Electrical basics - solutions - Huaqiang Electronic Network

Photocoupler

First, the circuit

The formation of current: the directional movement of the charge forms a current. (The directional movement of any charge will form a current).

The direction of the current: from the positive pole of the power supply to the negative pole.

Power supply: A device that provides continuous current (or voltage).

The power source converts other forms of energy into electrical energy. For example, a dry battery converts chemical energy into electrical energy. A generator converts mechanical energy into electrical energy.

Conditions with continuous current: There must be a power supply and circuit closure.

Conductor: An easily conductive object is called a conductor. For example: metal, human body, earth, saline solution, etc.

Insulator: An object that is not easily conductive is called an insulator. For example: glass, ceramic, plastic, oil, pure water, etc.

Circuit composition: consists of power supply, wires, switches and electrical appliances.

The road has three states: (1) path: the circuit that is connected is called the path; (2) the open circuit: the open circuit is called the open circuit; (3) the short circuit: the circuit that directly connects the wire to the two poles of the power supply is called a short circuit.

1. A diagram showing a circuit connection by a symbol is called a circuit diagram.

Concatenation: Connect the components one by one, called series. (Anywhere disconnected, the current will disappear)

Parallel: connect the components side by side, called parallel. (Each branch does not affect each other)

Second, current

International unit: Ampere (A); commonly used: milliamperes (mA), microampere (A), 1 amp = 103 mA = 106 microamperes.

The meter for measuring current is: ammeter, its rules of use are: 1 the ammeter should be connected in series in the circuit; 2 the current should be from the "+" terminal and the "-" terminal; 3 the measured current should not exceed the range of the ammeter 4 is absolutely not allowed to connect the ammeter to the poles of the power supply without passing through the appliance.

The ammeters commonly used in the laboratory have two ranges: 10 to 0.6 amps, and the current value per small cell is 0.02 amp; 20 to 3 amps, and the current value per small cell is 0.1 ampere.

Three, voltage

Voltage (U): Voltage is the cause of the current in the circuit, the power supply is the device that provides the voltage.

International unit: Volt (V); commonly used: kilovolt (KV), millivolt (mV). 1 kV = 103 volts = 106 mV.

The instrument for measuring voltage is: voltmeter, rules of use: 1 voltmeter should be connected in parallel in the circuit; 2 current should be from the "+" terminal, from the "-" terminal; 3 the measured voltage should not exceed the range of the voltmeter ;

The voltmeter commonly used in the laboratory has two ranges: 10 to 3 volts, and the voltage value per small cell is 0.1 volt;

20 to 15 volts, the voltage value per small cell is 0.5 volts.

Memorized voltage: 11 volts of dry battery voltage; 21 volts lead battery voltage is 2 volts; 3 home lighting voltage is 220 volts; 4 safety voltage is: no higher than 36 volts; 5 industrial voltage 380 volts.

Fourth, the resistance

Resistance (R): indicates the resistance of the conductor to the current. (The greater the resistance of the conductor to the current, the greater the resistance and the smaller the current through the conductor).

International unit: ohm (Ω); commonly used: megohm (MΩ), kiloohm (KΩ); 1 megohm = 103 kohm;

1 thousand ohms = 103 ohms.

Factors that determine the size of the resistor: material, length, cross-sectional area, and temperature (R is independent of its U and I).

Sliding rheostat:

Principle: Change the length of the resistance wire in the circuit to change the resistance.

Role: change the current and voltage in the circuit by changing the resistance in the access circuit.

Nameplate: If a sliding rheostat is marked with "50Ω2A", the meaning is: the maximum resistance is 50Ω, and the maximum current allowed to pass is 2A.

Correct use: a, should be used in series in the circuit; b, wiring should be "one up"; c, the resistance should be adjusted to the maximum place before power.

Five, Ohm's law

Ohm's law: The current in a conductor is proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor.

Formula: The unit in the formula: I→A (A); U→V (V); R→Europe (Ω).

The understanding of the formula: 1 I, U and R in the formula must be in the same circuit; 2I, U and R are known to be any two quantities can be used to find another quantity; 3 the unit should be unified in the calculation.

