I. Voltage
Knowledge point 1- voltage
● Voltage is the cause of current formation.
Water pressure is the reason for the directional movement of water to form water flow; Voltage is the reason for the directional movement of free charge to form current.
The voltage of (1) causes a current to form in the circuit.
(2) The difference between voltage and current: ① The voltage is meaningful between two points in the circuit, while the current corresponds to a certain place or point in the circuit, generally referring to the current somewhere and the voltage at both ends of the electrical appliance. ② Voltage is the cause and current is the result.
● Voltage unit
The unit of voltage is Volt (V), abbreviated as Volt (V). In addition, common voltage units are kilovolts (kV), millivolts (mV) and microvolts (μV).
1kV= 103V， 1mV= 10-3V， 1μV= 10-6V
● Power supply is a device that provides voltage.
(1) The power supply converts other forms of energy into electrical energy.
When supplying power to the outside world, the power supply converts electric energy into other forms of energy by using electrical appliances.
(2) Voltage value of common power supply:
① The voltage of dry cell is1.5v; ;
② The voltage of a battery is 2V; The anode and cathode of each battery are connected in turn to form a battery pack called series battery pack, which can meet the different requirements of electrical appliances for DC voltage. Because the voltage of each battery is the same, the total voltage of the battery pack is U=nU 1 after N batteries are connected in series.
(3) The voltage safe to human body shall not exceed 36V；;
④ The voltage of household circuit is 220V (lighting circuit).
⑤ The voltage between clouds with lightning can reach 103kV.
● Division of common voltage value
(1) The voltage whose safe voltage is not higher than 36V(2) 1000V is called low voltage; (3) The voltage above1000 V is called high voltage.
Knowledge point 2- voltmeter
● A voltmeter is an instrument for measuring voltage.
The current is measured by an ammeter, and the voltage is measured by a voltmeter, whose symbol in the circuit is.
In the circuit, the voltage across the power supply or electrical appliance can be directly measured by voltmeter.
V on the dial stands for DC voltmeter, which is used to measure the DC circuit voltage of power supply such as battery.
In the laboratory, the commonly used dual-range voltmeter has three terminals and two ranges. Generally, the "-"terminal is used, and the other two terminals are marked with the words "3" and "15" respectively, which together with the "-"terminal form two measuring ranges of 0 ~ 3v and 0 ~ 15v respectively.
The range of selection is different, and the dividing value is also different. When the range of 0 ~ 3v is selected, the dividing value is 0. 1V, and the reading should be based on the scale line below the dial. When the range of 0 ~ 15V is selected, the dividing value is 0.5V, and the reading should be based on the scale line above the dial.
● Voltmeter reading
(1) Measure the voltage with a voltmeter. When reading, first distinguish the range used by the voltmeter, so as to confirm each battery in the corresponding range of the voltmeter and the voltage value represented by each battery. Indicator = indicator value+number of units.
(2) Reading according to which scale the pointer is biased does not need to be estimated. When the pointer is in the middle of two scale lines, there are two correct values at which scale to read.
● Rules for the use of voltmeter
(1) Before use, check whether the pointer points to zero. If there is any deviation, turn the zero adjustment screw on the dial with a screwdriver and adjust the pointer to zero.
(2) The voltmeter must be connected in parallel with the tested electrical appliance.
(3) Connect the terminal correctly: the current "+"enters and exits.
(4) The measured voltage should not exceed the range of voltmeter.
(5) When the range of the voltage to be measured cannot be predicted, try a large range first and use the touch method. If the voltage indicator is in a small range, use a small range to improve the measurement accuracy.
Secondly, the voltage law in series and parallel circuits is explored.
Knowledge point 1- voltage law of series circuit (see experimental teaching)
The total voltage of the series circuit is equal to the sum of the voltages of all parts of the circuit, that is, U = U 1+U2+...+UN.
Knowledge Point 2-Voltage Law of Parallel Circuits
The voltage at both ends of each branch in parallel circuit is equal: u1= U2 = ... = un = u.
Three, resistance four, rheostat
Knowledge point 1- Conductor and insulator
● Conductor: An object that is easy to conduct electricity is called a conductor.
