Echo Measurement of Distance: 2s=vt
Section 1: The Production and Propagation of Sound
I: The Production of Sound
Highlights Definitions:
1 Sound is produced by the vibration of an object
2 Vibrations can be sounded
Key Points:
1 Everything that objects that make sound are vibrating
2 Sound is produced by vibrations of objects
3 Vibrations of the object where the sound is occurring stops and so does the occurrence
Doubt:
1 Everything that is making sound vibrates, solids, liquids, and gases can produce sound due to vibrations.
2 "When vibration stops, occurrence stops" is different from "When vibration stops, occurrence disappears". Vibration stops, just no longer sound, but the original sound will continue to exist and continue to spread outward.
Two: the propagation of sound
Key definitions:
1 The propagation of sound requires a medium
2 The propagation of sound in the form of a wave, which is called a sound wave
Key points:
1 The material that can propagate the sound is called the medium
2 The medium of sound: solid, gas, liquid
3 Vacuum can not transmit sound
3 The vacuum is not a sound. Cannot transmit sound
Focus:
Sound travels outward in waves. Because of the vibration of an object, the air on both sides of the object forms sparse and dense fluctuations that propagate over long distances, which are called sound waves
Three: Speed of Sound and Echo
Highlighted Definitions:
The speed at which sound travels is described by the speed of sound, which is equal to the distance that the sound travels in each second. The magnitude of the speed of sound is related to the type of medium and also to the temperature of the medium.
Key points:
1 The distance that sound travels per unit of time is called the speed of sound
2 The speed of sound is related to the type of medium. It generally travels fastest in solids, followed by liquids, and slowest in gases
3 The speed of sound is related to the temperature of the moderation. Generally in gases, the higher the temperature, the faster the speed of sound
4 Sound in the process of propagation, after encountering obstacles are reflected back, people can distinguish from the original, so that the sound waves reflected back is echo.
Focus:
Sound travels at a speed of 340m/s in air at 15°C
Expansion:
1 Conditions for distinguishing between the original sound and the echo:
1) the echo arrives at the human ear 0.1s or more later than the original sound; and 2) the source of the sound is at least 17m away from the obstacle
2 Role of echo:
1) to reinforce the original sound; and 2) to strengthen the original sound. p>① Strengthening the original sound; ② Echo localization; ③ Echo ranging
3 Echo ranging: 2s=vt
Section 2: How We Hear Sound
I: How to Hear Sound
Focus on the definition of:
In the whole process of transmission of sound to the brain, any part of the occurrence of an obstacle, a person will lose the sense of hearing. But if only the conduction is impaired, and can find a way to pass the vibration to the auditory nerve through other ways, people can also be able to perceive the sound
Essentials:
1 The structure of the human ear: the outer ear (the auricle, the external auditory canal) the middle ear (tympanic membrane, the auditory ossicles) the inner ear (the semicircular canals, the vestibule, the cochlea)
2 Pathways to hear the sound: vibration of an object → medium → the eardrum or skull → the auditory nerve → hearing
2 The pathway to hear sound: object vibration → medium → tympanic membrane or skull → the auditory nerve → produce hearing
Difficulties:
If the tympanic membrane and the auditory ossicles that conduct sound are damaged, hearing will be reduced, called conductive deafness, but there are also other ways to transmit the vibration to the auditory nerve, the person can continue to hear the sound; if the cochlea, the auditory center or the nerves related to hearing are damaged, hearing will be reduced, or even loss of hearing is called neurological deafness, generally incurable. deafness, generally incurable.
Expansion:
Conditions for hearing sound:
①normal auditory system; ② the vibration frequency of the object reaches the auditory range of the human ear; ③ the sound is sufficiently loud; ④ there is a medium of propagation
Two: Bone conduction and the binaural effect
Focus on the definition of:
Sound passes through the skull, and the jawbone penetrates into the auditory nerve, causing hearing. cause hearing. In science, this way of sound conduction is called bone conduction
Essentials:
Bone conduction pathway: vibration of an object → sound waves → skull or jawbone → auditory nerve
Essentials:
The conditions of the binaural effect:
1) for the same sound, the intensity of the two ears are different sizes; 2) for the same sound, the two ears are different times. Feel the time of the same sound is different; ③ for the same sound, the two ear rods are subjected to different vibration steps
Section 3: the characteristics of sound
One: Pitch
Focus on the definition of
1 object vibration of the fast, the tone issued by the high; vibration of the slow, issued by the tone is low
2 vibration of an object in each second -Frequency indicates how fast or slow an object vibrates. Frequency determines the pitch of a sound. The unit of frequency is Hertz, or Hz for short, and the symbol is Hz
3 A sound with a frequency higher than 20,000 Hz is an ultrasonic sound; a sound lower than 20 Hz is an infrasound sound
Questions:
1 Pitch refers to the height of the sound, which is what we normally refer to as the coarseness or thinness of the sound, not the size of the sound, or the timbre of the sound.
