Summary of ninth grade electricity

Knowledge points of electricity in junior high school

(A) simple electrical phenomena

1. Friction electrification

(1) Making an object charged by friction is called triboelectrification.

(2) Reasons for triboelectrification: Nuclei of different substances have different abilities to bind electrons. When two objects rub against each other, which object's nucleus binds electrons weakly, some of its electrons will be transferred to the other object; Objects that lose electrons are positively charged due to lack of electrons, and objects that gain electrons are negatively charged due to too many electrons.

(3) Friction charging, contact charging and neutralization are all electron transfer.

(4) The positive matter of an object is electron-deficient, and the negative matter of an object is right-excess electron.

Triboelectrification refers to the transfer of electrons from one object to another without generating or generating charges, and the total amount of charges remains unchanged.

2. Two kinds of accusations

(1) When an object has the property of attracting light and small objects, we say that it is charged, or charged, that is, charged objects can attract light and small objects.

(2) There are only two kinds of charges in nature: the charge on the glass rod rubbed with silk is called positive charge, and the charge on the rubber rod rubbed with fur is called negative charge.

(3) Interaction between charges: the same kind of charges repel each other and the different kinds of charges attract each other.

(4) People know that there are only two kinds of charges in nature, which is deduced on the basis of experiments.

A charged body attracts another object, and the other object must have a different charge from the charged body. It should be correct. Another object may have a different charge from a charged body. If it is light and small, it may not be charged. Because there are two possibilities of attraction, namely: charged bodies attract light and small objects; Heterogeneous charges attract each other.

An electroscope is an instrument to check whether an object is charged. Electrification of electroscope is a contact electrification phenomenon.

Methods to test whether an object is charged: ① Put the object close to a light and small object, and if it can attract, it means that the object is charged. (2) A charged body with a known charge.

If there is repulsion near the object, it means that the object is charged; (3) The object touches the metal ball of the electroscope. If two pieces of metal foil are opened, it indicates that the object is charged.

(2) Current

1. Formation of current

(1) The directional movement of charge forms current.

(2) The current in a metal conductor is formed by the directional motion of free electrons.

(3) The current in acid, alkali and brine solution is formed by the simultaneous movement of positive and negative ions in opposite directions.

(4) To obtain continuous current, there must be a power supply and the circuit must be closed.

2. Direction of water flow

(1) specifies that the direction of positive charge directional movement is the direction of current.

(2) Outside the power supply, the current direction is from the positive pole to the negative pole of the power supply.

In the actual general circuit, the free electrons in the metal wire move directionally to form a current, and the direction of the current is opposite to that of the free electrons. In acid, alkali and brine solutions, the direction of current is the same as that of positive ions, but opposite to that of negative ions.

3. The influence of current

When current passes through a conductor, it consumes electric energy and converts it into other forms of energy. What happens when a current passes through a conductor is called the current effect.

The thermal effect, magnetic effect and chemical effect of current transform electric energy into internal energy, magnetic energy and chemical energy respectively, which have been widely used in life and production.

The chemical effect of current must occur simultaneously with the thermal effect and magnetic effect of current.

(3) Circuit

1. Circuit and circuit diagram

A current path consisting of power supply, electrical appliances, wires and switches is called a circuit.

Power supply is a device to provide electric energy in the circuit, and it is a necessary condition to form continuous current in the circuit. All kinds of electrical equipment in the circuit are collectively called electrical appliances, which are devices that consume electric energy and convert it into other forms of energy. Wires transmit electric energy. The switch can make or break the circuit.

Circuit diagram: a diagram showing the connection relationship of various elements in a circuit with specific symbols.

2. Three states of the circuit

A short circuit of a broken road.

summary

Everywhere connectivity

The circuit is based on

Rest somewhere

Incomplete circuit without current

Conductors do not pass through electrical appliances.

Directly connected to the two poles of the power supply.

