I. Voltage
Voltage: when a current is generated in a circuit, there must be a voltage at both ends (voltage is the reason why the free charge in the circuit moves directionally to form the current). Power supply provides voltage, and voltage forms current. (If there is current, there must be voltage, and if there is voltage, there is no current. )
Symbol of voltage physical quantity: u.
Units: volts (V), kilovolts (kV), millivolts (mV) and microvolts (μV). 1kV = 103v； 1V = 103mv； 1mV= 103μV。
Common voltage value: dry battery:1.5v; ; Household circuit: 220 V; Mobile phone: 3.6V；; Lead storage battery: 2V; Safe voltage: Not higher than 36V.
Voltmeter: Measure the voltage (when analyzing the circuit, the position of voltmeter is equivalent to an open circuit).
Measuring range: 0-3V (large battery: 1V, small battery: 0. 1V)
0- 15V (large grid: 5V, small grid: 0.5V).
Usage: 1, voltmeter should be connected in parallel in the circuit; 2. Current should flow in from the "+"terminal and out from the "-"terminal; 3. Do not exceed the range of voltmeter. (Try to touch in a large range, not exceeding a small range, and measure in a small range)
Secondly, the voltage law of series-parallel circuit is explored.
Battery series: the voltage of series battery pack is equal to the sum of all battery voltages.
Parallel connection of batteries: the voltage of parallel battery pack is equal to the voltage of each battery.
Voltage of series circuit: In series circuit, the sum of the voltages of each part of the circuit is equal to the total voltage.
Voltage of parallel circuit: In parallel circuit, the voltage at both ends of each branch is equal.
Energy conversion of battery: chemical energy is converted into electrical energy. (chemical battery)
To prevent the harm of waste batteries to the environment: 1, use high-quality batteries; 2. Recycle waste batteries; Don't throw away the old batteries at will.
Third, resistance.
Resistance: Represents the resistance of a conductor to current. The greater the resistance of a conductor to current, the smaller the current passing through the conductor.
Symbol of physical quantity: R.
Unit: ohm (ω); Commonly used units are: Megaohm (mω) and Kilohm (kω). 1mω= 103kω； 1kω= 103ω.
Factors that determine the size of resistance: the resistance of a conductor is a property of the conductor itself, and its size depends on the material, length, cross-sectional area and temperature of the conductor (most materials increase in temperature, and the resistance increases). The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional product. The resistance is independent of the voltage across the conductor and the current through the conductor.
Control variable method: the method of controlling factors (variables) is often used in physics to transform multi-factor problems into multiple single-factor problems, study them separately and finally solve them comprehensively. This method is called the control variable method.
Four, rheostat
Sliding rheostat: structure: (resistance wire, insulating tube, slider, terminal, etc. )
Principle: Change the length of the resistance wire connected to the circuit to change the resistance, thus changing the current in the circuit.
Function: change the current and voltage in the circuit; It protects the circuit.
Nameplate: For example, a sliding rheostat marked with "50ω2A" indicates that the maximum resistance is 50Ω and the maximum allowable current is 2A.
Correct use: (1), should be used in series in the circuit; (2), wiring should be "one up and one down" (you can't treat the upper two terminals as wires and the lower two terminals as constant resistances at the same time; (3) Before closing the switch, adjust the resistance to the maximum position (the position with the minimum current) to protect the circuit.
Chapter VII Ohm's Law
Firstly, the relationship between the voltages at both ends of the current root on the resistor is explored.
Experimental inquiry method: controlled variable method
When the resistance is constant, the current in the conductor is proportional to the voltage across the conductor.
When the voltage is constant, the current in the conductor is inversely proportional to the resistance of the conductor.
Second, ohm's law and its application
Ohm's Law: The current in a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor.
Formula: (). Unit: I→ an (a); U→ volt (v); r→ω(ω)。
Understanding of the formula: ① I, U and R in the formula must be in the same circuit; ② If any two of I, U and R are known, the other quantity can be found; (3) Units should be unified when calculating.