2 In the same medium and temperature, sounds with different frequencies travel at the same speed.
Expand:
What is the pitch of a tone related to?
The level of pitch is related to a number of factors such as the shape and size of the sound producing body and the nature of the material used.
Two: Loudness
Focused Definitions:
1 The strength (size) of a sound is called loudness
2 Amplitude is used in physics to describe the magnitude of an object's vibration. The greater the amplitude of an object, the greater the loudness of the sound produced.
Key points:
1 Loudness in physics refers to the strength of a sound, and in life it refers to the size of the sound as perceived by the human ear.
2 The loudness of an object perceived by the human ear is related to the distance from the sound-producing body.
Focus:
1 Loudness is related to the amplitude of the sound source, the greater the amplitude, the greater the loudness; with the distance from the source, the greater the distance, the smaller the loudness.
2 Pitch and loudness are two fundamentally different characteristics that have nothing to do with each other.
Three: Tone
Focus on the definition:
1 Frequency determines the pitch of the sound, and the amplitude determines the loudness of the sound.
2 The material of different sound producing body, the structure is different, the timbre of the sound will be different.
Key points:
Tone is the quality of sound, that is, the quality of sound.
Expansion:
The timbre of a person's voice changes with age, as well as diet, and health factors. Exercise maintains a beautiful tone.
Section 4: Hazards and Control of Noise
I: Sources of Noise
Focused Definitions:
1 From the physical point of view, noise is the sound emitted by a sound-producing body when it vibrates in an irregular manner; from the environmental point of view, all the sounds that prevent people from resting, studying and working normally, and those that interfere with the sounds that people want to listen to, belong to Noise.
2 The waveform of noise is irregular and disorganized.
Difficulty:
The fundamental difference between musical sound and noise is that musical sound is produced by the regular vibration of a sound-producing body, and the waveform is regular; noise is produced by the irregular vibration of a sound-producing body, and the waveform is disorganized.
Two: the classification of the grade of noise
Focus on the definition:
1 People use the decibel (the symbol is dB) as a unit to express the grade of sound strength. Human hearing is 20Hz -----20000Hz. 0dB: people can just hear the faintest sound. 30-40dB: the more ideal quiet environment, in order to ensure rest and sleep, the sound can not exceed 50dB, in order to ensure work and study, the sound can not exceed 70dB, in order to protect the hearing, the sound can not exceed 90dB .
2 The three stages of sound from generation to cause hearing:
1) vibration of the sound source to produce sound; ② air and other media propagation; ③ vibration of the tympanic membrane
Expansion:
What are the hazards of noise can be divided into several categories?
The hazards of sound-making can be divided into physiological hazards, mental hazards and physical hazards. Not too strong noise, so that people feel bored; relatively strong noise, so that people feel harsh and uncomfortable, time will cause noise deafness, but also cause cardiac arrhythmia, elevated blood pressure, indigestion and other symptoms; stronger noise, a few minutes time will make people dizzy, nausea, vomiting, like seasickness; very strong noise will also affect the development of the fetus, impede the development of the child's intellect, and even the direct cause of human and animal deaths. animals' deaths.
Three: controlling noise
Focus on the definition:
Three aspects of controlling noise:
①Preventing noise from being produced; ②Blocking the propagation of noise; ③Preventing noise from getting into the ear
Points to note:
Anechoic (from the source of the sound); sound absorption (weakened in the process of propagation); sound isolation (weakened at the human ear)
Section 5: Utilization of Sound
I: Sound and Information
Essentials:
1 Echolocation
2 Sonar Ranging to Detect Fish
Questions:
The concept of sound is broader, including ultrasonic, infrasound, etc.; sound refers to the sound that a human being is able to perceive
Essentials:
Sound can Convey information
Difficulty:
Ultrasonic waves can be used to accurately obtain information about diseases inside the human body, which is called "ultrasound". When examining the body with ultrasound, because each part of the body's organs reflect sound waves differently, the use of computerized image display equipment can clearly display the structure of the body's internal organs on the screen, and based on the image, the doctor can quickly find out where the lesions are located, and ultrasound exploration does not cause harm to the human body. This is different from the "X-ray"
Two: sound and energy
Essentials:
Objects vibration → produce sound waves → energy transfer → sound waves can transfer energy
Essentials:
Ultrasonic waves can be used to clean precision machinery; surgeons can use the ultrasonic vibration to remove the human body. ultrasonic vibrations to remove stones from the body.
