point on the main optical axis, through which the propagation direction of light remains unchanged. Focus: The point at which the convex lens can converge the light parallel to the main axis on the main optical axis is called the focus of the lens, which is represented by "F".

Virtual focus: the light parallel to the main optical axis becomes divergent after passing through the concave lens, and the backward extension line of the divergent light intersects with a point on the main optical axis, which is not the convergence point of the actual light, so it is called virtual focus.

Focal length: The distance from the focal point to the optical center is called focal length, which is indicated by "F".

Each lens has two focal points, a focal length and an optical center.

6. The influence of lens on light

Convex lens: convex light.

Concave lens: It diverges light.

7, convex lens imaging law

The object distance (U) should be used for imaging size, virtual image position and image distance (V).

U & gt2f reduces both sides of the real-image lens F.

Oral memory method of convex lens imaging law

"One focus is virtual reality, and the other focus is size; The same side of the virtual image is positive, and the far image of the object becomes larger; The real image is on the opposite side and the object is much smaller. "

8. In order to make the image on the screen "vertical" (upward), the slide should be inserted backwards.

9. The lens of a camera is equivalent to a convex lens, and the film in the camera box is equivalent to a light screen. We adjust the focus ring, not the focal length, but the distance from the lens to the film. The farther the object is from the lens, the closer the film is to the lens.

Chapter IV Changes in the State of Matter

1, temperature: the degree of heat and cold of an object is called temperature.

2. Celsius temperature (symbol: T unit: Celsius)

Shertius of Sweden stipulates: ① The temperature of pure ice-water mixture is set at 0℃; ② The boiling temperature of pure water at 1 standard atmospheric pressure is set to100℃; ③ Divide the temperature of 0 ~ 100℃ into 100 equal parts, each equal part is 1℃.

3. Thermometer

Principle: It is made by using the property of liquid expanding when heated and contracting when cooled.

Structure: glass shell, capillary, glass bubble, scale and liquid.

Usage: Before using the thermometer, pay attention to the measuring range and recognize the dividing value.

When measuring the liquid temperature with a thermometer, the following three points can be achieved:

(1) The glass bulb of the thermometer should be completely immersed in the measured object; (2) Wait until the numbers are stable before reading; (3) When reading, do not take the thermometer out of the liquid, and the line of sight should be flush with the upper surface of the liquid level.

4. The main differences between thermometers, experimental thermometers and thermometers.

A construction method of range division value

When the thermometer is at 35-42℃ and 0. 1℃, there is a mark on the top of the glass bulb, which is convenient for reading when leaving the human body. It should be thrown away before use.

The experimental thermometer must be -20- 100℃, 1℃. You can't read without the measured object, and you can't throw it away.

The thermometer is not -30-50℃ 1℃.

5, melting and solidification

The change of a substance from solid to liquid is called melting, and melting requires heat absorption.

When a substance changes from liquid to solid, it is called solidification, and solidification gives off heat.

6. Melting point and freezing point

Solids can be divided into crystalline and amorphous.

Melting point: all crystals have a certain melting temperature, which is called melting point; Amorphous crystals have no melting point.

Freezing point: the crystal has a certain freezing temperature, which is called freezing point; Amorphous crystals have no freezing point.

The freezing point and melting point of the same substance are the same.

The conditions of crystal melting are: ① reaching the melting point temperature; ② continuing to absorb heat from the outside.

Conditions for liquid to solidify into crystals: ① reaching freezing point temperature; ② Continuously release heat to the outside.

Memorize some common crystals and amorphous.

7. Vaporization and liquefaction

The change of a substance from liquid to gas is called vaporization. There are two different ways of vaporization: evaporation and boiling, both of which absorb heat.

When a substance changes from gas to liquid, it is called liquefaction. There are two different ways of liquefaction: lowering the temperature and compressing the volume, both of which give off heat.

8, evaporation phenomenon

Definition: Evaporation is a phenomenon that liquid can occur at any temperature and only occurs on the surface of liquid.