Insulator: An object that does not conduct electricity easily is called an insulator.
An object whose conductivity is between conductor and insulator is called a semiconductor.
For example, metal, graphite, human body, earth and aqueous solution of acid, alkali and salt are all conductors;
Rubber, glass, ceramics and oil are insulators; Silicon and germanium are semiconductors.
Different materials have different conductivity.
There is no absolute boundary between conductor and insulator.
An originally non-conductive object may also become a conductor when conditions change. For example, under normal circumstances, glass is a good insulator. If glass is heated to a hot state, it will become a conductor. Pure water is an insulator, but water containing impurities is easy to conduct electricity and is a conductor; A dry stick is an insulator and a wet stick is a conductor.
An object with strong conductivity is a good conductor; An object with strong insulation is a good insulator. Good conductors and good insulators are good electrical materials. For example, in copper wire, copper wire is a good conductor and the outer insulating sheath is a good conductor.
● Factors affecting semiconductor conductivity: temperature, illumination and dopant.
Doping a small amount of other elements in the semiconductor will greatly improve its conductivity, so that it can be made into:
Photosensitive resistor: There is a great difference between the resistance values with and without illumination.
Thermistor: the temperature changes little, but the resistance changes obviously.
Varistor: When the voltage changes, the resistance value changes obviously.
Diode: unidirectional conduction.
Triode: It has the function of amplifying electrical signals.
Semiconductor components are widely used and have become important components of electronic computers and other electronic instruments.
Knowledge point 2-resistance
● Definition: The resistance of a conductor to current is called resistance.
Different conductors have different blocking effects on current. In physics, resistance is used to indicate the blocking effect of conductor on current. The resistance of a conductor is a characteristic of the conductor itself. Its size has nothing to do with whether it is connected to a circuit or not, and the voltage across it has nothing to do with the voltage passing through it.
The symbol of resistance is R.
● Resistance unit
In the international system of units, the unit of resistance is ohm, abbreviated as ohm, and the symbol is ω. Commonly used units greater than Europe are kiloohms (kω) and megaohms (mω), and their conversion relationship is1ω =10-3kω =10-6mω.
● Resistance: Components with a certain resistance value are often used in electronic technology, which are called resistors. Resistance is also called constant resistance, or resistor for short. When expressed in words, the symbol is R, and in the circuit diagram, the symbol is
Knowledge point 3-Factors affecting the size of conductor 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.
(1) The resistance of the conductor is related to the material of the conductor, and the resistance of different materials has different blocking effects on the current, that is, the resistance is different. Metals can conduct electricity, but nonmetals generally don't. Some elements, such as silicon and germanium, have conductivity between metal and nonmetal, worse than metal and stronger than nonmetal, and are often called semiconductors.
(2) The resistance of a conductor is related to the length of the conductor. The longer the conductor, the greater the resistance.
(3) The size of conductor resistance is related to the cross-sectional area of conductor. The smaller the cross-sectional area, the greater the resistance.
(4) The size of the conductor resistance is also related to the temperature of the conductor, and the resistance of most conductors increases with the increase of temperature. The resistance of some substances becomes zero at very low temperature, which is the phenomenon of superconductivity. The filament of a small bulb is made of tungsten, and its resistance changes with the temperature. When the current passing through it is large, it is brighter, the filament temperature is higher and the resistance is higher. When the current passing through it is small, it is dark, the filament temperature is low and the resistance is small.
Generally speaking, we think that the resistance of a conductor does not change with temperature.
Knowledge point 4-Explore the factors that affect the size of conductor resistance (see experimental teaching)
The size of the resistance is related to the length, cross-sectional area and material characteristics of the conductor, and also to the temperature of the conductor. Resistance is a characteristic of the conductor itself. When the temperature is constant, the resistance of the conductor does not change with the voltage applied to the conductor and the current flowing through the conductor.
Knowledge point 5- rheostat
The resistance of a conductor to current is called resistance.