Chapter 2: The phenomenon of light Basics
1. light source: its own light-emitting objects. The sun is a natural light source, electric lamps, candle flame is a man-made light source. The moon and all the stars are not light sources.
2. Light travels in a straight line in a homogeneous medium. Can explain the formation of shadows and imaging of small holes.
3. The speed of light in a vacuum is the fastest in the universe and is represented by the letter c: c=3×108 m/s The speed of light in water is about 3/4 of that in a vacuum
The speed of light in glass is 2/3 of that in a vacuum
4. Light reflects off water, glass, and the surfaces of many other objects. The reflection of light obeys the law of reflection. (1) reflected rays, incident light and the normal in the same plane (2) reflected rays, incident light on both sides of the normal (3) the angle of reflection is equal to the angle of incidence
5. In the phenomenon of reflection, the path of light can be reversed. Reflection is divided into specular reflection and diffuse reflection. Specular reflection: smooth surface, parallel light incident, the reflected light is still parallel. Diffuse reflection: rough surface, parallel light incident, reflected light in all directions.
6. When light is projected obliquely from one medium into another, the direction of propagation is deflected, and this phenomenon is called refraction of light. When refraction occurs, emission must also occur. Refraction phenomenon in the light path is also reversible.
7. Light from the air into water or other media, refracted light is deflected in the direction of the normal. Law of refraction of light: three lines **** surface, two lines on the side, the two angles are not equal (air in the larger angle) refraction phenomenon: pen misalignment, shallow pools, forked fish in the water, mirage, etc.
8. A beam of white light (sunlight) through the prism decomposed into red, orange, yellow, green, blue, indigo and violet seven colors of light is called the phenomenon of dispersion of light. That white light is not monochromatic light, but a variety of monochromatic light composite light. The rainbow is the sunlight is dispersed by the water droplets and become.
9. The three primary colors of light: red, green, blue pigment three primary colors: cyan, yellow, magenta The color of transparent objects through its color light to determine the color of opaque objects by the color of the light reflected from it.
10, infrared light is located outside the red light, all objects are constantly emitting infrared light, the higher the temperature of the object, the more infrared radiation, the object radiates infrared radiation is also absorbing infrared light. Infrared role:
① thermal role: heating food thermogram diagnosis ② infrared remote sensing: earth survey, search for water, monitoring forest fires, etc. ③ remote control: television, air conditioning, etc.
11. Ultraviolet is located in the ultraviolet outside the sun is an important source of natural ultraviolet light. Ozone can absorb ultraviolet light to avoid excessive ultraviolet damage to the human body. UV role: ① sterilization: hospital UV lamps ② UV fluorescence effect: banknote detectors, anti-counterfeiting ③ appropriate UV irradiation is conducive to the body's synthesis of vitamin D, to promote the body's absorption of calcium, the growth of the human skeleton and the health of the benefits.
Chapter III lenses and their applications
1. thick in the middle of the thin edge of the lens is called a convex lens, the edge of the thick middle of the thin lens is called a concave lens. Light rays passing through the center of light do not change the direction of propagation.
2. A convex lens has two real focal points and the distance from the focal point to the center of light is called the focal length. A concave lens has two imaginary focal points.
3. Convex lenses converge light rays and concave lenses diverge light rays.
4. Three special light rays: ① Light rays passing through the center of light do not change the direction of propagation. ② light rays parallel to the main optical axis after refraction over the focal point, for concave lenses, its focal point is the imaginary focal point, is the reverse extension of the refracted light rays over the focal point ③ light rays over the focal point after refraction parallel to the main optical axis. For concave lenses it is the imaginary focus, which is the forward extension of the incident light across the focal point.
5. The lens of the camera is a convex lens, the role of the focusing ring is to adjust the distance from the lens to the film, when shooting close-ups, the lens extends forward,
when shooting distant landscapes, the lens shrinks back, the aperture to control how much light enters the shutter to control the time of exposure.