Application of ohm's law;
Regardless of the current and voltage, the same resistor has the same resistance, but when the voltage across the resistor increases, the current passing through it will also increase. (R=U/I)
When the voltage is constant, the greater the resistance, the smaller the current. (I=U/R)
When the current is constant, the greater the resistance, the greater the voltage on the resistance. (U=IR)
The series connection of resistors has the following characteristics: (refer to R 1, R2 series connection)
Current: I=I 1=I2 (current is equal everywhere in the series circuit).
Voltage: U=U 1+U2 (the total voltage is equal to the sum of all circuit voltages).
Resistance: R=R 1+R2 (the total resistance is equal to the sum of all resistances), and the resistance of the total resistance of the series circuit is greater than that of any sub-resistance.
If n resistors with the same resistance value are connected in series, R total =nR.
Partial pressure action: =;
Resistance parallel connection has the following characteristics: (refer to R 1, R2 parallel connection)
Current: I=I 1+I2 (the main current is equal to the sum of the currents of each branch).
Voltage: U=U 1=U2 (main voltage is equal to each branch voltage).
Resistance: (the reciprocal of the total resistance is equal to the sum of the reciprocal of each parallel resistance), and the resistance of the total resistance of the parallel circuit is less than the resistance of any sub-resistance.
If n resistors with the same resistance are connected in parallel, then r = R
Shunt action:
Third, measure the resistance of the small bulb.
Experimental principle: ohm's law (R=U/I). (The resistance of a conductor is independent of voltage and current)
Experimental circuit:
Experimental steps: 1, draw the experimental circuit diagram; 2. Connect the circuit; (During the connection, the switch is disconnected; Before closing the switch, the sliding vane of the sliding rheostat slides to the position with the largest resistance; Reasonable selection of voltmeter and ammeter range). 3. Starting from the rated voltage, gradually reduce the voltage applied at both ends of the lamp to obtain several groups of voltage values and current values (the average value of multiple measurements can reduce the experimental error); 4. Calculate the resistance value; 5. Analyze the reason why the resistance value becomes smaller in the experimental data: the filament resistance is affected by temperature. The greater the current through the filament, the higher the temperature of the filament and the greater the resistance.
Four, ohm's law and the safe use of electricity
The higher the voltage, the more dangerous it is: according to ohm's law, the current in the conductor is directly proportional to the voltage at both ends of the conductor; The human body is also a conductor. The higher the voltage, the greater the current passing through it, and it is very dangerous to reach a certain level.
Don't touch electrical appliances with wet hands: for the human body, the resistance is small when it is wet, and the current passing through the human body will be large when it is electrocuted; In addition, touching the appliance with wet hands can easily make water flow into the appliance and connect the human body with the power supply.
Pay attention to lightning protection: lightning is a violent discharge phenomenon in the atmosphere. When discharging, the voltage and current are extremely high, which releases huge heat and causes air vibration. Lightning protection requires the installation of lightning rods.
Open circuit: A broken and unconnected circuit somewhere.
Short circuit: the phenomenon that two points in a circuit should not be connected together. Because the resistance of the wire is very small, the current will be large when the power supply is short-circuited, which will damage the power supply and the wire.
Chapter VIII Electric Power
I. Electric energy
Electric energy is a kind of energy. Such as: electric light: electric energy → light energy; Motor rotation: electric energy → kinetic energy; Working principle of rice cooker: electric energy → heat energy.
Electric energy unit: j, KWh. 1kWh=3.6× 106J .
Watt-hour meter: measures the electric energy consumed by users.
Several important parameters: "220V": This watt-hour meter can only be used if it is connected to a 220V circuit.
10(20)A: the calibration current is 10A. Short-term current is allowed to be larger, but it cannot exceed 20 A.. (For example, different electric meters are different)
50HZ: The electric energy meter is connected to the 50HZ circuit for use.