Resistors are divided into fixed resistance and variable resistance (adjustable resistance). Fixed value resistance is referred to as resistance for short, corresponding to resistance, and the symbol of the element is;
Variable resistance, adjustable resistance value, corresponding to rheostat, element symbol is,
Rheostats are generally divided into sliding rheostat, rheostat box and simple rheostat box. The most common is the sliding rheostat, which is often used in students' experiments.
● Structure of sliding rheostat
As shown in the figure, it consists of a coil, a porcelain tube, a slider and a metal bar.
The resistance wire is wrapped with an insulating layer and wound on the insulating tube, and its two ends are respectively connected with two terminals A and B, and the slider P is connected with two terminals C and D through a metal bar. On the outer side of the slider in contact with the resistance wire, the insulation layer is scraped off regularly to make good contact. When the slider P moves to different positions, the resistance in the access circuit can be changed.
● Working principle of sliding rheostat
By changing the length of the resistance wire in the access circuit, the resistance can be gradually changed, and then the current can be gradually changed.
● Symbol
The symbols of the structure diagram are shown in the figure, and the symbols of the circuit diagram elements are
In other words, there is no uniform provision for text symbols, and they are often expressed by Rx.
● Nameplate
By observing the nameplate on the sliding seat of the sliding rheostat, we can know its maximum resistance and maximum allowable current. For example, the nameplate is marked with the words "20Ω 1.5A", indicating that the maximum resistance of the sliding rheostat is 20Ω and the maximum allowable current is1.5a. ..
● Four connection modes of sliding rheostat
Four correct connection modes of sliding rheostat are shown in the figure.
The sliding rheostat has four terminals. If both terminals are connected to the circuit, it means that the resistor is not connected to the circuit or to the fixed resistor. If you want to change the access resistance, please follow the connection principle of "one up and one down".
Judging the change of the resistance value of the sliding rheostat connected to the circuit;
The first step is to determine the connection between the sliding varistor and the circuit.
Step 2, according to the current flowing through the sliding rheostat, judge which section of the rheostat is connected with the resistance.
Thirdly, according to the change of the position of the slider, the change of the length of the resistance wire passing through the current is judged.
Step 4, judging the size of the resistance in the path circuit according to the length change of the resistance wire path.
● Operating principle of sliding rheostat.
(1) shall be selected as required, and shall not exceed the maximum current allowed by the sliding rheostat.
(2) The sliding rheostat should be connected in series with the controlled circuit.
(3) When the sliding rheostat is connected to the circuit, the connection mode of "one up and one down" should be adopted.
(4) In order to protect the circuit, before turning on the power supply, the slider should be adjusted to the end with the largest resistance.
● Resistor boxes are sometimes used in laboratories, which are composed of 9 resistors1000Ω, 9 resistors100Ω, 9 resistors10Ω and 9 resistors10Ω. When in use, connect the terminals A and B into the circuit and adjust the four knobs on the panel to obtain any integer resistance between 0 and 9999 Ω. When reading, multiply the indication of the indication point corresponding to each knob by the multiple marked on the panel, and then add them to get the resistance value of the access circuit.
The resistance box is a kind of rheostat which can display the resistance value. It is similar to a sliding rheostat, which can change the resistance connected to the circuit; The difference is that the resistance box can read the output number, but the sliding rheostat can't.
Chapter VII Ohm's Law
Firstly, the relationship between the power supply on the resistor and the voltage at both ends is explored. Second, ohm's law and its application
Knowledge point 1 Relationship between current and voltage and resistance
The current in a conductor is related to the voltage across the conductor and the resistance of the conductor. When studying the qualitative relationship between them, we use the control variable method.
● When studying the relationship between current and voltage, keep the size of resistance unchanged, and study the relationship between current and voltage by changing the voltage at both ends of the conductor.
When studying the relationship between current and resistance, keep the voltage at both ends of the conductor constant, and observe the relationship between current and resistance by changing the resistance of the conductor. (See experimental teaching for the experiment)
Knowledge point 2 ohm's law
● 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.
(1) ohm's law formula. I =
U is the supply voltage in volts (V); I is the current passing through the conductor in ampere (A); R is the resistance of the conductor in ω.