6.
u>2f inverted zoomed out real camera
u=2f inverted equal real
f<u<2f inverted zoomed in real projector
u=f not imaging
u<f upright zoomed in virtual magnifying glass
double focal length divided into real and virtual, double focal length divided into small and large The image of the real is inverted and the imaginary is positive, the size of the image is determined by the distance, the image follows the object.
7. The eye is like a camera, the lens is equivalent to a convex lens, the retina is equivalent to a light screen. The distance is 25cm. Hyperopia can see objects in the distance but not near objects, the lens is too thin, the image in the retina after; myopia can see near but not far away objects, the lens is too thick, the image in the retina only before.
8. Myopic eyes should have concave lenses and farsighted eyes should have convex lenses. Degree of spectacles = 100 × focal length Focal length = 1/f
9. The eyepiece and objective lens of a telescope are both convex lenses; the eyepiece is equivalent to a magnifying glass, and the objective lens is equivalent to a camera lens. The eyepiece and objective lens of a microscope are also convex lenses; the eyepiece is equivalent to a magnifying glass and the objective lens is equivalent to a projector lens.
Chapter 4 Changes of State
1. Temperature is how hot or cold an object is.
2. Thermometer principle: the thermal expansion and contraction of the nature of the liquid made. Note before use: ① observe its range ② recognize its graduated value, when using Note: ① thermometer glass bubble all into the measured liquid, do not touch the bottom of the container or the container wall, ② thermometer glass bubble placed in the liquid for a while, after stabilization in the readings ③ readings, the thermometer can not be left (in addition to thermometers) the measured liquid and when the line of sight and the thermometer liquid column is level.
3. substances from one state to another state is called a change of state. Substances from a solid state into a liquid state called melting, from a liquid state into a solid state called solidification. Melting absorbs heat, solidification exothermic. Solids are divided into crystalline and amorphous.
4. The change of a substance from a liquid to a gas is called vaporization, and the change from a gas to a liquid is called liquefaction. Vaporization absorbs heat, liquefaction exothermic. Vaporization is divided into evaporation and boiling. Evaporation phenomenon: at any temperature, occurs on the surface of the liquid, slow vaporization phenomenon. Factors affecting evaporation: ① the temperature of the liquid ② the surface area of the liquid ③ the speed of air flow on the surface of the liquid Boiling: at a certain temperature, in the liquid and the surface of the phenomenon of intense vaporization.
5. Liquefaction has two methods: lowering the temperature, compressed volume.
6. substances from solid to gas is called sublimation, sublimation absorbs heat, from gas to solid is called condensation, condensation exothermic.
Chapter 5: Electricity and Circuits
1. An object becomes electrically charged by friction, which is called frictional electricity, and an electrically charged object attracts small, light objects. There are only positive and negative charges in nature. Silk rubbed glass rods with a positive charge, fur rubbed rubber rods with a negative charge, the same kind of charge repel each other, different kinds of charges attract each other.
2. The amount of charge is called charge. Unit: coulomb (c) meta-charge is the smallest charge e = 1.6 × 10-19 Atoms have a positively charged nucleus and negatively charged electrons. Normally the nucleus with a positive charge and the total number of electrons outside the nucleus **** with an equal number of negative charges, do not show electrical, but get the electron on the apparent negative, lose the electron on the apparent positive charge.
3. Directional movement of charge (positive or negative) to form an electric current. Positive charge direction of movement is specified as the direction of the current. Power supply is a device to provide electrical energy, electrical appliances are devices that consume electrical energy, switches control the on and off of the circuit, wires connect the circuit role.
4. Outside the power supply: the direction of the current from the power supply to the positive pole to the appliance and then to the negative pole, inside the power supply: the direction of the current from the negative pole of the power supply to the positive pole.
5. Open circuit: the circuit is connected everywhere, the appliances work normally. Open circuit: open circuit, there is no current in the circuit, the appliance can not work. Short-circuit: the wire is connected directly to both ends of the power supply without passing through the appliance.
6. Good conductor of electricity is called a conductor, not good conductor of electricity is called an insulator. Metals conduct electricity by free electrons, acid and alkali solutions conduct electricity by positive and negative ions.