600 rpm/kWh: For the electrical appliances connected to the watt-hour meter, the turntable of the watt-hour meter rotates 600 rpm for every 1 kWh of electric energy consumed.
Electric work: the work done by current is equal to the electric energy consumed by electrical appliances.
Second, electricity.
Electric power (P): indicates the speed of electric energy consumption and the electric energy consumed by electrical appliances per unit time.
Unit: w, kw; 1kw= 103w。
Electric power formula: (where the unit is powerW→ Coke (J); T→ seconds; U→ volt (v); I→ an (a)。
When calculating, the units should be unified. ① If W uses J and T uses S, the unit of P is W; ② If W uses kwh, T uses H, and the unit of P is KW.
Kwh means: the electric energy consumed by electrical appliances with power of 1kw1h.
This formula can also be used to calculate electric power: P=I2R, p = U2/r.
Rated voltage (U0): the voltage at which the electrical appliance works normally.
Rated power (P0): the power of electrical appliances under rated voltage.
Actual voltage (u): the voltage actually applied to the electrical appliance.
Actual power (P): the power of electrical appliances under actual voltage.
The brightness of the light bulb is determined by the actual electric power.
When U > U0, P > P0 is bright and easy to burn.
When u < U0, then p
When U = U0, then p = P0 emits light normally.
The same resistor or bulb connected to different voltages.
Third, measure the electric power of the small bulb.
Experimental principle: P=UI.
Experimental circuit: (with measuring resistance)
Experimental steps: 1, draw the experimental circuit diagram; 2. Connect the circuit (measure the resistance of the small bulb at the same time) 3. Close the switch, adjust the sliding rheostat, make the voltmeter read the rated voltage of the small bulb, read the reading of the ammeter, and observe the light emission of the bulb; 4. Make the voltage at both ends of the small bulb 1.2 times the rated voltage, observe the brightness of the bulb and measure its power; 5. Make the voltage at both ends of the small bulb lower than the rated voltage (about 0.8 times), observe the brightness of the small bulb and measure its power.
Note: During the experiment, the power supply voltage is higher than the rated voltage of the bulb.
Fourth, electricity and heat.
Thermal effect of current: the phenomenon that electric energy is converted into heat when current passes through a conductor.
Joule's Law: Heat and current generated when an electric current passes through a conductor.
Proportional to the square of, proportional to the resistance of the conductor, and
The power-on time is proportional.
Note: Don't simply think that the greater the resistance, the more heat will be released in the same time.
Joule's law formula: Q=I2Rt, (where the unit is q → j;
I→A； r→ω； t→S .)
When current passes through a conductor, all the work (electric work) is used to generate heat.
Quantity (electrothermal), W=Q, q can be calculated by electric power formula.
(such as electric heaters, the resistance is like this. )Q = UItQ=U2t/R .
Utilization of electric heating: heating (rice cooker, electric iron)
Prevent electric heating: too high temperature will damage electrical appliances and cause fire (cooling windows, cooling fins, cooling fans).
(series)
(parallel)
Verb (abbreviation of verb) electricity and safe use of electricity
Danger of excessive current: fuse blowing or even fire.
Cause of excessive current: 1, short circuit; 2. The total power of electrical appliances is too large.
Fuse: Fuse is made of lead-antimony alloy, with high resistance and low melting point (material characteristics). When the current is too large, its temperature rises and fuses, cutting off the circuit and protecting the circuit. (function)
Air switch: when the current is too large, the electromagnet in the switch works, the switch is turned off, and the circuit is cut off.
Note: 1, copper wire, iron wire, etc. Can't be used instead of fuse. 2. When the safety device in the circuit is cut off, don't rush to replace the fuse or reset the air switch. Find out the cause of the fault first, and then restore power supply after troubleshooting.
Sixth, the common sense of electricity consumption in life
Composition of home circuit: live wire and neutral wire → watt-hour meter → master switch → safety device (fuse or air switch) → socket, electrical appliance, switch, etc.