Note that when using the formula of ohm's law to calculate, it must be unified to the international system unit before calculation. All physical quantities in ohm's law formula are identical, that is, I, U and R are for the same conductor at the same time.
(2)U=IR
When applying ohm's law formula and two variants to solve problems, as long as two of the three quantities I, U, R U and R are known, the third unknown physical quantity can be obtained. In the process of calculating and understanding problems, we must pay attention to the fact that the calculation of physical quantities is different from that in mathematics, and it is only meaningful to use the corresponding units of physical quantities. In order to avoid mathematizing physical problems, we should understand the physical meaning of each quantity.
● Physical meaning of the formula
The formula I = of (1) ohm's law shows that when the voltage across the conductor increases several times, the current in the conductor also increases several times. When the voltage at both ends of the conductor remains constant, the resistance of the conductor increases several times and the current in the conductor decreases to a fraction of the original.
(2) The derived formula U=IR indicates that the voltage across the conductor is equal to the product of the current passing through the conductor and its resistance.
(3) The derived formula R = indicates that the resistance of a conductor is numerically equal to the ratio of the voltage applied across the conductor to the current it passes through. Because the resistance of the same conductor is certain (the nature of the conductor itself), it cannot be said that "the resistance of a conductor is directly proportional to the voltage passing through it and inversely proportional to the current passing through it".
● Using the formula of Ohm's Law to solve problems.
When solving a problem, in order to analyze the problem easily, we should first draw a circuit diagram according to the meaning of the problem, and indicate the symbols of known physical quantities, numerical values and symbols of unknown physical quantities in the diagram. The units of the three physical quantities in the formula all adopt international (system) units.
Knowledge point 3 rated voltage
● Rated voltage: The voltage required for normal operation of electrical appliances is called rated voltage. If the actual voltage is much higher than the rated voltage, it is likely to damage the electrical appliance; If the actual voltage is much lower than the rated voltage, the electrical appliance will not work normally and sometimes it will be damaged.
● Rated current: The current flowing through electrical appliances at rated voltage is called rated current. For example, the light bulb is marked with the words "3.8V 0.3A", "3.8V" is the rated voltage of the small light bulb and "0.3A" is the rated current of the small light bulb. Generally, each electrical appliance is marked with rated voltage and rated current. The cause of electrical damage is often excessive current. When the actual current is greater than the rated current, it is easy to damage the electrical appliance. When the actual current is less than the rated current, the electrical appliance can not work normally, and sometimes it is easy to damage the electrical appliance.
Knowledge point 4 series-parallel connection of resistors
● Series connection of resistors
The total resistance of (1) series resistor is greater than that of any one sub-resistor.
(2) The total resistance of series resistors is equal to the resistance of each fractional R string = r 1+R2+...rn.
● Parallel connection of resistors
The total resistance of (1) parallel resistors is less than that of any one sub-resistor.
(2) The reciprocal of the total resistance of parallel resistors is equal to the sum of the resistances of individual resistors, that is,1/r =1/r1/R2+...1/rn.
Third, measure the resistance of the small bulb.
Knowledge point 1 ohm's law to measure resistance
By modifying the formula of Ohm's Law, we can get: r =, as long as the voltage across the inverter and the current through the conductor are measured, the resistance of this conductor can be measured (by calculation), and the unity of units is strictly required in measurement and calculation, that is, the unit of resistance is ω, the unit of voltage is V, and the unit of current is A. This method of measuring resistance is called voltammetry.
This method of measuring resistance by measuring voltage and current is an indirect measurement method.
Knowledge point 2 Voltammetry to measure the resistance of small bulbs
Whether a small bulb emits light or not, the filament temperature changes greatly when the brightness changes, and the temperature has a great influence on the filament resistance.
Small bulbs emit light normally at rated voltage, with high filament temperature and high resistance. When the actual voltage is lower than the rated voltage, it cannot emit light normally, and the filament microcomputer is low and the resistance is small. When the actual voltage is higher than the rated voltage, the filament is easy to burn out, which should be avoided.