Connecting circuit

Phenomenon (power consumption

Current in the device)

These appliances include

Current transmission

No electrical appliances

Current transmission

The current is very large and the power supply is on again.

The wire is stuck by overheating.

burn-up

Short circuit and local short circuit-the former is a circuit in which wires are directly connected to the two poles of the power supply without electrical appliances, and the latter is a circuit in which wires are directly connected in parallel at both ends of the electrical appliances. If there are other appliances in the circuit, it will not cause a short circuit of the power supply. This circuit is a local short circuit. When there is a local short circuit, there is almost no current flowing through the short-circuited electrical appliance, and this electrical appliance will no longer work and will not be damaged. When other electrical appliances in the circuit work, the current will increase.

3. Circuit connection mode

Name series circuit parallel circuit

concept

Connect the electrical appliances one by one.

The connected circuit is connected in parallel with the electrical appliance.

The circuit is based on

connect

There is only one current path.

There are two or more paths of current.

path

Consume electricity

Mechanic

state

also

Work interaction of various electrical appliances

Affect each other. When a person uses

When an electrical appliance is broken or damaged,

The other uses electrical appliances.

Can not work

Electrical appliances can work independently,

Do not affect each other. household appliances

Other electrical appliances are damaged or broken.

It can still work normally.

1 switch

1

F is used to control the whole circuit.

When disconnected, the position of the switch will change.

The switch on the main road controls the whole.

On-off of circuit, on-off of branch

Close only controls the on-off of this branch.

Examples of the causes and functions of the concept of name

conductor

An object that conducts electricity easily.

have ability

By moving

electric charge

Used to conduct electricity

Metal, graphite, human

Body, dirt and acid,

Aqueous solution of alkali and salt

They are all conductors.

insulator

An object that does not conduct electricity easily

Almost charged

Inevitably

Atomic specification

Inside, no.

free movement

Prevent leakage

Electricity and people

get an electric shock

Rubber, glass, pottery

Porcelain, plastic, oil, pure

water

Circuit on-off method: an electrical appliance can be removed from the circuit. If it affects the normal operation of other electrical appliances, this circuit is a series circuit, otherwise it is a parallel circuit.

Current direction method: the current in the series circuit starts from the positive pole of the power supply, and then flows back to the negative pole of the power supply without branching, passing through various electrical appliances in turn; The current in the parallel circuit starts from the positive pole of the power supply, divides into several branches from the "branch point", then gathers at the "confluence point" and flows back to the negative pole of the power supply.

Equivalent circuit method: sometimes the circuit diagram is complicated, so it is difficult to see the connection relationship between several electrical appliances and draw the flow direction of current. At this time, we can change the original circuit into an intuitive and easy-to-understand form, but we can't change the functions of each component.

1. We often see an eye-catching slogan "It is forbidden to transport gasoline in plastic barrels" at gas stations. Please tell us the reason.

2. The experimental circuit connected by Xiao Ming is shown in Figure 6- 18-3. The circuit diagram of his connection to this circuit is () in Figure 6- 18-4.

3. Among the four circuits shown in Figure 6- 18-4, the one corresponding to the physical diagram shown on the right is ().

As shown in Figure 6- 18-6, if the switch S is closed, the lights Ll and L2 will not light up. Xiaohua used a wire to find the circuit fault. When she connects the wires to both ends of the lamp L 1, neither lamp will light up. When wires are connected to both ends of the lamp L2, the lamp L 1 will be lit, but the lamp L2 will not be lit. It can be judged from this ()

A. the lamp Ll 8 is short-circuited. L2 lamp short circuit

C. the light L 1 is turned on. The light L2 is on.

4, use a switch to control two lights and a bell at the same time, three electrical appliances in the circuit ().

A. 8 must be connected in series. It must be connected in parallel C. It can be connected in parallel or in series D. It can be connected in series.