7. The current represents the strength of the current physical quantity, expressed in I. (Single A) 1A = 1000 m A 1m A = 1000uA
8. Ammeter Note (two to two do not): ① ammeter to be connected in series in the circuit ② current from the "+" terminal into the ammeter, from the "-" terminal flow at the ammeter. "terminal flow from" + "ammeter ③ measured current do not exceed the range of the ammeter ④ do not go through the electrical appliances and the ammeter is connected directly to the ends of the power supply. It should also be noted: ① before using the ammeter, you should observe the ammeter pointer is pointing to zero, if not pointing to zero, should be adjusted to zero ② with the test contact method of selecting the range, to start with a large number of ranges of the terminals.
Current in a series circuit is equal everywhere, and the current in a parallel circuit is equal to the branch current
Summary of second-year physics
A, the circuit
The formation of electric current: the directional movement of charge to form an electric current. (Any directional movement of charge creates an electric current).
Direction of current: from the positive pole of the power supply to the negative pole.
Power supply: a device that provides a continuous current (or voltage).
Power supply is the conversion of other forms of energy into electrical energy. For example, a dry cell converts chemical energy into electrical energy. A generator converts mechanical energy into electrical energy.
Conditions for a continuous current: there must be a power source and a closed circuit.
Conductor: An object that conducts electricity easily is called a conductor. Such as: metal, human body, earth, salt water solution.
Insulator: not easily conductive objects called insulators. Such as: glass, ceramics, plastic, oil, pure water.
Circuit composition: by the power supply, wire, switch and electrical components.
There are three states of the circuit: (1) through: the circuit is called through; (2) open: the circuit is called open; (3) short circuit: directly connected to the wire on the power supply poles of the circuit is called a short circuit.
Circuit diagram: A diagram that shows the connections of a circuit using symbols is called a circuit diagram.
Series: Connecting components one by one in sequence is called series. (When disconnected at any point, the current disappears.)
Parallel: Connecting components in parallel is called a parallel connection. (Each branch is unaffected by each other.)
Two, current
International unit: Ampere (A); commonly used: milliampere (mA), microampere (A), 1 Ampere = 103 milliampere = 106 microampere.
Measurement of current instrumentation is: ammeter, it is the use of the rules:
① ammeter to be connected in series in the circuit;
② current from the "+" terminal into the "-" terminal;
③ measured current do not exceed the range of the ammeter;
4 not allowed to connect the ammeter to the power supply poles without the use of electrical appliances.
4) Never connect the ammeter to the poles of the power supply without passing through the appliance.
The ammeter commonly used in the laboratory has two ranges: ① 0 ~ 0.6 amps, the current value expressed in each cell is 0.02 amps;
② 0 ~ 3 amps, the current value expressed in each cell is 0.1 amps.
Three, voltage
Voltage (U): Voltage is the cause of current in a circuit, the power supply is a device that provides voltage.
International units: volts (V); commonly used: kilovolts (KV), millivolts (mV). 1 kV = 103 volts = 106 mV.
Measurement of voltage instrument is: voltmeter, the use of rules:
①voltmeter to be connected in parallel in the circuit;
②current to be from the "+" terminal into the "-" terminal;
③measured voltage do not exceed the range of the voltmeter;
Laboratory commonly used voltmeter has two ranges: ① 0 to 3 volts, each cell represents the voltage value is 0.1 volts. voltage value is 0.1 volts;
② 0 to 15 volts, each cell indicates the voltage value is 0.5 volts.
Memorize the voltage values: ① 1 dry cell voltage 1.5 volts; ② 1 lead battery voltage is 2 volts; ③ home lighting voltage is 220 volts; ④ safety voltage is: not higher than 36 volts; ⑤ industrial voltage 380 volts.
Four, resistance
Resistance (R): represents the resistance of a conductor to current
. (The greater the resistance of a conductor to the flow of current, the greater the resistance and the lesser the current through the conductor).
International unit: ohm (Ω); commonly used: megohm (MΩ), kilohm (KΩ); 1 megohm = 103 kilohm; 1 kilohm = 103 ohm.
Factors that determine the size of a resistor: material, length, cross-sectional area and temperature (R is independent of its U and I).
Sliding Resistor:
Principle: Changing the length of the resistor wire in the circuit changes the resistance.
Function:To change the current and voltage in a circuit by changing the resistance in the circuit.
Nameplate: such as a sliding varistor labeled "50Ω 2A" means: the maximum resistance is 50Ω, the maximum current allowed to pass through the 2A.