The factors that affect the evaporation rate are: liquid temperature, liquid surface area and air velocity on the liquid surface.

9, boiling phenomenon

Definition: Boiling is a violent vaporization phenomenon both inside and on the surface of liquid at a certain temperature.

Conditions for liquid boiling: ① The temperature reaches the boiling point; ② Continue to absorb heat.

10, sublimation and solidification

The direct change of matter from solid to gas is called sublimation, and the direct change from gas to solid is called sublimation.

Sublimation and condensation in daily life (frozen wet clothes dry, frost in winter)

Sublimation absorbs heat and condensation releases heat.

mnemonics

Steaming and boiling

discrepancy

Influencing factors of temperature change under severe temperature conditions in the place where it occurred

similar

sublimate

┌—————————┐

Melting vaporization

Solid → liquid → gas (endothermic)

- - - - - - - - - - -

Gas → liquid → solid (endothermic)

Liquefaction and solidification

└—————————┘

sublimate

Chapter V Current and Circuit

Simple electrical phenomenon circuit

1, charge is also called electricity, which is a property of matter.

There are only two kinds of charges: positive charge and negative charge. The same charge as the glass rod rubbed by silk is called positive charge. The same charge as the rubber rod rubbed by fur is called negative charge.

The same charges repel each other, but different charges attract each other.

(3) charged body has the characteristics of attracting light and small objects.

The amount of charge is called electricity.

⑤ Electroscope: an instrument used to check whether an object is charged. Its working principle is that like charges repel each other.

2. Conductors and insulators that are easy to conduct electricity are called conductors, and aqueous solutions of metals, human bodies, the earth, acids, alkalis and salts are common conductors. Objects that do not conduct electricity easily are called insulators. Rubber, plastic, glass and ceramics are all common insulators.

The boundary between conductor and insulator is not absolute. When conditions change, insulators can also become conductors. For example, glass is a good insulator at room temperature and becomes a conductor at high temperature. For example, under normal circumstances, there are few charged particles (free electrons and positive and negative ions) that can move freely in gas, so gas is a good insulator. However, under the action of strong electric field force or when the temperature rises to a certain extent, gas discharge occurs due to gas ionization. At this time, the gas changes from insulator to conductor. Therefore, there is no absolute boundary between conductor and insulator. When conditions change, insulators and conductors can be transformed into each other.

3. The circuit will be a current path connected with electrical appliances, power supplies and switches by wires.

Three states of a circuit: a circuit that is connected everywhere is called a path or a closed circuit, at which time a current flows; Open circuit is called open circuit, when there is no current in the circuit; A circuit that directly connects the two poles of the power supply with wires is called a short circuit.

4. Circuit connection mode Series circuit and parallel circuit are the basic ways of circuit connection.

The basic method to identify the circuit is the current method, that is, when the current passes through the components on the circuit, there is no shunt phenomenon, and the connection relationship of these components is series. If there is shunt phenomenon, the connection relationship between elements on several shunt branches is parallel.

5. Circuit diagram symbols are used to display the circuit connection diagram.

15. Ohm's law of current and voltage resistance

1. Generation of current: A current is formed due to the directional movement of charges.

Current direction: ① The direction of positive charge directional movement is the direction of current.

The current formed in a metal conductor is the directional motion of charged free electrons, so the direction of current in metal is opposite to that of free electrons. The current formed in the conductive solution is formed by the directional movement of positive and negative ions, so the direction of current in the conductive solution is the same as that of positive ions, but opposite to that of negative ions.

The current in the circuit starts from the positive pole of the power supply and flows back to the negative pole of the power supply through appliances, switches and wires.

There are three effects of current: thermal effect, magnetic effect and chemical effect, among which thermal effect and magnetic effect are inevitable.

2. Current intensity: the physical quantity representing current, referred to as current for short.

① Definition: The charge passing through any section of a conductor every second is called current intensity, which is called current for short. I = q/t。

② Unit: Ampere (a) Common units are milliampere (mA) and microampere (μA).

