Some questions about short circuits in junior high school physics and electricity

Short circuit, to put it bluntly, means that your simplified circuit diagram directly has a wire connected to both ends of the power supply without any electrical appliances (such as resistors, bulbs, motors, heating wires, etc.), such a circuit will burn out the power supply.

PS: The current in the line is too large.

Local short circuit: In fact, it is not essentially the same property as the above short circuit, for example (in an arbitrary circuit diagram containing multiple electrical devices, you take a wire and connect it in parallel with one of the things, such as a light bulb, then the light bulb will not turn on, then the light bulb is partially short-circuited).

PS: There is almost no current in the line in the short-circuited part.

When the circuit is formed, if there is a conductor with resistance close to zero and the power supply forms a closed loop, then the current will all pass through this conductor, another element with resistance will be short-circuited, the short circuit is relative, it requires that the resistance of one of the components must be extremely close to zero ohms, and the other element must have resistance, then the element with resistance will be short-circuited by the element without resistance.

Power short-circuit: If you can start from the positive pole of the power supply, if you can directly connect to the negative pole by the wire without passing through any resistance (electrical appliance), it is a power short-circuit.

Local short circuit: If there is a direct wire communication between the circuits of the short circuit, it is a local short circuit.

This is actually super simple, and if I haven't done it for a long time, it's a bit difficult to express.

Middle School Physics. Summary of "short circuit" cases in electricity.

What are the common short-circuits caused by the phenomenon is a common fault in junior high school physics and electricity, and there are two types of common short-circuits from the structure of short-circuits: power short-circuits and local short-circuits. That is, in the circuit, the two ends of the electrical appliance are directly connected by wires, which is called local short circuit; In the other case, the power supply is directly connected with a wire without any electrical appliances, and the power supply is short-circuited.

In the two figures below, the power supply is short-circuited.

Common phenomenon:1Power Short Circuit:

The electrical appliances in the circuit do not work (e.g. small light bulbs do not emit light). There is a danger that the power supply and the entire circuit will burn out. 2.

Local short circuit: Local short circuit involves specific circuits, and the situation is varied. The following diagram is an example to illustrate a local short circuit, when the switch S is disconnected, two small bulbs are connected in series, and emit light.

When the switch S is closed, the bulb L1 is short-circuited, the bulb L1 does not emit light, while the bulb L2 still emits light (L2 brightness becomes larger, using Ohm's law.

and electrical power knowledge explained), ammeter.

The indication becomes larger. In the experimental problem of junior high school physics, a local short circuit only causes the electrical appliance to not work, but does not damage the electrical appliance, so it is allowed. Because it is variable, it is a difficult problem in electrical problems.

Other cases:1Excessive current in a circuit with an ammeter can damage the ammeter and form a short circuit. 2.From the components used to cause short circuits, common short circuits are: short circuit with wire, short circuit with switch, short circuit with ammeter, and sliding rheostat.

short circuit of the slide and so on.

Cause analysis: In the event of a short circuit, the current passes directly through the wire without passing through the electrical appliance. Qualitatively explained with the concept of resistance.

Resistance is the conductor's obstruction of the current, the electrical appliance always has a certain resistance, and the resistance of the connected wire can be regarded as zero, after the two ends of the electrical appliance are connected with the wire, the current will pass through the wire without resistance, and will not pass through the electrical appliance with resistance. A short circuit in the power supply is prone to fire. The current flows directly from the positive pole of the power supply to the negative pole of the power supply without passing through the electrical appliance, and the current in the circuit is extremely large.

Ohm's law: i=u r can be seen that when the power supply voltage u is certain and r is very small, the current in the circuit will increase instantaneously a lot, by Joule's law q=i2rt, the time t is certain, and the wire will heat up.

Heating up, and even fires.

(1) A short circuit means that the power supply is directly connected to a closed circuit by the wire without passing through the load. The short-circuit current refers to the current when the electrical appliance is not connected, which is equivalent to the current when the positive and negative phases of the battery are connected directly by finding a wire. (Usually this is a serious fault of the circuit that should be avoided as much as possible, which will cause the circuit to burn out due to excessive current and cause a fire).