5. In the circuit shown in Figure 6- 18-7, how are the three light areas L 1, L2 and L3 connected?

6, as shown in figure 6- 18-8, in the horizontal

There are three identical wires in the hard plastic pipe crossing the river. In order to distinguish which two wires are the two ends of the same wire, you can use the detector as shown in the figure to test, and its operation process is as follows:

(1) Connect A and B, and connect M of the detector to D. When N is connected to F, the small bulb emits light, and when E is connected, the small bulb does not emit light. From this, it can be determined that CE is both ends of the same wire (fill in letters, the same below).

(2) In order to find out the two ends of the other two wires, A and C can be connected. When one end of the probe must be connected to E, the other end only needs to touch one wire to distinguish the other two wires.

7, the following phenomenon, can't use electrostatic knowledge to explain is ().

A. lightning rods should be installed on tall buildings.

B.an oil tanker often has an iron chain at its stern.

C, install an insulating sleeve on the handle of the wire cutters.

D. Dust will be adsorbed on the TV screen.

8. As shown in Figure 6- 18- 10, carbon rods A and B are immersed in copper sulfate solution, and red copper adheres to carbon rod A after being electrified. The following statement is true ()

Alternating current is the positive pole of power supply.

B.d is the positive pole of the power supply.

The free electrons in the C.C.AC line move from A to C D, and the current direction in the BD line is from B to D.

9. In the circuit shown in Figure 6- 18-ll, when both s 1 and S2 are closed, then ().

A.l 1, L2 series connection

B.L 1 and L2 are connected in parallel.

C. lamp L 1 short circuit.

D.L2 lamp short circuit

10, as shown in Figure 6- 18- 12. When the switch S is closed, the possible phenomenon is ().

A.ll emits light, while L2 does not.

B.Ll burned out. L2 is exhausted. D. The battery is burnt out.

1 1. In general, the following objects belong to conductors: () A. Salt B. Distilled water C. Silicon D. Graphite.

12, 8. Find a thick copper wire with a length of 5 cm, and then cut a zinc skin with a width of 2 mm from the waste dry battery. After scraping, insert copper wire and zinc skin into fruits (apples, tomatoes, etc.). ) respectively. Then the two wires of the earphone are in continuous contact with the copper wire and the zinc skin respectively, and a "giggle" sound can be heard from the earphone, indicating that there is a continuous current passing through the earphone. Try to tell the direction of the current in the earphone at this time.

13. The circuit of an instrument is shown in Figure 6- 18- 13, in which a metal slider M with a thin metal rod is connected with metal springs on both sides and connected to the circuit, and the metal elbow and spring are sleeved on a smooth and insulated horizontal thin rod. Now the instrument is fixed on a car, and the _ _ _ _ _ light is on when the car starts (choose "red light" or "green light"); When the car brakes suddenly, _ _ _ _ _ _ _ lights up (choose "red light" or "green light").

14, Xiaoli wants to explore what factors are related to the interaction between charges. She put a positively charged object in position A, and then hung the charged ball on the silk thread in positions P 1, P2 and P3. The movement of the force on the ball is shown in Figure 6- 18- 14. It is known that the ball belt is charged _ _ _ _ _ _ (choose "positive" or "negative"), and the preliminary conclusion is: _ _ _ _ _ _ _ _ _ _ _ _

15, 1 1. Connect the elements in Figure B according to the circuit diagram shown in Figure 6- 18- 15A. (Draw lines to indicate conductors)

13. Figure 6- 18- 17 is a closed control box, and A, B and C are three terminals. M and n are electric light and electric bell respectively. When the current line is connected to A and B, the lamp will not ring; When connecting B and C with wires, the bell light does not light up; When connecting a and c with wires, the light does not light up and the bell does not ring. Try to draw this circuit diagram.

