Composition of 1. circuit

A correct circuit should have the following basic elements. Power supply, electrical appliances, switches and wires. The function of power supply is to convert other forms of energy into electric energy and provide electric energy; Wires play the role of connecting circuit elements and transmitting electric energy to electrical appliances; The switch controls the transmission of electric energy (the on-off of current); Convert electric energy into other forms of energy with electrical appliances. If a circuit lacks some of these four basic elements, the circuit will not work or be wrong or dangerous (short circuit).

2. Three states of the circuit

(1) path: connected circuit. Features: There is current in the circuit and the electrical appliance works normally. (2) Open circuit: Open circuit. Features: There is no current in the circuit, so the appliance can't work. (3) Short circuit: Definition: The two ends of a power supply or an electrical appliance are directly connected by wires (the current does not pass through the electrical appliance). Features: The power supply is short-circuited, and there is a large current in the circuit, which may burn out the insulation of the power supply or conductor and easily cause a fire. In parallel circuit, once one branch is short-circuited, the whole circuit will be short-circuited.

Open circuits and paths are common states of circuits, for example, lights are turned on and off. Short circuit is a wrong dangerous state and should be absolutely avoided.

3. Local short circuit in series circuit

In a series circuit, for some reason or practical need, one electrical appliance in the circuit is short-circuited, while other electrical appliances are still working. In the circuit shown in the figure, when the switch S is closed, L 1 is short-circuited, and L2 still has current, so it can emit light.

4. Connection mode of the circuit

The series-parallel connection of circuits is the electrical knowledge that junior high school must master, and it is the basis of circuit analysis and calculation.

Comparison of (1) circuit connection modes.

series connection

Parallel connection method

pressure

A circuit that connects elements end to end.

A circuit that connects elements end to end in parallel.

architectural feature

There is only one current path in the circuit, and there are no branches.

There are branches in the circuit (there are separation points and convergence points) and at least two current paths.

operating characteristic

All electrical appliances are interactive. When they are turned on in one place, all electrical appliances stop working.

Electrical appliances in each branch work independently and do not affect each other.

change

use

Control the whole circuit

The switch in the main circuit controls the whole circuit. The switch in the branch controls the branch.

circuit diagram

Real example

Decorate small colored lights, switches and electrical appliances

Household appliances and street lamps.

(2) Common methods for judging the series and parallel connection of circuits.

Based on the learning requirements of junior high school, this paper only introduces the following common judgment methods, and you can skillfully choose the method that suits you in practice.

① Definition method: number the electrical terminals, with the current inflow end as "head" and the current outflow end as "tail". Observe all electrical appliances, if the "head → tail → head → tail" connection is in series; If "head → head" and "tail → tail" are connected, they are parallel.

② Structural characteristic analysis method: When there is a circuit diagram or physical connection diagram, the circuit can be identified along the current direction: positive power supply → all electrical appliances → negative power supply. If there is no node (bifurcation point) on the way, the current will go along a path without shunt, and the appliances in this circuit are connected in series; If the circuit has nodes, the currents are separated in one place and merged in another place, these appliances are connected in parallel.

③ Analysis of working characteristics: Remove any electrical appliance, and connect two electrical appliances in series if the other electrical appliance doesn't work; If the other appliance is still working without being affected, the two appliances are connected in parallel. In the absence of circuit diagram or physical circuit diagram (such as street lamp and household circuit), the connection can be judged according to the working characteristics of electrical appliances.

Second, the application example

1.(09 Jining) In the circuit shown in Figure 1, when the switch S 1 and S2 are closed, the correct statement is ().

A.l 1 and L2 can both emit light.

B.l 1 and L2 cannot emit light.

C.Ll can shine, but L2 can't.

D.Ll can't shine, but L2 can.

Analysis: When S 1 is off and S2 is closed, L 1 is connected in parallel with L2; When S 1 is closed, L 1 is short-circuited. Because it is a parallel circuit, a short circuit occurs in one place, and the whole circuit is in a short circuit state. So L 1 and L2 can't emit light, and b is correct.

2.(09 Lanzhou) As shown in Figure 2, when the switch S is closed, the correct circuit for two small bulbs to emit light at the same time is ().

Analysis: Among the four circuits, in circuit A, two lamps are connected in series, and switch S is connected in parallel with one lamp. When the switch is closed, the lights connected in parallel with the switch will be partially short-circuited, so only one light will be on after the switch is closed. In circuit b, the switches are connected in series in the circuit. When the switch is closed, the circuit is a path. So two lights in series will emit light, and B is correct. There is no power supply in circuit C, which is an error circuit with incomplete basic components. In D circuit, two lamps are connected in parallel, and when S is closed, both lamps will emit light, but one of them has no switch control, which does not meet the basic connection requirements of the circuit.

3.(09 Chengdu) As shown in the figure, it is a simplified circuit of hair dryer, which can blow both cold air and hot air. In the picture, A is a hair dryer and B is a heating wire. Plug the plug into the socket. If only the switch S 1 is closed, the blower will blow out _ _ _ _ wind; If the switches S 1 and S2 are closed, the blower will blow out _ _ _ _ wind. (Fill in "hot" or "cold")

Analysis: In this device, the fan and the heating wire are connected in parallel, and S 1 is the main switch. Once turned off, the blower will start to work and blow out cold air. Only when S 1 and S2 are closed at the same time can the heating wire cooperate with the blower to blow out hot air. So, the answer is: cold, hot.

Further analysis: since the hair dryer and the heating wire are connected in parallel, why not install a switch to control them separately? Because the heating wire can't work alone. If you work alone, the heat generated cannot be dissipated in time, and the internal temperature of the hair dryer will be very high, which will burn out the shell of the hair dryer and even bring danger. Therefore, this design can prevent the electric heating wire from working independently, so that the hair dryer can blow out both cold air and hot air.

4.(09 Anshun, Guizhou) In the circuit shown in the figure, when _ _ _ _ _ is closed, L 1 is connected in parallel with L2; When _ _ _ _ _ _ is closed, the circuit will be short-circuited and the power supply will be damaged.

Analysis: This question examines the knowledge of series connection, parallel connection and short circuit. Only when S2 is closed, L 1 is connected in series with L2; When S2 is off and S 1 and S3 are closed at the same time, L 1 and L2 are connected in parallel; S 1 cannot be opened. When S2 and S3 are closed at the same time, the circuit is short-circuited.

Answer: S 1S3, S2S3.