Power supply: 220V AC from power plant, two-wire system.
Fire line: there is a voltage of 220V relative to the earth (neutral line).
Neutral wire: grounding of power plant and outdoor.
Watt-hour meter: measure the electricity consumption of users; The unit is kwh, and the difference between the two readings is the power consumed during this time.
Master switch: replace the safety circuit for maintenance. (The air switch can also play an insurance role. )
Safety device: fuse (box) → fuse when the current is too large to cut off the circuit. Air switch → overcurrent trips, and the circuit is cut off.
Three-wire plug (socket): one wire is connected to live wire (L), one wire is connected to neutral wire (N), and one wire (E) is connected to electrical appliance shell (ground); Use electricity safely.
Note: All appliances in the home circuit are connected in parallel (including sockets), and the controlled appliances and switches are connected in series.
Electric probe: function → distinguish between live wire and neutral wire. Use → Hold the pen card with your fingers, and touch the measured wire with the tip of your pen. The glowing wire is the live wire.
Electric shock: 1, single-line electric shock: people standing on the ground touch the fire line. 2, people come into contact with the live line and the zero line at the same time.
Electric shock first aid: first cut off the power supply; Save the electrocuted again.
Chapter 9 Electricity and magnetism
I. Magnetic phenomena
China's earliest compass → Sina.
Magnetism: the property of magnets to attract iron, cobalt, nickel and other substances.
Magnets: magnetic objects, magnets have iron absorption and directivity.
Magnetic poles: the strongest part of a magnet (two magnetic poles). South pole: the magnetic pole of the guide rail of a small magnetic needle that rotates freely at rest (geographical south pole); North Pole: The magnetic pole (n) pointing north at rest.
Interaction between magnetic poles: magnetic poles with the same name repel each other and magnetic poles with different names attract each other.
Magnetization: the process of making an object without magnetism acquire magnetism.
Second, the magnetic field
Magnetic field: There are invisible and intangible substances around the magnet (or current), which can exert a force on the magnet (or current). There is a magnetic field around the magnet, and the interaction between magnetic poles occurs through the magnetic field.
Basic properties of magnetic field: the effect of magnetic force on magnets entering it.
Magnetic field direction: at a certain point in the magnetic field, the direction that the north pole points to when the small magnetic needle is at rest is the magnetic field direction of that point.
Magnetic induction line: an imaginary curve with arrows, describing the strength and direction of magnetic field. The magnetic induction line around the magnet comes from its north pole and returns to the south pole. (The magnetic induction line does not exist, and it is indicated by a dotted line and does not intersect. Inside the magnet, the magnetic induction line extends from the south pole to the north pole. The direction of the magnetic field and the direction of the magnetic induction line at a certain point in the magnetic field are the same as the direction of the north pole when the small magnetic needle is at rest.
Geomagnetic field: the magnetic field existing in the space around the earth.
The geomagnetic north pole is near the geographical south pole; And geomagnetism
The South Pole lies near the North Pole. (The north and south poles of geomagnetism do not coincide with the north and south poles of geography, and their intersection angle is called magnetic declination, which was first described by China scholar Shen Kuo. )
3. electromagnetism
Oster (Denmark) first discovered the magnetic effect of current.
Magnetic effect of current: there is a magnetic field around the electrified wire, and the direction of the magnetic field is related to the direction of current.
Magnetic field of energized solenoid: (If it is made into a solenoid coil, the magnetic field generated by each wire will be superimposed together, and the magnetic field will be much stronger). 1. The magnetic field outside the energized solenoid is the same as that of the bar magnet. 2. Ampere's Law: Hold the solenoid with your right hand and bend four fingers in the direction of the current in the solenoid, then the end pointed by your thumb is the north pole (N pole) of the solenoid.
Fourth, electromagnets
Electromagnet: a solenoid (with an iron core inside) that is magnetic when energized and not magnetic when de-energized.
