Knowledge points of physics and electricity in grade three 1. Preliminary knowledge of circuits.
1. The international unit of current I is ampere (A); Commonly used units are: milliampere (mA) and microampere (? Answer: 1 amp =103mA =106mA.
2. The instrument for measuring current is ammeter, and its usage rules are as follows: ① ammeter should be connected in series in the circuit; (2) The terminals should be connected correctly, so that the current enters from the "+"terminal and exits from the "-"terminal; (3) The measuring current should not exceed the range of ammeter; ④ Never connect the ammeter to the two poles of the power supply without using electrical appliances.
3. Voltage (U): Voltage is the cause of current in the circuit, and power supply is the device that provides voltage.
4. The unit of voltage U is: the international unit is volts (V); Commonly used units are: kilovolt (KV), millivolt (mV), microvolt (? Chapter 5.1kv =103v =106mv =109mv.
5. The instrument for measuring voltage is voltmeter, and its usage rules are as follows: ① The voltmeter should be connected in parallel in the circuit; (2) The terminals should be connected correctly, so that the current enters from the "+"terminal and exits from the "-"terminal; (3) The measured voltage should not exceed the range of voltmeter.
6. Resistance (R): indicates the resistance of the conductor to the current. The greater the resistance of a conductor to current, the smaller the current passing through the conductor.
7. Unit of resistance (R): international unit: ohm (ω); Commonly used units are: Megaohm (mω) and Kilohm (kω).
8. 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 (the resistance has nothing to do with the voltage applied across the conductor and the current passing through it).
9. Rheostat: sliding rheostat and resistance box.
Second, ohm's law.
1. 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.
2. Formula: (I=U/R), where the unit is: i→ an (a); U→ volt (v); r→ω(ω)。 1 A = 1 V/O。
3. The application of ohm's law.
(1) The same resistor has the same resistance regardless of current and voltage, but when the voltage across the resistor increases, the current passing through it also increases. (R=U/I).
(2) When the voltage is constant, the greater the resistance, the smaller the current. (I=U/R).
(3) when the current is constant, the greater the resistance, the greater the voltage across the resistance. (U=IR).
4. The series connection of resistors has the following characteristics.
① Current: I=I 1=I2 (the currents in all parts of the series circuit are equal).
② voltage: U=U 1+U2 (the total voltage is equal to the sum of all voltages).
③ Resistance: R=R 1+R2 (total resistance is equal to the sum of all resistances). If n resistors with the same resistance are connected in series, r =nR.
④ Partial pressure.
5. The parallel connection of resistors has the following characteristics.
① Current: I=I 1+I2 (the main current is equal to the sum of the currents of each branch).
② voltage: U=U 1=U2 (main voltage is equal to each branch voltage).
③ Resistance: (the reciprocal of the total resistance is equal to the sum of the reciprocal of the parallel resistances) If n resistors with the same resistance are connected in parallel, the sum is1/r =1/r1+1/r2.
④ Shunt function: I1:I2 =1/r1:1R2.
Third, electricity and electric heating.
1. Electric work (w): The work done by current is called electric work.
2. Electric power unit: international unit: Joule. Commonly used units are: kWh, 1 degree = 1 kWh =3.6× 106 joules.
3. Electric power calculation formula: W=UIt (where the unit is W→ coke (j); U→ volt (v); I→an(a)； T→ seconds).
4. The following formula can also be used to calculate electric work: w = i2rtW = PtW=UQ(Q is electric quantity).
5. Electric power (P): the work done by current in unit time. Unit: Watt (international); Commonly used units are: kilowatts.
6. Electric power calculation formula: (where the unit is powerW→ Coke; T→ seconds; U→ volt (v); I→ an (a)。
7. The correct formula can also be used to calculate electric power: P=I2R, p = U2/r. ..
8. Rated voltage (U0): the voltage at which the electrical appliance works normally.
9. Rated power (P0): the power of electrical appliances at rated voltage.
10. actual voltage (u): the voltage actually applied to the electrical appliance.