Conversion between them:1a =103ma =106μ a.

③ Measurement: ammeter.

In order to measure the current intensity of a certain part of the circuit, an ammeter must be connected in series with this part of the circuit. When the ammeter is connected in series in the circuit, the current must flow into the ammeter from the "+"terminal and flow out from the "-"terminal.

Estimate the current intensity before and after measurement, and then connect the ammeter with appropriate range into the circuit. When turning off the electric key, you must try to touch it first. If the pointer of the ammeter swings violently and exceeds the full scale, you must change to an ammeter with a larger range.

When using an ammeter, it is absolutely forbidden to connect the two terminals of the ammeter directly to the poles of the power supply through electrical appliances to prevent the ammeter from burning out due to excessive current. Because the resistance of ammeter is very small, it is absolutely forbidden to connect ammeter in parallel at both ends of electrical appliance or power supply, otherwise it will cause short circuit and burn out ammeter.

When reading, before reading the value displayed by the pointer, you must first read the corresponding range and the minimum scale value of the range.

3. Characteristics of series circuit current: The currents in all parts of the series circuit are equal. I = I 1 = I2。

Characteristics of parallel circuit current: the current in the main circuit of parallel circuit is equal to the sum of currents in each branch I = I 1+I2.

4. Voltage is the cause of current formation, and power supply is the device that provides voltage.

Unit of voltage: Volt, abbreviated as Volt, with symbol V. 。

The commonly used units are: Megavolt (mV), kilovolt (KV), millivolt (MV) and microvolt (μV).

Conversion between them:1mv =103kv1kv =103v1v =103mv1mv =103μ v.

② Some common voltage values: one dry battery 1.5 volts, one lead battery 2 volts, the human safe voltage is not higher than 36 volts for lighting circuit, and the voltage of 220 volts for power supply circuit is 380 volts.

③ Measurement: voltmeter

To measure the voltage at both ends of a certain part of a circuit or electrical appliance, a voltmeter must be connected in parallel with this part of the circuit or electrical appliance, and the "+"terminal of the voltmeter must be connected to one end of the current inflow circuit.

Each voltmeter has a certain range, that is, the range. When using, it must be noted that the measured voltage shall not exceed the measuring range of voltmeter. If the voltage value of the tested circuit or electrical appliance is not accurate enough, try to touch it when closing the electrical key. If it is found that the pointer of the voltmeter swings quickly and exceeds the maximum range, a voltmeter with a larger range must be selected for measurement. Before measuring the voltage with a voltmeter, carefully observe the voltmeter to see how many ranges it has.

6. Voltage characteristics of series circuit: The total voltage of series circuit is equal to the sum of the voltages of all parts. U = U 1+U2。

Characteristics of parallel circuit voltage: the voltage at both ends of each branch of parallel circuit is equal. U = U 1 = U2。

7. Resistance: Resistance is a property of the conductor itself and a physical quantity that represents the resistance of the conductor to current. Regardless of the voltage across the conductor and the current through the conductor.

The unit of resistance: ohm, which stands for the symbol ω.

Commonly used units are: Megaohm (mω) Kilohm (kω). Their conversion is:1mω =106ω1kω =103ω.

8. Factors that determine the size of resistance: The resistance of a conductor is related to its length, cross-sectional area, materials constituting the conductor and the temperature of the conductor.

9. Sliding rheostat: an instrument that changes the resistance value by changing the length of the wire connected to the circuit.

Connect: one action at a time: change the current in the circuit.

Nameplate meaning: "100Ω 2A" means that the maximum resistance is100Ω and the maximum allowable current is 2a.

Note: When the sliding rheostat is connected to the circuit, the slider P should be moved to the position with the largest resistance value of the rheostat to limit the current in the circuit and protect the circuit.

10. Varistor box: an instrument that changes the resistance by changing the number and resistance of fixed-value resistors connected to the circuit. There are two kinds of rheostat: knob type and plug-in type, which are assembled by a group of resistance wires with different resistance values. By adjusting the knob on the rheostat box or pulling out the copper plug, the resistance can be changed discontinuously, and the resistance value can be read directly.