Principle of electromagnet: the magnetic effect of current (the iron core is magnetized, and the magnetic field of the iron core and the coil works together).
The factors that determine the magnetic induction intensity of electromagnet are: 1, whether there is an iron core inside; It has an iron core and strong magnetism. 2. Current size; A certain shape, the same number of turns, the greater the current, the stronger the magnetism. 3, coil turns; The shape is certain, the current is the same, and the more turns, the stronger the magnetism.
Features of electromagnet: ① Magnetism can be controlled by the on-off of current; ② The magnetic force can be adjusted by changing the current and the number of turns of the coil; ③ The current direction can change the magnetic pole.
Five, electromagnetic relay speaker
Electromagnetic relay: In essence, it is a switch that uses electromagnet to control the on-off of working circuit. It indirectly controls high-voltage and high-voltage circuits by using the on-off of low-voltage and weak-current circuits.
Working circuit: consists of low-voltage control circuit (composed of low-voltage power supply and electromagnet) and high-voltage working circuit (composed of electromagnetic relay contact, high-voltage power supply and electrical appliances).
Usage: It can realize remote operation and automatic control.
Speaker: Principle: Convert electrical signals into acoustic signals.
Structure: permanent magnet, coil, cone-shaped paper basin. Sounding process: when the current passes through the coil, the coil will be attracted or repelled by the permanent magnet, and the coil will constantly vibrate back and forth to drive the paper basin to make sound.
Six, the motor
Effect of magnetic field on current: charged conductor will be acted by force in magnetic field (motor principle), and the direction of force is related to the direction of current and magnetic induction line. (When the direction of current or magnetic induction line changes, the stress direction of live wire also changes. )
Motor structure: rotor (rotating part), stator (fixed part) and commutator.
Energy conversion: electric energy → kinetic energy.
Commutator structure: Two (or more) copper half rings are connected to the motor coil and insulated from each other.
The function of commutator: when the coil turns to the balance position, it automatically changes the direction of current in the coil and makes the coil rotate continuously.
Motor type: DC motor, AC motor.
Advantages of motor: simple structure, convenient control, small volume, high efficiency and no pollution.
Seven, magnetoelectricity
Faraday (England) discovered electromagnetic induction, which further revealed the connection between electricity and magnetism.
Electromagnetic induction: the phenomenon that a conductor (a part of a closed circuit) moves in a magnetic field (cutting the magnetic induction line) and generates current; The generated current is called induced current (the direction of induced current is related to both the direction of conductor movement and the direction of magnetic induction line).
Generator: kinetic energy → electric energy. (energy conversion)
Principle; Electromagnetic induction.
Structure: stator and rotor.
Alternating current: (AC) Current whose magnitude and direction change periodically.
DC: The direction of water flow will not change.
Frequency: the number of times the current changes periodically in1s. (China power grid frequency is 50HZ).
Energy conversion of generator:
Thermal power generation: chemical energy → internal energy → kinetic energy → electric energy.
Hydroelectric power generation: kinetic energy → electric energy.
Chapter 10 Information Transmission
First of all, modern clairaudient-telephone
Bell invented the telephone in 1876.
Basic structure and principle of telephone: the simplest telephone consists of a microphone and an earpiece, and a pair of telephone lines are connected between the microphone and the earpiece; Microphones convert sound into electric current, which transmits information to far places along wires. At the other end, the current vibrates the diaphragm of the receiver, and the current carrying information becomes sound again.
Telephone exchange: Connect the telephone to improve the line utilization.
Two kinds of signals: analog signals and digital signals.
Advantages of digital signal: strong anti-interference ability; Convenient for computer processing; Good confidentiality.
Second, the ocean of electromagnetic waves.
Electromagnetic wave: There are electromagnetic waves around the rapidly changing current, which can transmit information.