Knowledge point 3 Voltammetry to measure the resistance of resistance
The resistance value of the constant value resistor is usually fixed. Under the rated voltage or when the actual voltage is lower than the rated voltage, the temperature change of the fixed resistance is very small, and the influence of temperature on the fixed resistance is very small, which can generally be ignored.
Four, ohm's law and the safe use of electricity
Knowledge point 1 The higher the voltage, the more dangerous it is.
● The human body directly or indirectly touches the fire line, and a large current passes through the human body, causing electric shock.
The cause of electric shock is that the human body directly or indirectly touches the fire line. The real cause of electric shock danger is that there is a large current passing through the human body.
Because the human body is a conductor, when the human body touches a charged body, there is a current flowing through the human body. The harm of electric current to human body is related to factors such as the magnitude of electric current and the length of electrifying time. When the current flowing through the human body is 8 ~1000 ma, it is difficult for the human hand to leave the charged body. When the current flowing through human body reaches 100 Ma, it will make people suffocate in a short time and stop their heartbeat, that is, when an electric shock accident occurs, the greater the current.
The voltage not higher than 36V is called safe voltage.
● The principle of safe electricity use is: Don't touch the low-voltage charged body and don't approach the high-voltage charged body.
● Division of high pressure and low pressure
The boundary between low voltage and high voltage is 1000 V, with low voltage below 1000V and high voltage above1000 v. Low voltage is not safe voltage for human body. To prevent low-voltage electric shock, do not touch low-voltage charged objects (mainly live wires). There are two kinds of high voltage electric shock: high voltage arc electric shock and step voltage electric shock. In order to prevent electric arc shock, you should stay away from places where electric arc is easy to occur. To prevent stepping voltage from getting an electric shock, you should squat down with your feet together or jump off the high-voltage charged body with your feet together.
Knowledge point 3 short circuit
● Definition: For some reason, two points in a circuit that should not be connected are directly connected together, which is called short circuit.
● Hazard of short circuit: Short circuit of power supply is very dangerous. Because the resistance of a wire is much smaller than that of a light bulb, the current passing through it will be very large. Such a large current, the battery or other power supply can not bear, the power supply will be damaged; What's more, due to the excessive current, the temperature of the conductor will rise, which may cause a fire in severe cases. In daily life, we often use fuses, air switches and fuses to prevent the harm caused by short circuit or overload.
● Short circuit is divided into power short circuit and electrical short circuit. When an electrical appliance is short-circuited, it is generally believed that there is no current flowing in the electrical appliance, which will not damage the circuit.
● Three connection states of the circuit.
(1) path: The connected circuit is called a path, that is, a closed loop.
(2) Short circuit: directly connecting the wires at both ends of the power supply is called power supply short circuit; Connect the wires directly to both ends of the appliance, which is called short circuit.
(3) Open circuit: Open circuit is called open circuit (or short circuit).
Chapter VIII Electric Power
I. Electric energy
Knowledge point 1 electric energy
● Electric energy and ordinary energy.
Electric energy is a form of energy, and the acquisition of electric energy is transformed from various other forms of energy, and the transformation process of these energies is provided by various power devices and various batteries. For example, power plants include: thermal power generation, hydropower generation, wind power generation, solar power generation and so on. All kinds of batteries are: dry cells, storage batteries, silicon photovoltaic cells, solar cells and so on. , are devices that provide electrical energy, the essence of which is to convert other forms of energy into electrical energy.
Kinetic energy: refers to the energy that an object has when it moves. All moving objects have kinetic energy, such as cyclists, pedestrians, birds and spinning windmills.
Internal energy: in a sense, if an object is heated, we say that it has internal energy, such as fire heating, electric heating, electric furnace heating and so on.
Light energy: the energy that an object has when it emits light, such as the light of an electric lamp and the light of the sun.
Chemical energy: the energy possessed by an object when it undergoes chemical changes, in which chemical changes refer to the process in which an object generates a new substance from a substance. For example, thermal power generation depends on the combustion of fuel, and chemical changes have taken place during the combustion process, so chemical energy is converted into electrical energy during thermal power generation. The internal structure of dry battery is carbon rod and zinc plate, and chemical reaction can take place between them to make it have chemical energy, and then supply power to the outside.
