Resistance, current and voltage are the basic physical quantities in a circuit. Why not

Resistance, current and voltage are the basic physical quantities in a circuit. Why not

National standard SI system: seven basic units: length m, time s, mass kg, thermodynamic temperature (Kelvin temperature) k, current unit a, light intensity unit CD (candela), material mass mol. the resistance R in the circuit is derived from U / I. voltage may be regarded as the basic quantity of the circuit, but it can not be regarded as the seven basic physics

What is the function of zero resistance of ohmmeter

For zero adjustment, you need to make clear the structure and principle of the ohmmeter
The specific function is as follows
Because the ohmmeter actually measures the current, and then reads different resistance values according to the dial
I = E / R + R '+ R measurement
That is, the current is equal to the sum of the electromotive force of the battery in the meter divided by the internal resistance of the power supply plus the zero resistance plus the resistance to be measured
Assuming that E = 1.5V, r = 1 Ω, the full bias current of the ammeter is 0.6A, then only when the resistance value of the zeroing resistor is adjusted to 1.5 Ω, the ammeter is full biased. At this time, the indication of the ohmmeter is 0
Because the resistance value of internal resistance will change for various reasons, a zero resistance is needed

What is the role of a zero ohm resistor in a circuit?

Zero ohm is used in the design of printed circuit board, which is used as jumper or crossing between two points in series
(1) Because other circuit wire diameter must be across the circuit board,
Two points cannot be connected
(2) When a new contact point needs to be changed after the design is completed
(3) By changing the point of contact, and the need to change the circuit

What is the function of a 220 ohm resistor in series with an LED? How to understand the current limiting resistor, voltage dividing resistor and pull up (down) resistor? What is the difference

The current limiting resistor is used to limit the current flowing through the LED and prevent the LED from being damaged by excessive current;
In fact, it is also partial pressure, because it has the function of partial pressure
But this is mainly to protect led, so we call it current limiting resistor
Pull up and pull down are for IO port. If a resistance is connected between the port and the power supply, it is pull-up, otherwise it is pull-down