The propagation of electromagnetic waves does not need medium; Vacuum can spread. C = λ F. (C = 3×108m/s). (λ wavelength of electromagnetic wave; Unit m). Frequency; Hertz). 1 MHz = 103 khz = 106hz .
Radio wave: The part of electromagnetic wave whose frequency is between several hundred thousand hertz and several hundred megahertz is called radio wave (transmitting all kinds of information).
Visible light is a member of the electromagnetic wave family.
Microwave oven: Using microwave to make food molecules vibrate violently under the action of microwave, thus increasing internal energy and raising temperature.
Three. Radio, television and mobile communications
Transmission and reception of radio broadcast signals: the microphone converts sound signals into electrical signals, which are loaded on high-frequency electromagnetic waves by the modulator and then transmitted into the air through the antenna. The receiver tunes a signal with a specific frequency, selects a sound signal through an electronic circuit, amplifies it and sends it to a speaker to convert the electric signal into sound.
The transmission and reception of television is similar to broadcasting, which transmits both sound signals and image signals.
Working principle of mobile phone: mobile phone and base station (station) transmit radio signals.
The working principle of cordless telephone:
Four. Wider and wider information road
The higher the radio frequency, the more information is transmitted at the same time. Microwave communication: the wavelength is 10m- 1mm and the frequency is 30mhz-3x105mhz. Microwave is close to light wave in nature and travels in a straight line. Microwave is blocked by the ground during transmission, so a relay station should be built every once in a while to "relay".
Satellite communication: using satellites as relay stations; Three satellites can cover the whole world.
Optical communication: let the laser carrying information spread in the optical fiber; Laser has a high frequency and carries a lot of information.
Network communication: connect computers together and use the network to communicate.
formula
Speed v (m/s) v = s/t; S: distance, t: time
Gravity g (n) g = mg; M: quality; G: 9.8n/kg or 10N/kg.
Density ρ(kg/m3) ρ= m/V m: mass; Five: volume
The resultant force f and (n) are in the same direction: f and =F 1+F2.
Opposite direction: f = f1-when F2 is opposite, f1> Second generation
Buoyancy f float (N) F float =G object -g sight; G-vision: gravity of objects in liquid.
Buoyancy f float (N) F float =G object; This formula only applies to floating or suspended objects.
Buoyancy F float (N) F float =G row =m row g=ρ liquid gV row; G row: gravity of displacement liquid; Line M: mass of displaced liquid, ρ liquid: density of liquid, and line V: volume of displaced liquid (i.e. volume immersed in liquid).
The equilibrium condition of lever f1* l 1 = F2 * l2f1:power, l1:power arm F2: resistance L2: resistance arm.
Crown block F=G object, S=h, f: the tension of the free end of the rope, g object: the gravity of the object, s: the distance that the free end of the rope moves, and h: the distance that the object rises.
Moving pulley F= (G object +G wheel) /2, S=2 h, g object: object gravity, g wheel: moving pulley gravity.
Pulley block F= (G object +G wheel) /n, S=n h, n: the number of segments bearing the weight of the object.
Mechanical work W(J) W=FS F: force s: distance moved in the direction of force.
Useful work: w available, total work: w is total, w is =G *h, w is total =Fs, and the mechanical efficiency η=W available /W is total × 100% when the pulley block is placed vertically.
Work W = F S = P t 1J = 1N? m = 1W？ s
Power P = W/t = F*v (uniform straight line) 1kw = 10 3w, 1mw = 10 3kw.
Useful Work W Useful = G h= W Total–W Quantity =ηW Total
Extra work w amount = W total amount–w has = G movement h (ignoring friction between shafts) = f L (inclined plane)
Total workload W Total = W Useful +W Quantity = F S = W Useful/η
Mechanical efficiency η = useful power/total power
η = g/(n f) = g-object/(g-object+g-motion) This definition applies to moving pulleys and pulley blocks.
Power p (w) p = w/t; W: work; T: time
Pressure p(Pa) P= F/S F: pressure /S: stress area.