1 1, ohm's law

Content: The current in a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. Formula: I = u/r.

12. series resistance: the total resistance of the series circuit is equal to the sum of all series resistances. R total = r 1+R2.

13. Resistance parallel connection: the reciprocal of the total resistance of the parallel circuit is equal to the sum of the reciprocal of all parallel resistances. 1/RTotal =1/r1+1/r2.

14, series voltage division, the voltage division is proportional to the resistance; Shunt shunt, shunt is inversely proportional to resistance.

Method introduction

Method of distinguishing series circuit from parallel circuit

(1) element connection method analyzes the connection method of circuit elements in the circuit. Series circuits are one after another, and parallel circuits are connected between two points.

(2) The current path method starts from the positive pole of the power supply and analyzes the current path along the current direction until the negative pole of the power supply. If there is only one loop, it is series; If the current path has several branches, it is a parallel circuit.

(3) Remove an element in the circuit by the element elimination method, and connect it in series if there is an open circuit; If one component in the circuit is removed, other components can still work normally.

Sixteen, electricity, electricity, domestic electricity

1. Electric work: The work done by current is called electric work. The process of electric current doing work is the process of converting electric energy into other forms of energy.

Calculation formula: W = UIT = PT = T = I2RT = UQ (where W = T = I2RT is only applicable to pure resistance circuits).

Unit: Joule (j) Common unit kWh1kwh = 3.6×106 j.

Measurement: watt-hour meter (instrument for measuring the power consumption of household appliances)

Connection: ① series connection in the main circuit of the household circuit; ② "1, 3" in "2,4" out; "1, 2" fire "3,4" zero

Parameter: "220V 10A(20A)" means that the electric energy meter should be used in the 220V circuit; The rated current of the watt-hour meter is 10A, and the current cannot exceed 20A. The total power of electrical appliances in the circuit cannot exceed 2200 W；; "50Hz" means that the electric energy meter should be used in the circuit with AC frequency of 50Hz; "3000R/KWh" means that the dial of the electric energy meter rotates 3000 revolutions every time the working circuit consumes 1KWh.

The formula for indirectly measuring electric power by electric energy meter is p = × 3.6× 106 (w).

2. Electric power: Electric power is the work done by current in unit time, which is equal to the product of current and voltage. The unit of electric power is watt. Calculation formula: p = w/t = ui = = i2r (where p = = i2r is only applicable to pure resistance circuits).

3. The difference and connection between rated power and actual power: the rated power is determined by the electrical appliance itself, and the actual power is determined by the actual circuit. Connection: P real = () 2p, which can be understood as 1/n when the voltage across the electrical appliance becomes 1/n2 of the original power.

The brightness of a small light bulb is determined by the actual power of the light bulb.

5. Joule's Law: The heat Q generated by current passing through a conductor is proportional to the square of current I, the resistance R of the conductor and the energization time T. The calculation formula is: Q = I2RT = UIT = T (where Q = UIT = T is only applicable to pure resistance circuits).

6. Electric heater: The main components are heating elements, which are made of materials with high resistance and high melting point. Its principle is the thermal effect of current.

7. Household circuit: It consists of power cord, watt-hour meter, switch, fuse, electrical appliance, socket and other components.

① The incoming line of home circuit is equivalent to the power supply of home circuit, and consists of two wires, one is live wire and the other is zero wire, and there is a voltage of 220V between live wire and zero wire.

Switches and fuses must be connected to the live wire of the circuit. This switch is connected to the live wire. When the switch is pulled to cut off the circuit, all parts of the circuit are separated from the live wire, so that the human body will not get an electric shock when touching these parts, and it is more convenient to maintain the circuit, which can make the whole circuit safer.