Application of Ohm's Law:

1 The resistance of the same resistor is constant, independent of current and voltage, and its current increases with increasing voltage. (R=U/I)

2 When the voltage is constant, the larger the resistance, the smaller the current passed. (I=U/R)

3 When the current is constant, the greater the resistance, the greater the voltage across the resistor. (U=IR)

The series connection of resistors has the following characteristics: (refers to R1, R2 in series, the more strings, the greater the resistance)

1 current: I=I1=I2 (the currents in the series circuit are equal)

2 voltage: U=U1+U2 (the total voltage is equal to the sum of the voltages)

3 Resistor: R=R1+R2 (total resistance equals the sum of the resistors) If n equivalent resistors are connected in series, then R total = nR

4 partial pressure effect: =; calculate U1, U2, available:;

5 Proportional relationship: Current: I1: I2 = 1:1 (Q is heat)

The parallel connection of the resistor has the following characteristics: (refers to R1, R2 in parallel, and the more, the smaller the resistance)

1 current: I=I1+I2 (the main current is equal to the sum of the currents of the branches)

2 voltage: U=U1=U2 (the main circuit voltage is equal to the voltage of each branch)

3 resistance: (the reciprocal of the total resistance is equal to the reciprocal sum of each resistor) If the n equivalent resistors are connected in parallel, there is R total = R

4 shunt action:; calculate I1, I2 available:;

5 proportional relationship: voltage: U1: U2 = 1:1, (Q is heat)

Sixth, electric power and electric power

1. Electrical work (W): How much electrical energy is converted into other forms of energy,

2. International unit of work: Joule. Commonly used: degree (kWh), 1 degree = 1 kWh = 3.6 × 106 joules.

3. Tools for measuring electrical work: electric energy meter

4. Electric work formula: W = Pt = UIt (in the formula, unit W → focus (J); U → volt (V); I → An (A); t → seconds).

When using W=UIt calculation, pay attention to: WUI and t in equation 1 are in the same circuit; 2 units should be unified in calculation; 3 can be found in the third quantity by any three arbitrary quantities. There is also formula: = I2Rt

Electric power (P): indicates the speed of current work. International unit: Watt (W); Commonly used: kW

Formula: unit P → watt (w); W → focus; t → seconds; U → volt (V), I → An (A)

When using the calculation, the unit should be unified. 1 If W uses coke, t uses seconds, then P is in watts; 2 if W uses kWh, t uses hours, then P is in kilowatts.

10. Calculate the electric power and use the right formula: P=I2R and P=U2/R

11. Rated voltage (U0): the voltage at which the appliance is working normally. Another: rated current

12. Rated power (P0): The power of the appliance at the rated voltage.

13. Actual voltage (U): The voltage actually applied to the ends of the appliance. Another: actual current

14. Actual power (P): The power of the appliance under actual voltage.

When U > U0, the P > P0 lamp is very bright and easy to burn out.

When U < U0, the P < P0 lamp is very dark.

When U = U0, then P = P0 is normally illuminated.

15. The same resistor, used under different voltages; if: the actual voltage is half of the rated voltage, then the actual power is 1/4 of the rated power. Example "220V100W" if connected at 110 volts In the circuit, the actual power is 25 watts.)

16. Thermal power: The thermal power of a conductor is proportional to the square of the current and proportional to the resistance of the conductor.

17.P thermal formula: P = I2Rt, (in the formula P → watts (W); I → An (A); R → Europe (Ω); t → seconds.)

18. When the work done by the current through the conductor (electrical work) is all used to generate heat (electrical heat), there are: thermal power = electric power, the electric power formula can be used to calculate the thermal power. (such as electric heater, the resistor is like this.)

Seven, living electricity

The household circuit consists of: the incoming line (fire and neutral) → electric energy meter → main switch → fuse box → electrical appliances.

All household appliances and sockets are connected in parallel. The appliance is connected in series with its switch.

Fuse: It is made of lead-bismuth alloy with high resistivity and low melting point. Its function is that when there is excessive current in the circuit, it rises to the melting point and melts, automatically cuts off the circuit and plays the role of insurance.