● Electrical appliances
We call the equipment that works with electricity electrical appliances. Such as household lamps, televisions, refrigerators, electric fans, electric kettles, electric stoves, etc. They are all electrical appliances, and their characteristics are: consuming electric energy and converting it into other forms of energy. As shown in the figure:
● Unit of electric energy
(1) "Joule" and "kWh" with symbols of "J" and "kW" respectively. 6? 1h ",kwh is much larger than joule, and their conversion relationship is 1 kW? 6? 1h=3.6× 106J, and kWh is also called "degree".
(2) Perceptual knowledge of Joule and kWh,
The energy required to lift an apple from the ground to the desktop is about1j.
The electric energy consumed by flashlight 1s is about 1J,
The power consumption of microwave oven 1min is about1j.
The electric energy consumed by the tram running 0.85km is about 1 kW? 6? 1h，
1.6kg electric furnace steelmaking consumes about 1 kW? 6? 1h .
● Watthour meter: commonly known as watt-hour meter.
(1) The function of the electric energy meter is to measure the electric energy consumed by electrical appliances in a period of time.
(2) The external structure of the watt-hour meter is as shown. The top number shows the used electric energy in kWh, and the aluminum plate in the middle rotates when the measuring instrument consumes electric energy. When reading, note that the rightmost number is the number after the decimal point.
(3) Calculation method: The difference between the two readings on the counter of the electric energy meter is the power consumption degree of the electrical appliances during this period.
【 Note 】 When reading the numbers on the electric energy meter, whether the rightmost box is marked with double marks indicating the numbers after the decimal point.
(4) Several important parameters of electric energy meter:
① "220v" —— This watt-hour meter should be used in a 220v circuit.
② "10 (20) a" —— The rated current of this watt-hour meter is 10A, and the allowable current is large in a short time, but it cannot exceed 20A.
③ "50hz" —— This watt-hour meter is used for 50h AC circuit.
(4) "600 rpm? 6? 1h "-each electric appliance connected to this watt-hour meter consumes electricity 1 kW? 6? 1h electric energy, and the turntable on the electric energy meter turns 600 revolutions.
⑤IC card watt-hour meter and new watt-hour meter.
There are IC card watt-hour meters at present. The user buys an IC card and inserts it. The meter reads the amount in the card. Once the dosage is used up, the watt-hour meter will cut off the circuit. At this time, you need to go to the bank to store the value for the IC card and plug in the watt-hour meter again.
There is also a new type of electric energy meter, which has no rotating aluminum disk. The electric energy is calculated by the internal electronic circuit, and the reading is displayed by the LCD panel.
● Power classification
Power supply is divided into DC power supply and AC power supply. All batteries are DC power supply, and the direction of current obtained from batteries is constant, usually called DC. All generators (various power plants) are AC power supply, and the current that changes direction periodically when supplying power is called AC power supply. The application of alternating current is very common. The current in the household circuit and the current in the power supply circuit for production are alternating. Usually, the symbol marked with 50H indicates that the frequency of alternating current is 50H, that is, there are 50 cycles in 1; Whether it is DC power supply or AC power supply, the electric energy meter must be connected in series in the household circuit. Their essence is to convert other forms of energy into electrical energy when storing electrical energy, and to convert electrical energy into other forms of energy when supplying external power.
Knowledge point 2 electric power
● Electric work: The work done by current is called electric work. What is current doing work? For example, current makes the electric lamp glow through the electric lamp, current makes the motor rotate through the motor, and current makes the water temperature rise through the electric water heater. , are the current process of doing work. From the point of view of energy, when current does work, it consumes electric energy and obtains other forms of energy. The light emitted by an electric lamp is the process of converting electric energy into light energy. The brighter the electric light, the more work the current does, and the more light energy it gets. Electric motors and electric water heaters are similar to electric lamps, and electric energy is converted into kinetic energy and thermal energy respectively. If the current does more work, the motor will turn faster and the water temperature will rise higher. So the essence of work done by current is: how much work is done by current when electric energy is converted into other forms of energy.
