I. Requirements for knowledge and skills
1, know what is charged object and triboelectrification;
2. Recognize that there are two kinds of charges in nature; Know how positive and negative charges are defined; Know the interaction between charges; Know the electric quantity and its unit, and know the structure and principle of electroscope;
3. Understand the formation of current, simple circuit and circuit diagram;
4. Understand the power supply and electrical appliances, and understand their functions from the perspective of energy conversion;
5. Understand the connection characteristics of series circuit and parallel circuit, and connect simple series circuit and parallel circuit.
Second, analysis of key points and difficulties
1. When an object has the property of attracting light and small objects, we say it is charged, or it is charged. It is customary to call a charged object a charged body.
2, make the object charged method:
① triboelectrification: The method of charging an object by friction is called triboelectrification.
② Contact electrification: An object is electrified by contact.
All glass rods rubbed with silk repel each other, and they are bound to be attracted by rubber rods rubbed with wool. These facts make people realize that there are only two kinds of charges in nature.
American scientist Franklin stipulated:
Positive charge: the charge of the glass rod after wiping the wire is positive charge, which can be represented by "+";
Negative charge: the charge brought by rubber rod rubbing fur is negative charge, which can be represented by "-".
Charge interaction: charges of the same kind repel each other, but charges of different kinds attract each other. Electroscope is an instrument commonly used in the laboratory to test whether an object is charged. Its principle: Like charges repel each other.
1, charging amount: the charging amount is called the charging amount (q). The unit is coulomb, and the symbol is C. Neutralization: the phenomenon that the same amount of heterogeneous charges are put together to completely cancel is called neutralization. Meta-charge: an electron with an electric quantity of1.6×10-19 coulombs is regarded as meta-charge.
2, the directional movement of the charge forms a current, and the condition of maintaining a continuous current in the circuit is:
① There must be a power supply.
(2) The circuit should be connected.
Function of power supply: It is a device that provides continuous current and converts other forms of energy into electrical energy. General power supply: dry battery, storage battery and generator. There are positive and negative electrodes on dry batteries and storage batteries. The anode of the dry battery is a carbon rod, which is one end with a copper cap when viewed from the outside; The negative electrode is a zinc tube (showing the cross section of the dry cell).
In electrical phenomena, there are some similar terms about charge, such as positive charge, negative charge, electron, free electron, positive ion and negative ion. These nouns have different meanings. When using these nouns, they can be distinguished as follows:
(1) In the problems of triboelectrification and contact electrification, it should be noted that electrons are shifting, and cannot be said to be "free electrons" or "free charges".
(2) Note that when discussing the microscopic mechanism of conductors and insulators, it is necessary to explain "free charge", that is, conductors can conduct electricity because there are a lot of free charges in conductors, while insulators are not easy to conduct electricity because there are few free charges in them. Metal conductors conduct electricity by "free electrons", while acid, alkali and brine solutions conduct electricity by "free ions". These "free electrons" or "free ions" are all free charges.
(3) Use "expense" in the definition. For example, current is defined as the directional motion of charge, and current direction is defined as the directional motion direction of positive charge.
3, the provisions of the current direction:
There are two kinds of charges. When there is current in the circuit, the directionally moving charge may be positively charged or negatively charged. Positive and negative charges may also move in opposite directions at the same time. The directional motion direction of positive charge is defined as the direction of current. The direction of current outside the power supply is from the positive pole to the negative pole of the power supply; The current direction inside the power supply is from the negative electrode to the positive electrode.
Knowledge of circuit diagram will require you to draw a simple circuit diagram of series circuit and parallel circuit.
Extended reading: analysis of junior high school physics circuit diagram
1, circuit diagram analysis method
First of all, when you see the circuit diagram to judge whether it is a series or parallel circuit, you must remember the characteristics of current and voltage of the two circuits and mark the known conditions on the diagram. Also, do more typical exercises in this area, especially the teacher's example. Ask students and teachers the same circuit diagram, they may have different views or analysis methods, but the result is that the same physics can't just rely on rote memorization, so you should observe more, be good at thinking about the circuit diagram and don't deliberately communicate with the teacher to draw the circuit diagram: First, distinguish whether it is series or parallel, if,
First of all, keep in mind the representative significance of electrical symbols, understand the functions of various components in the circuit, and combine with the circuits that are often encountered in life, see more, draw more, and connect with reality. When you meet the circuit diagram, look at it in the order of power supply+-switch-electrical appliance-power supply, and pay attention to the functions of series and parallel devices in the circuit (voltage division, shunt, current limiting, etc.). ). This still needs more practice.
The most basic thing is to find the positive pole of the power supply first. The current will start from the positive pole of the power supply and flow along the conductor. When you meet a fork in the road, look at it this way: if one of the fork in the road has no electrical appliances, but only a wire, or only an ammeter (the ammeter in junior high school can be considered to have no resistance, so it is equivalent to a wire), then all the current will definitely only take this fork in the road and will not go anywhere else. If one of the branches has a voltmeter (the voltmeter in junior high school is regarded as infinite resistance, which is equivalent to an open circuit), then the current will definitely not go in this direction.
In addition to the above two special cases, the current will flow to each branch at the fork in the road, and the current of each branch can be calculated by ohm's law.
Through the explanation and study of the knowledge of circuit diagram analysis method in physics, I hope students can master the above knowledge well, and I believe students will learn it well.
2. The circuit diagram is actually to see whether it is series or parallel.
Step 1: Cover all voltmeters and their left and right leads to the node.
Step 2: Look for other nodes. If there are any, look at what is between the two nodes, such as what current is measured by the ammeter (connected in series with it), and so on. (The resistance or ammeter from the power supply to the node is on the main road. To understand this, draw the main road with a pencil first. )
Step 3: Check again, check the circuit again, or cover the circuit with a voltmeter.
Step 4: See which electrical appliances or resistors are connected to the wires at both ends of the voltmeter, that is, which voltage is measured.
End. (PS。 This is all my experience when I first learned circuit. I hope it works for you. )
I hope students can master the knowledge of circuit diagram in physics well, and I believe students will get good grades in the exam.