Arrangement of basic electrical knowledge (II)

Basic knowledge of electricity （ Two ）

Capacitors and capacitors

• Capacitor

  • Two conductors close to and sandwiched with insulating dielectric , Such a system is called a capacitor .
    

  • A capacitor can store electric charge , So that energy can be stored .

• capacitance

  • Refers to the storage capacity of free charge under a given potential difference , Write it down as C, The international unit is farad （F）.
  • Definition ：$$C=\frac{Q}{U}$$
  • Generally speaking , The electric charge will be moved by force in the electric field , When there is a medium between the conductors , It prevents the charge from moving and causes the charge to accumulate on the conductor , Cause the accumulated storage of electric charge , The amount of stored charge is called capacitance .
  • Physical meaning ： Describe the capacitor at a given voltage , The ability to store the amount of charge .

• Capacitor energy storage
  

  • Formula derivation ：
    • According to the power formula $P=U\ast I$ In the capacitor, the current and voltage are variable , Its energy expression ：$$E_p=\sum _{\Delta t}P\ast \Delta t=\sum _{\Delta t}u\ast i\ast \Delta t$$
    • It is known that $$I=\frac{\Delta Q}{\Delta t},C=\frac{Q}{U}$$
    • therefore $$E_p=\sum _{\Delta q}u\ast \Delta Q$$
    • capacitance C Is the fixed value , You know Q And u The relationship is one with 1/C Is a straight line with a slope .
      
  • E_p Is the area of a small triangle , Then there are $$E_p=\frac{1}{2}Qu=\frac{1}{2}C{u}^2=\frac{1}{2}\frac{Q^2}{C}$$

Power and current

Power Supply

• Micro explanation
  
• Steady electric field （ Constant electric field ）
  • The field strength is parallel to the conductor , The induced electric field balances the vertical conductor component .
  • Achieve dynamic balance , Steady electric field （ Constant electric field ） Does not change over time .
  • Similar to electrostatic field ,E、φ unchanged .

Current intensity （ electric current ）

• Definition ： The ratio of the time taken to pass through a certain cross-section .$$I=\frac{Q}{t}$$
• Direction ： The direction in which the positive charge moves
• Company ： Ampere $1A=1C/s$ （ Basic unit )
• The current has a direction , But the current is scalar .
  
• Micro explanation $$I=\frac{Q}{t}=nq\ast S\ast v$$
  • $n$ Free charge number density ;
  • $q$ The amount of electric charge ;
  • $S$ Cross sectional area of conductor ;
  • $v$ The speed at which the free charge moves directionally ;

emf

• emf ： The ability to convert other forms of energy into electrical energy .
• Electromotive force generates voltage at both ends of the power supply , In the circuit , Electromotive force is commonly used E Express , The unit is v （V）.
• expression ：$$E=\frac{W_{Not}}{q}$$
• $W_{Not}$： Non electrostatic force （ Other forces that act on charge flow ） Work done .
• The direction of the electromotive force is specified as from the negative pole of the power supply to the positive pole of the power supply through the interior of the power supply , That is, it is opposite to the voltage at both ends of the power supply .
• measurement ： The electromotive force of the power supply can be measured with a voltmeter . When measured , Do not connect the power supply to the circuit , Use a voltmeter to measure the voltage at both ends of the power supply , The voltage obtained can be regarded as equal to the electromotive force of the power supply . If the power supply is connected in the circuit , The voltage at both ends of the power supply measured by the voltmeter will be less than the electromotive force of the power supply . Because the power supply has internal resistance , In a closed circuit , Current flows through an internal resistor r There is an internal voltage drop , Through external resistance R There is an external voltage drop . Then the electromotive force of the power supply E Equal to the internal voltage U_r And external voltage U_R The sum of the , namely E=U_r+U_R .

Summary

• A few confusing concepts