1 1. Joule's Law: The heat generated by a current passing through a conductor is proportional to the square of the current, to the resistance of the conductor, and to the electrifying time.
12. Joule's law formula: Q=I2Rt, (where the unit is Q→ coke; I→an(a)； R→ Europe (ω); T→ seconds).
Expanding reading: the way to learn physics 1. Do the problem independently.
Do some problems independently with good quality and quantity (meaning not relying on others). There must be a certain number of topics, not too few, but also a certain quality, that is to say, there must be some difficulty. Anyone who studies mathematics and physics can't learn this level well. Solving problems independently may sometimes be slow, sometimes you have to take detours, and sometimes you can't even solve them, but these are normal and the only way for any beginner to succeed.
2. Physical process.
To clarify the physical process, there must be hidden dangers in solving problems if the physical process is not clear. No matter how difficult the topic is, try to draw as much as possible, some can draw sketches, and some need to draw accurate pictures, using compasses, triangles, protractors and so on. To represent geometric relationships. Drawing can turn abstract thinking into image thinking and master physical process more accurately. With this diagram, we can make state analysis and dynamic analysis. State analysis is fixed, dead and discontinuous, while dynamic analysis is alive and continuous.
3. attend class.
Listen carefully in class and don't think or think as little as possible. Don't be self-righteous, learn from the teacher modestly. Don't give up listening just because the teacher speaks simply. If this happens, it can be regarded as review and consolidation. Try to be consistent and synchronized with the teacher, and you can't do it yourself, otherwise it will be self-study. After getting started, with a certain foundation, you are allowed to have your own activity space, that is, you are allowed to have something of your own. The more you learn, the more things you have.
4. Notebook.
Class is mainly based on listening, and there are notebooks, and some things should be written down. Knowledge structure, good problem-solving methods, good examples, places you don't understand, etc. Should be recorded. You should sort out your notes after class, on the one hand, for digestion, and on the other hand, for supplementary notes. Notebook is not only to remember what the teacher said in class, but also to make some reading notes. Good problems found in homework and good solutions should also be recorded in a notebook, which is what students often say is a good problem book. Hard-won notebooks should be numbered, studied later and kept for life.
5. Learning materials.
Learning materials should be well preserved, classified and marked. The classification of learning materials includes exercises, papers, experimental reports and so on. Marking refers to, for example, marking exercises. General questions are not marked. Good questions, valuable questions and error-prone questions are marked differently for future reading. Marking can save a lot of time.
6. Time.
Time is precious. Without time, there is no time to do anything, so we should pay attention to making full use of time, which is a very superb art. For example, we can use memory learning to save time, such as going to bed, waiting for the bus, walking on the road and so on. And we can recall the lessons of the day section by section, so that we can learn again and again to achieve the purpose of strengthening. Some physics problems are more difficult, and some problems may be solved on foot. People who study physics often have several problems in their minds that they can't do and won't forget. I don't know when they will make a breakthrough and find the answer to the question.
7. Learn from others.
We should learn from others, from our classmates, from people around us and see how others learn. We should always have academic exchanges with them, teach each other, learn from each other and improve together. We must never be self-righteous. Cann't be conservative Tell others if you have a good method, so that others will tell you that you have a good method. You should have some good friends in your study.
8. Knowledge structure.
We should attach importance to the knowledge structure, master it systematically, and systematize scattered knowledge. From the whole knowledge structure of physics to the knowledge structure of mechanics, even to chapters, such as the knowledge structure of statics and so on.
9. Use math.
The calculation of physics depends on mathematics, which is too important for learning physics. Physics is difficult to do without mathematics as a calculation tool. In the physics department of the university, mathematics and physics are equally important. To learn mathematics well, we must make good use of this powerful tool.
Junior high school physics memory formula 1. Circuits and characteristics.
The triboelectric capacity is large, and the electron transfer changes; Attract and repel electroscope, and pay attention to electrostatic discharge.