There are two reasons for the excessive current of the circuit: First, the circuit is short-circuited; second, the total power of the appliance is too large.

The principle of safe use of electricity is: 1 does not touch the low-voltage charged body; 2 is not close to the high-voltage charged body.

Eight, electricity and magnetic

Magnetic: The object attracts the properties of iron, nickel, cobalt and other substances.

Magnet: A magnetic object is called a magnet. It has directivity: Guide to the North.

Magnetic pole: The most magnetic part of the magnet is called the magnetic pole.

Any magnet has two poles, one is the North Pole (N pole); the other is the South Pole (S pole)

The role of the magnetic pole: the same name magnetic poles are mutually exclusive, the different name magnetic poles attract each other.

Magnetization: The process of bringing an object that is not magnetically magnetic.

There is a magnetic field around the magnet, and the interaction between the poles occurs through the magnetic field.

The basic properties of a magnetic field: the magnetic force acting on the magnets entering it.

The direction of the magnetic field: the direction pointed by the north pole when the small magnetic needle is stationary is the magnetic field direction at that point.

Magnetic line: Describes the strength of the magnetic field, the imaginary curve of the direction. Does not exist and does not intersect, north and south.

The direction of the magnetic field at a certain point in the magnetic field, the direction of the magnetic line, and the direction of the north pole when the small magnetic needle is stationary.

10. The geomagnetic north pole is near the south pole of the geographical position; while the geomagnetic south pole is near the geographical north pole, but it does not coincide. Their intersection angle is called magnetic declination. Chinese scholar Shen Kuo first described this phenomenon.

11. Oster experiment proves that there is a magnetic field around the energized wire.

12. Ampere's rule: Hold the solenoid with your right hand and bend the four fingers toward the current in the solenoid.

Then the end pointed by the thumb is the north pole (N pole) of the solenoid.

13. The nature of the energized solenoid: 1 the greater the current, the stronger the magnetic force; 2 the more coil turns, the stronger the magnetic; 3 the soft core is inserted, the magnetism is greatly enhanced; 4 the polarity of the energizing solenoid can be current Direction to change.

14. Electromagnet: The solenoid with iron core inside constitutes the electromagnet.

15. Characteristics of electromagnet: 1 The presence or absence of magnetism can be controlled by the on/off of current; 2 The strength of magnetic can be adjusted by changing the magnitude of current and the number of turns of the coil; 3 The magnetic pole can be changed by the direction of current.

16. Electromagnetic relay: It is essentially a switch controlled by electromagnet. Its function can realize long-distance operation, using low voltage and weak current to control high voltage and strong current. It can also realize automatic control.

17. Basic principles of telephone: vibration → strong and weak current → vibration.

18. Electromagnetic induction: When a part of the conductor of the closed circuit moves in the magnetic field to cut the magnetic induction line, a current is generated in the conductor. This phenomenon is called electromagnetic induction, and the generated current is called induced current. Application: Generator

The conditions of the induced current: 1 circuit must be closed; 2 is only a part of the conductor of the circuit in the magnetic field; 3 this part of the conductor to cut the magnetic line motion.

The direction of the induced current: related to the direction of the conductor movement and the direction of the magnetic induction line.

The principle of the generator: electromagnetic induction phenomenon. Structure: stator and rotor. It converts mechanical energy into electrical energy.

The effect of the magnetic field on the current: the energized wire is subjected to the magnetic force in the magnetic field. It is converted from electrical energy to mechanical energy. Application: Electric motor.

The direction of the force of the energized conductor in the magnetic field: related to the direction of the current and the direction of the magnetic line.

Motor principle: It is made by the principle that the energized coil is rotated in the magnetic field.

Commutator: to achieve the exchange between AC and DC.

AC: periodically changes the current in the direction of the current.

DC: The current whose current direction does not change.

experiment

Volt-ampere resistance

Experimental principle: (experimental equipment, circuit diagram as shown on the right) Note: The sliding rheostat should be adjusted to the maximum resistance before the experiment.

The role of the sliding rheostat in the experiment is to change the voltage across the measured resistance.

2. Measuring the electric power of the small bulb - experimental principle: P = UI