2) In a hybrid circuit, the two ends of a certain circuit element or load are directly connected with a wire or switch. (This is a local or partial short circuit that is required for a safety connection that does not cause burnout due to excessive current.) For example, in order to prolong its service life, when one of the filaments is broken and damaged, the special structure inside it will automatically connect the two ends of the small bulbs to make the other small bulbs work normally.

Forget to adopt, thank you!

That is, there is a wire that directly connects the positive and negative poles of the power supply, which is called a short circuit of the power supply, and the current in the circuit is particularly large after the short circuit of the power supply, which will damage the power supply. I sorted out the difference between a short circuit and an open circuit, and the difference between a short circuit and an open circuit.

A short circuit means that in a circuit, the current does not flow through the electrical appliance, but is directly connected to the two poles of the power supply, and the power supply is short-circuited. According to Ohm's law i=u r, we know that since the resistance of the wire is small, the current on the circuit will be very large when the power supply is short-circuited. Such a large current, the battery or other power sources cannot withstand it, which will cause damage to the power supply; What's more, because the current is too high, the temperature of the wire will rise, and in severe cases, it may cause a fire.

A short circuit is when the positive and negative poles of a power supply are directly connected. There is voltage in the short circuit, and the power supply in the short circuit is extremely harmful. Because the resistance of the wire is very small according to i=u r, there is a large current in the circuit, which will burn out the electrical appliances.

An open circuit is when a normal circuit is broken somewhere. So there is no current, but the power supply is still there, so there is voltage.

The so-called open circuit is a state in which a certain part of the closed circuit is disconnected and the current cannot be turned on, which is also called the open circuit state. When a circuit breaker occurs, the electrical equipment does not work, and the equipment in operation falls into a standstill or abnormal state.

A short circuit is a state in which the wires leading from a power source to an electrical device (also known as a load) are directly connected to each other without passing through the load (or the load is zero), also known as a short circuit state. That's the definition in the book. There is also a kind of short circuit that we often use, that is, there are more than two electrical appliances in series, and one is short-circuited with a wire (connected to both ends of the electrical appliance) to increase the current of the other.

1.Short circuit junior high school stage book refers to the short circuit of the power supply: that is, there is a wire that directly connects the positive and negative poles of the power supply, which is called a short circuit of the power supply, and the current in the circuit after the short circuit of the power supply is particularly large, which will damage the power supply.

When teaching, the teacher will short-circuit the power supply and local circuits (i.e. short-circuits of electrical appliances).Short circuit of electrical appliances refers to the use of wires (theoretically think that the resistance is zero) to directly connect the two binding posts of electrical appliances, and all the current passes through the wires, and the electrical appliances do not have current to pass through, and will not be damaged and will not work.

When judging whether there is a short circuit in the circuit, see if there is a pure wire between the two points in the circuit, if there is, no matter what kind of components between the two points will be short-circuited by the wire and will not work.

2.Open circuit (also known as open circuit): refers to a circuit that is disconnected somewhere in the circuit, and there must be no current in the open circuit, and the electrical appliances do not work. A common example in life is poor exposure.

There are two types of short circuits: power supply short circuit and local short circuit, power supply short circuit is the process of current flowing from the positive electrode to the negative electrode without passing through any electrical appliances. A local short circuit is between two points in a circuit, one of which passes through the non-consumer branch, and all the current flows through the branch without the consumer. The short-circuited electrical appliances have no current, and the entire circuit is still working with electrical appliances.

In a circuit, if the current does not flow through the consumer and is directly connected to the two poles of the power supply, the power supply is short-circuited. According to Ohm's law i=u r, we know that since the resistance of the wire is small, the current on the circuit will be very large when the power supply is short-circuited. Such a large current, the battery or other power sources cannot withstand it, which will cause damage to the power supply; What's more serious is that because the current is too large, the temperature of the wire will rise, which will often cause damage to the machine, and in serious cases, it may cause ** or fire.