The fur rubs against the rubber rod, and the negative electricity on the rod is relatively strong; Silk rubs the glass rod, while silk rubs the glass.
Directional motion is converted into current, and the direction of current is specified; The outside of the power supply is positive to negative; Free electrons flow backwards.
Conductor is easy to conduct electricity, and insulation is difficult to conduct electricity; Insulation free charge is less, to prevent leakage and electric shock.
Learn to draw components before drawing circuits, and careful standardization is the key; The whole figure is a long frame, and the components are evenly placed in four directions.
Leave the corner blank and let the standard appear; The path is open and short-circuited, and the power supply is burned all the way at last.
The basic circuits are connected in series and parallel, and distinguishing the characteristics is the key; Series connection is a path, starting from the positive pole and returning to the negative pole.
Yi Deng burns the whole road, and one switch controls the whole situation; The switch position has no influence, and the local short circuit is special.
Parallel circuit is like a river, which divides the main road into branches, the main road switch is fully controlled and the branch appliances are independent.
Series current sharing and voltage dividing, parallel shunt voltage dividing, etc. The bright resistance of series lamps is large, and the bright resistance of parallel lamps is small.
2. Lighting circuit and safe electricity use.
The zero line of fire line should be distinguished and always parallel on the schematic diagram; The watt-hour meter is used to measure electric energy, and the fuse is on the main road.
All kinds of sockets should be connected in parallel, and the electrical room should also be included; The bulb switches are connected in series, and the switches are connected with live wires.
The tail metal wire is the safest method; The zero line should be connected with a spiral sleeve, and remember to prevent electric shock.
For electrical appliances with metal shell, the middle pin should be grounded; Three hole socket uses two holes, and the insulation damage is too dangerous.
Excessive power will overload and short circuit will be more dangerous. If the safety device works, the nameplate of the electric meter will be calculated.
Safe voltage should keep in mind that the path will get an electric shock and the high voltage will not approach. If you get an electric shock, cut off the power first.
Be careful to shelter from the rain under the tree, and install lightning rods for tall objects; Don't pull the switch when your hands are wet, and always replace the aging parts.
3. Voltammetry is used to measure resistance and power supply connection circuit.
In the process of connection, draw the circuit, connect the components and turn off the switch.
Slide to the maximum end, and the voltmeter and ammeter are connected in series.
"Positive" and "negative" needles are connected by mistake, and then the instrument arranges and calculates.
"The three quantities of the same conductor, I and U are directly proportional to I and R. No matter how many transformations I and U have, it is understood that R is a constant.
W=UIt, which can be recorded in a homophonic way: "Kick again if it's too big".
4. Electricity and magnetism.
There is a magnetic field around the magnet, and the direction of magnetic induction is from north to south, as is the off-site North Pole.
When the closed conductor cuts the magnetic induction line, the induced current will appear. Change direction, change direction.
Mechanical energy is converted into electrical energy. Faraday's contribution to electromagnetic induction power generation is extraordinary.
Judge the screw with amperes and hold the solenoid with your right hand.
The current direction is four fingers, and the N pole points to the thumb.
5. mechanical movement.
Motion and stillness are important in choosing reference objects, speed and direction, and so is stillness.
"Objects have inertia. Inertia is an attribute. Its size depends on mass, regardless of motion and stillness.
6. Calculation of average speed.
Schematic diagram of the movement route, the movement problem is clearer.
Remember clearly when crossing the bridge and piercing the hole. The length of the bridge is the length of the bus.
Have the knack of meeting and chasing, and find a good path sequence equation.
Echo laser ranging, twice the distance is the distance.
7. Conditions for the balance of two forces.
One thing and two forces can be balanced and in opposite directions.
A straight line is a condition, and the resultant force must be equal to zero.
8. Steps to try.
Draw a sketch, two fixed points, three fixed proportions and four lines.
Draw five line segments with six points, and mark the final data with sharp edges.