Correct use: a, should be used in series in the circuit; b, wiring should be "one on the next"; c, before power should be adjusted to the maximum resistance.
V, Ohm's law
Ohm's law: the current in a conductor, with the voltage at the ends of the conductor is proportional to the resistance of the conductor is inversely proportional.
Formula: in units: I → A (A); U → volts (V); R → ohms (Ω).
The understanding of the formula:
①I, U and R in the formula must be in the same section of the circuit;
②I, U and R in the known any two quantities can be another quantity;
3 calculations should be unified units.
Application of Ohm's law:
①The resistance of the same resistor is constant, independent of current and voltage, and its current increases with voltage. (R=U/I)
②When the voltage is constant, the larger the resistance, the smaller the current through. (I=U/R)
3) When the current is constant, the higher the resistance, the higher the voltage across the resistance. (U=IR)
Resistors in series have the following characteristics: (refers to R1, R2 in series, the more the series, the greater the resistance)
①Current: I=I1=I2 (the current in the series circuit is equal to the current at each place)
②Voltage: U=U1+U2 (the total voltage is equal to the sum of the voltages at each place)
③Resistance: R=R1+R2 (the total resistance is equal to the sum of the resistances)
3 Resistance: R=R1+R2 (the total resistance is equal to the sum of the resistances)
This is a series circuit with the following characteristics: (I=U/R)
3 When the current is certain, the greater the resistance, the greater the voltage across the resistor. Resistance: R = R1 + R2 (total resistance is equal to the sum of the resistors) If n equivalent resistors are connected in series, there are R total = nR
④ voltage divider: =; calculate U1, U2, can be used:;
⑤ proportionality: the current: I1: I2 = 1:1 (Q is the amount of heat)
Parallel resistors have the following characteristics: (refers to the R1, R2 in parallel, and the more the more, the lower the resistance)
①Current: I = I1 + I2 (dry circuit current is equal to the sum of the branch current)
②Voltage: U = U1 = U2 (dry circuit voltage is equal to the branch voltage)
③Resistance: (the inverse of the total resistance is equal to the inverse of the resistance of the sum of the) if n equivalent resistors are connected in parallel, there is a total of R = R
④Shunt:; calculate I1, I2 can be used:.
⑤ proportionality: voltage: U1: U2 = 1:1, (Q is heat)
Six, electric power and electric power
1. electric power (W): the conversion of electrical energy into other forms of energy is called how much electric power,
2. work of the international unit: joule. Commonly used: degrees (kWh), 1 degree = 1 kWh = 3.6?06 joules.
3. Measurement tools: energy meter
4. Power formula: W = Pt = UIt (where the unit W → joule (J); U → volts (V); I → ampere (A); t → seconds).
Calculation using W = UIt Note:
①W.U.I and t is in the same section of the circuit;
②Calculation of the units should be unified;
3 known any three quantities can be found in the fourth quantity. There is also the formula:=I2Rt
Electrical power (P): indicates how fast the current does work. International units: watts (W); commonly used: kilowatts
Formula: formula units P → watts (w); W → joules; t → seconds; U → volts (V), I → ampere (A)
Use of the calculation units to be unified
① if W with joules, t with seconds, the unit of P is watts;
② if W with kilowatt-hours, t with hours, then the unit of P is kilowatts. .
10. Calculation of electric power can also be used in the right formula: P = I2R and P = U2 / R
11. Rated voltage (U0): the voltage at which an appliance normally operates. Another: rated current
12. rated power (P0): the power of the appliance at the rated voltage.
13. Actual voltage (U): the actual voltage applied to the terminals of the appliance. Another: the actual current
14. actual power (P): the power of the appliance at the actual voltage.
When U > U0, then P > P0 ;lamp is very bright, easy to burn out.
When U < U0, then P < P0 ;the lamp is very dim,
When U = U0, then P = P0 ;normal light.
15. The same resistor, connected to different voltages, there are; such as: when the actual voltage is half of the rated voltage, the actual power is 1/4 of the rated power. Example: "220V100W" if connected to a 110-volt circuit, the actual power is 25 watts.)
16. Thermal power: the thermal power of a conductor is proportional to the quadratic of the current, proportional to the resistance of the conductor.
17.P heat formula: P = I2Rt, (where the unit P → watts (W); I → ampere (A); R → ohms (Ω); t → seconds.)