(3) The switch of the electric lamp should be connected between the live wire and the lamp holder (or lamp holder), and whether the switch is installed correctly can be checked by electroprobe. Unscrew the light bulb, close the switch, touch the two terminals of the lamp holder with the tip of electroprobe, and one neon tube will glow, then turn off the switch, and then touch the two terminals with electroprobe. If both of them don't shine, the switch is installed correctly; If there is a lamp, the switch is connected between the zero line and the lamp holder and should be corrected.

④ Fuses used in general lighting circuits are made of lead-antimony alloy with high resistivity and low melting point. When the current in the circuit exceeds the fusing current of the fuse, the fuse will fuse immediately, thus disconnecting the circuit, thus protecting the electrical appliance and avoiding fire.

The principle of selecting fuse should use its rated current slightly greater than or equal to the normal working current of the circuit.

If the fuse is replaced by copper wire in the lighting circuit, when the current exceeds the rated current, the copper wire will not fuse and will not play a safety role.

Electric shock: an injury accident caused by a certain intensity of current passing through the human body.

9. Common sense of safe electricity use: Don't touch charged bodies with voltage higher than 36V or close to high-voltage charged bodies. Surface-mounted sockets should be installed at a distance of 1.8m from the ground, and household appliances such as electric fans and washing machines should be grounded.

mnemonics

Seventeen, electricity and magnetism

1, magnet: the property that an object can adsorb iron, cobalt, nickel and other substances is called magnetism, and an object with magnetism is called a magnet.

Magnets have iron attraction and directionality.

2. magnetic pole: the place with strong magnetism on the magnet is called magnetic pole. Magnets have two magnetic poles, called North Pole, South Pole or North Pole and South Pole. Magnetic poles of the same name repel each other, but magnetic poles of different names attract each other.

3. Magnetic field: There is a magnetic field around the magnet, and the basic property of the magnetic field is to generate magnetic force on the magnet placed in it. The magnetic field is directional, and the magnetic field direction at a certain point in the magnetic field is the direction pointed by the north pole when the small magnetic needle is at rest at that point.

4. Magnetic induction line: a curve that vividly describes the space magnetic field, referred to as magnetic induction line. The density of magnetic induction line indicates the strength of magnetism, and the arrow of magnetic induction line indicates the direction of magnetic field.

5. Geomagnetic field: The earth is a huge magnet, and the magnetic field around the earth is called geomagnetic field. The south pole of the geomagnetic field is near the geographical north pole, and the north pole of the geomagnetic field is near the geographical south pole. Shen Kuo first proposed the magnetic declination.

6. Oster experiment: it shows that there is a magnetic field around the current, so as to find the magnetic effect of the current. The magnetic field distribution of electrified spiral tube is similar to that of bar magnet. The distribution of magnetic poles can be judged by the right-handed spiral rule.

Electromagnet: It consists of iron core and coil. It is made according to the principle of magnetic enhancement after the electrified coil is inserted into the iron core.

Its magnetic field strength is related to the presence or absence of iron core, current and coil turns.

7. Electromagnetic induction phenomenon: When a part of the conductor of the closed circuit cuts the magnetic induction line in the magnetic field, there is induced current in the conductor. The direction of induced current is related to the direction of conductor movement and the direction of magnetic induction line. Faraday discovered it.

Generator: a machine that converts mechanical energy into electrical energy. The principle is electromagnetic induction.

9. Effect of magnetic field on charged conductor: Charged conductor is forced in magnetic field, and the direction of force is related to the direction of current in conductor and the direction of magnetic induction line.

10, DC motor: a machine that converts electrical energy into mechanical energy. The DC motor is manufactured according to the principle that the energized coil is forced to rotate around the shaft in a magnetic field. The reason why the coil can rotate continuously is that ① the coil has inertia, and when it reaches the equilibrium position, it can cross the equilibrium position due to inertia; ② When it crosses the equilibrium position, the commutator can change the current direction in the coil in time.

1 1, direct current: current with constant direction alternating current: current with periodically changing magnitude and direction.

The frequency of alternating current in China is 50Hz, which means that the current changes 50 cycles per second and the direction changes 100 times.