1.Damage to electrical equipment and components. For example, the insulation part of the equipment is naturally aging or the equipment itself is defective, and it is broken down and short-circuited during normal operation; and equipment defects caused by improper design, installation, and maintenance, which eventually develop into short circuits, etc.

2.Natural causes. For example, the weather is bad, and the overhead line is broken due to strong winds and low temperature wire icing; Due to direct lightning or lightning induction, equipment overvoltage, insulation breakdown, etc.

3.Man-made accidents. Such as:

The staff violated the operating procedures and pulled the brake with load, resulting in a short circuit of the interphase arc light; Violation of electrical safety work regulations with grounding knife switching, resulting in gold dust short circuit, human negligence to connect the wrong line caused by short circuit or poor operation management caused by small animals entering live equipment to form a short circuit accident, etc.

Open circuit (open circuit) refers to the state that the circuit is not closed, or the wires are not connected properly, or the electrical appliances are burned out or not installed well (such as connecting the voltmeter in series in the circuit), that is, the entire circuit is disconnected somewhere.

1. Determine whether the fault is open or short circuit according to the conditions given by the question: when the two lights are connected in series, if only one light is not on, the light must be short-circuited, and if the two lights are not on, the circuit must be open-circuited; Two lights in parallel, if only one light is not on, it must be this branch road is broken, if both lights are not on, it must be a trunk road broken. In a parallel circuit, the fault cannot be a short circuit because if it is short, the power supply will burn out.

2. According to the first step, determine which part is open or short-circuited.

Example 1: L1 and L2 are connected in series in the circuit, the voltmeter measures the voltage at both ends of L2, after the switch is closed, it is found that the two lights are not on, and the voltmeter has an indication, what is the cause of the fault? Solution:

You first draw a circuit diagram: if neither light is on, it must be an open circuit. The voltmeter has an indication, which means that the two binding posts of the voltmeter are connected to the two poles of the power supply, and this part of the wire is not broken, so only L1 is broken.

Example 2: L1 and L2 are connected in series, and the voltmeter V1 measures the L1 voltage, and V2 measures the L2 voltage. When the switch is closed, neither light is on. The following statement is correct:

A. If V1=0 and V2 is large, L1 is short-circuited and L2 is normal; b. If V1=0 and V2 is large, L2 is broken.

Solution: You may make the wrong choice A. Actually, the answer is b.

First of all, according to the question, the conditions are given: if the two lights are not on, the circuit is open, and A is definitely incorrect. When L2 is disconnected, V2 is connected to the power supply poles, and it measures the supply voltage, so the indication is large.

At this time, L1 has no voltage at both ends because the current is measured to pass through, so the indicator of V1 is zero.

One more mantra for you.

Formula for analyzing circuits 1. Methods for analyzing circuits: first judge series and parallel; The meter measures and then shuts off. All the way to the end must be a string; If there are branches, they are in parallel.

2. Please also pay attention to the following points: table A is equivalent to a wire; A short circuit will occur when paralleling. If it is found and the source; The destruction of the table and the source is really miserable.

If there is an electrical appliance that is merged with it; The circuit occurs locally short.

The V table can not be stringed; String time is quite circuit-breaking.

If it is found to be stringed; Zero current should be taken for granted.

Connection circuit formula 1, what to do with connecting the circuit: series connection is very simple, each component is connected in turn; It is a bit difficult to connect in parallel, connecting trunk roads and marking nodes; The branch roads can be connected one by one, and then inspected. 2. And how to connect the meter:

A table is strung among them; v table and both ends. The line post is carefully connected; Positive (in) negative (out) cannot be reversed. The range must not be forgotten; The size is carefully broken.

I'll also help you find a more complete information:

Opening; From the + pole of the power supply, it is not possible to reach the -pole of the power supply through the wire [regard the electrical appliance as a wire].

Short circuit; From the + pole of the power supply through a wire to the -pole directly to the power supply [the ammeter is regarded as a wire, and the electrical appliance is regarded as an open circuit].

The positive and negative poles are short, and they don't know how to come to me directly.