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What is the unit of resistivity? What is the symbol?
The resistivity sign ρ reads Rou four times!
Physical meaning: it is a measure of resistance conductivity, is the characteristics of the material, does not change with shape, size
Unit: Ω * m!
Is there a difference between M Ω and m Ω, the unit of resistivity
Some people say that there is a difference between M Ω and lowercase,
The capital M is a megaohm
The lower case is milliohm
How to understand the unit of resistivity
The unit of resistivity is Ohm · M
Then the unit of resistivity does not include the element of conductor section,
According to r = ρ (L / s), the resistance per unit length of a conductor can only be known if the resistivity, length and cross section of the conductor are known, and the unit of cross section also needs to be known
If there are 2 conductors:
What is the resistance of a wire with a resistivity of 1 Ω· m, a length of 3 meters and a cross section of 1 square meter?
What is the resistance of a wire with a resistivity of 1 Ω· m, a length of 3 meters and a cross section of 1 square millimeter?
So if I understand the unit of resistivity as Ω * m / mm2, I don't know whether it is right or not
In addition, ρ = RS / L, and its unit looks like Ω * mm2 / m
Sincerely looking forward to your answer
The concept of resistivity is for materials. It is defined as the resistance of a conductor made of a certain material with a length of 1 meter and a cross-sectional area of 1 square millimeter at room temperature. Resistivity is for materials. For wires with the same material and different area, the resistance is different. It can not be understood as Ω * m / mm2, because it is generally used to calculate a certain
Resistance calculation formula
The torque formula is t = 9550x power P / speed n. in case of multi-stage transmission, the torque of each stage should be multiplied by the reduction ratio. The lower the speed is, the greater the torque is
The power calculation formula is as follows:
p=w/t
p=UI
P=I^2 *R
P=Fv
P=U^2 /R
The calculation formula of work is as follows
W=Fs
W=UIt
W=I^2 *Rt
W=U^2 *t /R
1. The power calculation method of two-phase household appliances is as follows:
P = current * voltage * power factor
For example, 5A current * 220V AC voltage * 0.9, power factor = 990w
1 kwh = 1000W
2. The power calculation method of symmetrical three-phase AC household appliances is as follows:
Active power (W) P = follow sign 3 * current * AC voltage * power factor (COS)
Reactive power (VaR) q = follow sign 3 * current * AC voltage * power factor (SIN)
Apparent power (VA) s = follow sign 3 * current * AC voltage
P is power, unit is Watt, abbreviation is Watt, symbol is w. W is work, unit is Joule, abbreviation is focal, symbol is J. t is time, unit is second, symbol is s. because w = f (f force) * s (s distance) (the definition of work), the formula of power can also be deduced P = f · V (F is force, V is velocity)
The higher the power, the higher the speed, the higher the maximum speed of the car. The maximum power is commonly used to describe the dynamic performance of the car. The maximum power is generally expressed in horsepower (PS) or kilowatt (kw), 1 horsepower is equal to 0.735 kilowatt
1w=1J/s
P=W/t=FV=FL/t
1. The current and voltage of series circuit have the following rules: (for example: R1, R2 Series)
① Current: I = I1 = I2 (the current is equal everywhere in the series circuit)
② Voltage: u = U1 + U2 (the total voltage is equal to the sum of all voltages)
③ Resistance: r = R1 + R2 (the total resistance is equal to the sum of the resistances). If n resistances with the same resistance value are in series, then r = Nr
2. The current and voltage of parallel circuit have the following rules: (for example: R1, R2 in parallel)
① Current: I = I1 + I2 (main current equals to the sum of branch currents)
② Voltage: u = U1 = U2 (main circuit voltage equals to branch voltage)
③ Resistance: (the reciprocal of the total resistance equals the sum of the reciprocal of the parallel resistances) or
If n resistors with the same resistance are connected in parallel, then r = R
Note: the total resistance of parallel circuit is smaller than that of any branch circuit
The calculation formula of electric work: w = uit (where the unit is w → J; u → V; I → a; t → s)
5. When using w = uit to calculate electric work, we should pay attention to: (1) W, u, I and T in the formula are in the same section of circuit; (2) the unit should be unified in calculation; (3) the fourth quantity can be obtained if any three quantities are known
6. The following formula can also be used to calculate electric work: w = i2rt; w = Pt; w = UQ (q is the quantity of electricity);
[electrical part]
1 current intensity: I = q electric quantity / T
2 resistance: r = ρ L / S
Ohm's Law: I = u / R
4 Joule's law
(1) q = i2rt universal formula)
(2) q = uit = Pt = UQ, electric quantity = u2t / R (pure resistance formula)
5 series circuit:
⑴I＝I1＝I2
⑵U＝U1＋U2
⑶R＝R1＋R2
(4) U1 / U2 = R1 / R2 (partial pressure formula)
⑸P1/P2＝R1/R2
6 parallel circuit:
⑴I＝I1＋I2
⑵U＝U1＝U2
⑶1/R＝1/R1＋1/R2 [ R＝R1R2/(R1＋R2)]
(4) I1 / I2 = R2 / R1 (shunt formula)
⑸P1/P2＝R2/R1
7 constant resistance:
⑴I1/I2＝U1/U2
⑵P1/P2＝I12/I22
⑶P1/P2＝U12/U22
8 electric power:
(1) w = uit = Pt = UQ (universal formula)
(2) w = I ^ 2rt = u ^ 2T / R (pure resistance formula)
9 electric power:
(1) P = w / T = UI (universal formula)
(2) P = I2 ^ R = u ^ 2 / R (pure resistance formula)
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