14. Traffic safety can be ensured by installing traffic lights at the intersection of two roads, as shown in Figure 6- 18- 18A. When the L2 green light in AB direction is on, cD. The direction red light L4 is on at the same time; When the red light L4 in AB direction is on, the green light L2 in cD direction is on at the same time. Please draw the connection diagram of L 1, L2, L3 and L4 in the dotted box in Figure B according to the above requirements. Complete a "traffic light" analog circuit (S in Figure B is a single-pole double-throw switch. Note that the codes L 1, L2, L3 and L4 of the four lights must be marked on the drawing.

15. Figure 6- 18- 19 is a hair dryer circuit, in which the heating wire will generate heat when energized, and the motor will supply air when energized. When selecting which two contacts the switch S touches at the same time, what hot air does the blower emit?

Current-voltage resistance

Name current I voltage u resistance r

concept

Indicates the strength of the current.

Causes of current formation

A pair of conductors representing electricity.

The size of the flow obstacle

Unit ampere volt ohm

unit

convert

1 ma = 10-3 amp

1μ= 10-6A

1 kv =103v

1 mv =10-3v

1μV = 10—6V 1mω= 106ω

1kω= 103ω

Measuring ammeter voltmeter voltammetry

① Voltage is the cause of current formation.

There are a lot of free charges in the conductor, which are constantly moving irregularly in all directions. So objectively speaking, there is no current in the conductor, and the free charges everywhere in the conductor move almost simultaneously under the push of voltage, forming a current at the same time in the whole circuit. Voltage is used to generate electricity.

The directional movement of free charge in the path causes the current.

② The function of power supply is to provide voltage.

A power supply is a device that provides voltage in a circuit. In the process of operation, the positive charge is continuously accumulated at the positive pole of the power supply, and the negative charge is continuously accumulated at the negative pole of the power supply, thus generating voltage between the two poles of the power supply. From the perspective of energy conversion, power supply is a device that converts other forms of energy into electrical energy.

Where there is voltage, there is current. It should be correct to have voltage and no current. For example, there is voltage at both ends of the power supply. If it is not connected in the circuit to form a closed loop, there will be no current.

① Determinant-The resistance of a conductor is a property of the conductor itself, and its size depends on the material, length and cross-sectional area of the conductor, and has nothing to do with the applied voltage and the current passing through the conductor. The same voltage is applied to different conductors, and the current in the conductors is different, which means that the conductivity of different conductors is different, or they have different obstacles to the current. Experiments show that for the same conductor, the ratio of the voltage across it to the current intensity passing through it is certain, but for different conductors, the ratio is different. It can be seen that this ratio (U/I) reflects the conductivity of the conductor, which we call the resistance of the conductor. Resistance is a physical quantity representing the conductivity of a conductor and a property of the conductor itself.

② The influence factor-the resistance of the conductor is also related to the temperature. The resistance of most conductors increases with the increase of temperature, and the resistance of a few conductors (such as carbon) decreases with the increase of temperature.

③ Superconducting phenomenon: For most conductors, the higher the temperature, the greater the resistance. When the temperature of some metals or alloys drops to a certain value, their resistance becomes zero. This phenomenon is called superconductivity.

(2) Law

1. Current Law in Series and Parallel Circuits

(1) The currents everywhere in the series circuit are equal. I=I 1=I2.

(2) The parallel circuit main current is equal to the sum of each branch current l = i 1+I2.

2. Rules for series and parallel battery packs

(1) The voltage of the series battery pack is equal to the sum of the voltages of each battery.

(2) The voltage of the parallel battery pack is equal to the voltage of one battery.

3. Voltage law in series and parallel circuits

(1) The total voltage of the series circuit is equal to the sum of the voltages of all parts of the circuit U=U 1+U2.

(2) The voltages at both ends of each branch of the parallel circuit are equal U=U 1=U2.

(3) Instruments

1. ammeter and voltmeter

Both ammeter and voltmeter used in laboratory have three terminals and two measuring ranges. Pay attention to the zero line, range and dividing value before use.