18. When the current through the conductor to do the work (electric power) all used to produce heat (electric heat), then there: thermal power = electric power, can be used to calculate the electric power formula for thermal power. (Such as electric heaters, resistance is such.)
Seven, life with electricity
Home circuit by: into the house line (fire wire and zero wire) → energy meter → main switch → fuse box → appliances.
All appliances and sockets are connected in parallel. The appliance is connected in series with its switch to the fire wire.
Fuses: are made of lead-antimony alloy with high resistivity and low melting point. Its role is when there is excessive current in the circuit, it heats up to the melting point and melt, automatically cut off the circuit, play the role of insurance.
Two reasons for the circuit current is too large: one is a short circuit; the second is the total power of the appliance is too large.
The principle of safe use of electricity is: ① do not touch the low-voltage charged body; ② not close to the high-voltage charged body.
Eight, electricity and magnetism
Magnetism: the object attracts iron, nickel, cobalt and other substances.
Magnet: An object with magnetic properties is called a magnet. It is directional: it points north and south.
Magnetic poles: The most magnetic part of a magnet is called a pole.
Any magnet has two poles, one is the North Pole (N pole); the other is the South Pole (S pole)
The role of magnetic poles: the same name magnetic poles repel each other, the different name magnetic poles attract each other.
Magnetization: the process of making a non-magnetic object magnetic.
There is a magnetic field around a magnet, and the interaction between magnetic poles occurs through the magnetic field.
The basic property of a magnetic field: it exerts a magnetic force on the magnet into which it is inserted.
Direction of the magnetic field: The direction of the magnetic field at a point where a small magnetic needle comes to rest is the direction of the north pole.
Magnetic lines of inductance: imaginary curves that describe the strength and direction of a magnetic field. They do not exist and do not intersect, and run north out and south in.
The direction of the magnetic field at a point in a magnetic field is the same as the direction of the magnetic lines of inductance and the direction of the north pole of a small magnetic needle at rest.
10. The geomagnetic north pole is near the geographic south pole; and the geomagnetic south pole is near the geographic north pole. However, they do not coincide, their intersection angle is called the magnetic declination, China's scholars Shen Kuo first recorded this phenomenon.
11. Auster's experiment proved that there is a magnetic field around an energized wire.
12. Ampere's rule: hold a solenoid in your right hand and bend your four fingers to the direction of the current in the solenoid,
then the end pointed to by your thumb is the north pole of the solenoid (N pole).
13. Properties of an energized solenoid: ① the higher the current, the stronger the magnetism;
② the more turns of the coil, the stronger the magnetism;
③ inserted into a soft iron core, the magnetism is greatly enhanced
④ the polarity of the energized solenoid can be changed by the direction of the current.
14. electromagnet: solenoid with an iron core inside constitutes an electromagnet.
15. Characteristics of electromagnets:
① the availability of magnetism can be controlled by the current on and off;
② the strength of the magnetism can be adjusted by changing the size of the current and the number of turns of the coil;
③ magnetic poles can be changed by the direction of the current.
16. Electromagnetic relay: essentially the use of an electromagnet to control the switch. Its role can be realized long-distance operation, the use of low voltage, weak current to control high voltage, strong current. Can also realize the automatic control.
17. Telephone basic principle: vibration → strong and weak changes in current → vibration.
18. Electromagnetic induction: a part of the closed circuit conductor in the magnetic field to do cutting magnetic susceptor movement, the conductor in the current, this phenomenon is called electromagnetic induction, the current generated is called induction current. Application: generator
Conditions for induced current:
①The circuit must be closed;
②Only a part of the circuit conductor in the magnetic field;
3This part of the conductor to do the cutting of magnetic lines of motion.
The direction of the induced current is related to the direction of motion of the conductor and the direction of the magnetic field.
Principle of generator: electromagnetic induction. Structure: stator and rotor. It converts mechanical energy into electrical energy.
The effect of a magnetic field on the electric current: an energized wire is subjected to magnetic force in a magnetic field. It converts electrical energy into mechanical energy. Application: electric motors.
The direction of the force on an energized conductor in a magnetic field is related to the direction of the current and the direction of magnetic inductance.
Principle of electric motor: It is made by using the principle of rotating an energized coil of wire in a magnetic field.
Commutator: to realize the exchange between alternating current (AC) and direct current (DC).
AC: A current that periodically changes direction.
Direct current: a current that does not change direction.
Direct current: a current that does not change direction.