Liquid pressure p (pa) p = rhogh rho: liquid density h: depth (vertical distance from liquid surface to required point)
Heat Q(J) Q=cm△t c: specific heat capacity m: mass, △t: change value of temperature.
Heat released by fuel combustion q (j) q = MQ; M: mass, q: calorific value.
series circuit
Current I (a) I = I 1 = I2 = ... Current is equal everywhere.
The voltage u (v) u = u 1+U2+ ... The series circuit acts as a voltage divider.
The resistance r (ω) r = r 1+R2+ ...
parallel circuit
Current I (a) I = i 1+I2+ ... The main current is equal to the sum of the currents of each branch (shunt).
Voltage u (v) u = u 1 = U2 = ...
Resistance1/r (ω) =1/r1+1/R2.
Ohm's law I= U/R
The current in the circuit is directly proportional to the voltage and inversely proportional to the resistance.
Current definition formula I= Q/t: charge (coulomb) t: time (second)
Electric work w (j) w = uit = pt; U: voltage I: current t: time p: electric power
Electric power p = ui = I 2r = u 2/r u: voltage I: current r: resistance.
Electromagnetic wave velocity and wave
The relationship between length and frequency c=λf
C: wave velocity (the wave velocity of electromagnetic wave is constant, equal to 3× 10 8m/s) λ: wavelength f: frequency.
2. Knowledge points
1. Several values to remember:
A. The propagation speed of sound in the air: 340m/s;
B the propagation speed of light in vacuum or air: 3x108m/s.
C density of water: 1.0× 10 3kg/m3 d specific heat capacity of water: 4.2× 10 3j/(kg? ℃)
E. dry battery voltage:1.5v f. household circuit voltage: 220V.
G. safe voltage: not higher than 36V.
2. Density, specific heat capacity and calorific value are the characteristics of substances, and the values of these three physical quantities of the same substance are generally unchanged. For example, a glass of water and a bucket of water have the same density and specific heat capacity.
3. A virtual image of the same size formed by a plane mirror, in which the image and the object are symmetrical about the plane mirror.
Sound cannot travel in a vacuum, but light can travel in a vacuum.
4. Ultrasonic waves: sounds with frequencies higher than 20000Hz, such as bats, ultrasonic waves, dolphins, etc.
Infrasound: Infrasound can be produced by volcanic eruption, earthquake, wind explosion and tsunami below 20Hz, nuclear explosion and missile launch.
6. Light travels in a straight line in the same homogeneous medium. Shadow, pinhole imaging, solar eclipse and lunar eclipse are all formed by light propagating along a straight line.
7. When light is refracted, the angle in the air (the angle with the normal) is always slightly larger. When you look at something in the water, you see a shallow virtual image (conversely, the trees on the shore become taller when you look at it in the water).
8. Convex lens converges light and concave lens diverges light.
9. The law of convex lens imaging: the object becomes a reduced and inverted real image (camera) at a distance of twice the focal length. Between the focal length of 2 times and the focal length of 1 times, it becomes an inverted magnified real image (projector). In the focal length range of 1 times, it becomes an upright magnified virtual image (magnifying glass).
10. Sliding friction is related to pressure and surface roughness. Rolling friction is smaller than sliding friction.
1 1. Pressure is a physical quantity that compares the effect of pressure, and the effect of pressure is related to the size of pressure and the stress area.
12. When transmitting electric energy, high voltage power transmission should be adopted. The reason is that the power loss on the transmission line can be reduced under the same transmission power.
13. The principle of the motor: the electrified coil is forced to rotate in the magnetic field. It is the conversion from electric energy to mechanical energy.
14. Generator principle: electromagnetic induction phenomenon. Mechanical energy is converted into electrical energy. Microphones and transformers are based on the principle of electromagnetic induction.
15. Optical fiber is the medium for transmitting light.
16. The magnetic induction line emanates from the N pole of the magnet and finally returns to the S pole.