● Symbol of electrical work: W.
● Unit: According to how much work the current has done and how much electric energy it has consumed, how much work the current has done is the same as how much electric energy it has consumed. So the unit of electric work is the same as the unit of electric energy, which is "Joule" and "kWh".
Second, electricity.
Knowledge point 1 What is electricity?
The rotating speed of the turntable of electric energy meter is related to the use of different electrical appliances. The turntable connected to the water heater rotates quickly, indicating that the water heater consumes energy quickly. The turntable connected to an ordinary light bulb rotates slowly, indicating that an ordinary light bulb consumes slow power. To be exact, different electrical appliances consume electricity at different speeds. In order to express the speed at which electrical appliances consume electric energy, the concept of electric power is introduced into physics.
● Physical meaning of electric power: Electric power is a physical quantity indicating the speed at which electrical appliances consume electric energy (also the speed at which current does work).
【 Note 】 The speed of power consumption can directly reflect the size of power. If an electrical appliance consumes electric energy faster and converts electric energy faster, its electric power will be greater. The slower the electric energy consumption, the slower the electric energy conversion and the smaller the electric power.
● Symbol and unit of electric power.
(1) symbol of electric power: p
(2) Unit of electric power: "Watt", abbreviated as "Watt", with symbol "W". For example, the bulbs marked 100W, 40W, 15W that we often observe refer to the electric power of the bulbs. The other unit is kilowatt, and the symbol is kilowatt. The conversion relationship between kilowatt and watt is 1kW= 103W.
[Note] Different electrical appliances have different power, so all kinds of electrical appliances have signs or instructions. We can know their power according to the parameters provided by the signs and descriptions.
● Definition and definition of electricity
(1) The power of an electrical appliance is equal to the electric energy it consumes in1s.
(2) definition of electric power: P=W/t
(3) the meaning and unit of the symbol
W- work done by current (electric energy consumed), in joule (j)
T- time spent-seconds
P- Electric power of electrical appliances-Watt (W)
[Note] The unit of electric power is Watt. According to P=W/t, if the work done by the current is 1J and the time taken is 1s, then p =1j/1s. It can be seen that Watt = Joule/sec, where Joule/sec consists of the unit of electric work and the unit of time, which is called synthetic unit. Its pronunciation is Joule per second.
The origin of knowledge point 2 kWh
● Deduction of electric power formula: ∵P=W/t ∴W=Pt.
● The meaning of 1 kWh:1kw electrical appliance use 1 h electric energy consumed. ..
[Description] 1, kW is used for the measurement of large electric energy in daily life? 6? 1h .
2.kW and kWh are two different units, which are easily confused in application and must be distinguished.
Knowledge point 3 How to measure electric power
The common formula for measuring electric power is P=IU.
Meaning and unit of symbols
Current current
Voltage-volt
Electric power -w
● Rated voltage and rated power
(1) The rated voltage refers to the voltage value when the electrical appliance works normally.
(2) Rated power refers to the electric power when an electrical appliance works at rated voltage.
(3) The actual voltage refers to the actual working voltage of the electrical appliance.
(4) The actual power refers to the electric power obtained from the actual voltage.
(5) Rating: An electrical appliance has only one rating; Practical means: an electrical appliance can have multiple practical means, mainly depending on the circuit where the electrical appliance is located.
It can be seen from the relationship of P=IU that if the voltage of an electrical appliance changes, the power it consumes will also change. Therefore, the normal operation of the electrical appliance means that the rated voltage of the electrical appliance is equal to the voltage across the connected circuit. When the electrical appliance works normally, U real =U amount, P real =P amount.
Knowledge point 4 summarizes the formulas of electric power and electric work.
● Electric power formula
(1) define the formula p = w/t.
(2) The formula for measuring electric power is P=IU.
(3) The deformation formula derived from Ohm's law is P=U2/R or P=I2R.
Low electric power calculation formula
(1)W=Pt, according to the formula defined by electric power.
(2)W=UIt, according to P=UI and the derived formula.
(3) Deformation formula: W=I2Rt or w = u2t/r.
I hope it helps you! ↖(^ω^)↗