阻力公式
如果要控制电线内流动的电流,则需要了解电阻。在电路中,它是电流的对立面。它以欧姆(Ω)为单位测量。它可以使用欧姆定律计算。它被定义为施加的电压与电流的比值。所以,
R = V/I
Where,
R = resistance
I = current
V = voltage
可以使用导线的长度和面积来计算电阻公式。该公式表明,通过导线的电阻值与长度成正比,与横截面积成反比。
在数学上,这种关系通常写成,
Resistance of the conductor is directly proportional to it’s length,
R α L
Resistance of the conductor is inversely proportional to it’s area of cross-section,
Removing the proportionality and the constant is called as Resistivity,
Where,
R = resistance
ρ = resistivity of the conductor
l = length of the conductor
A = area of the cross-section of the conductor
上式也可以写成,
电阻取决于其制成的材料。由橡胶等电绝缘体制成的物体往往具有非常高的电阻,而由金属等电导体制成的物体往往具有非常低的电阻。
电阻的水管类比
- 当管道长度较长时,对水流的阻力会很大。
- 管道截面积大时,水流阻力小。
电阻与功率的关系
电功率 (P) 是电流乘以电压的乘积。功率的 SI 单位是瓦特 (W)。电力是根据欧姆定律和电压、电流和电阻的值计算的。
P = VI
Where,
P = Electric Power
V = Voltage
I = Current
From Ohm’s Law we know that,
V = IR
Replacing the value of V in the above equation,
P = I2/R
When the values for current and resistance are given, equation is,
P = V2/R
温度对电阻的影响
材料的电阻随温度的变化而变化。变化量因材料类型而异。
- 金属
纯金属的电阻随温度升高而增加。因此,金属具有正的电阻温度系数。例如,铜、铝、银等。
- 合金
合金的电阻随着温度的升高而增加,但合金的正温度系数值较低。例如,镍铬合金等。
- 半导体、绝缘体和电解质
半导体和绝缘体的电阻随着温度的升高而降低。电阻值下降。因此,这些材料具有负的电阻温度系数。
影响电阻的因素
- 导体长度
电线的长度越大,电线提供的电阻就越大。
- 导体横截面面积
导线的电阻随着导线横截面积的增加而减小。
- 导体材料。
不同的材料有不同的电阻。对于金属,提供的电阻非常低,但对于绝缘体,提供的电阻非常大。
- 材料温度
当温度升高时,纯金属和合金的电阻会增加,但对于绝缘体,电阻会随着温度的升高而降低。
电阻率
电阻率是一种材料的特性,它在自然界中是基本的,它衡量该特性抵抗电流的强度。电阻率的 SI 单位是欧姆表,符号是 row(ρ)。
对于理想情况,材料的横截面和物理成分在整个样品中是均匀的。电阻率可以写成:
对于不太理想的情况,电流和电场在材料的不同部分会有所不同。我们使用通用表达式,
Where,
ρ is that the electrical resistance of metal Ω.m
E is that the electric field in V.m-1
J is that the current density in A.m-2
示例问题
问题1:什么是欧姆定律?
回答:
Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
问题 2:长度为 50m 和 40m 的两根电线分别具有相同的横截面积且由相同的材料制成。哪根电线的电阻更高?
回答:
We know that R α L, wire of length 50m will offer higher resistance. The resistance of a small wire is low while the resistance of a long wire is high. t
问题 3:两条线 L1 和 L2 的长度分别为 L 和 2L。横截面积分别为 2A 和 A。两根电线由相同的材料制成。求导线 L 1和 L 2的电阻比?
解决方案:
So,
⇢ (1)
⇢ (2)
Dividing equation 1 by equation 2 above two equations,
问题4:计算长度为5m,横截面积为2×10 -6 m²的铜线的电阻。铜的电阻率为1.7 × 10 -8 Ωm。
解决方案:
Length of copper wire = 5m
Area of cross-section of copper wire = 2 10-6 m2
Resistivity of copper wire = 1.7 × 10-8Ωm
We know that,
So putting the values of ρ, l and A in the above equation,
R = 4.25 × 10-2Ω
问题 5:什么是电阻率?
回答:
Electrical resistivity is a fundamental property of a material that measures how strongly it resists electric current. The SI unit of electrical resistivity is ohms meter and the symbol is row (ρ).
问题6:解释力量与阻力的关系?
回答:
Power is Voltage times Current,
P = VI ⇢ (1)
From Ohm’s Law,
V = RI ⇢ (2)
Replacing value of equation (2) in equation (1),
P = I2R
It can also be rewritten as,
问题 7:如果电源电压为 220V,1500W 的电器会消耗多少电流?
解决方案:
Power of the appliance = 1500W
Voltage supply to the appliance = 220V
From Power- current relation we know that,
P = VI
The above equation can be rewritten as,
I = 6.81A
问题8:计算50W、100V电灯泡的电流和电阻。
解决方案:
Power of the appliance = 100W
Voltage = 100V
From Power-current relation we know that,
P = VI
Also,
I = 0.5A
So, the current = 0.5A
From Ohm’s Law,
R = 200Ω
The Resistance offered by the Electric Bulb = 200Ω
问题9:绝缘子的电阻随着温度的升高会发生什么变化?
回答:
With an increase in temperature, the resistance of insulators decreases. Thus, such material has a negative temperature coefficient of resistance.
问题10:电阻率为6×10 -6 Ωm、长度为20m的金属线的电阻为10Ω。计算它的半径。
回答:
Resistivity of the wire = 6 × 10-6Ωm
Length of the wire = 20m
Resistance of the wire = 10Ω
The above equation can be rewritten as,
⇢ (1)
Replacing the values in equation (1),
A = 12 × 10-6m2 ⇢ (2)
Generally the area-cross section of a wire is a circle. So the area of a circle is,
A = πr2 ⇢ (3)
r = radius of the cross-section of the wire
Putting the value of of (2) in (3),
πr2 = 12 × 10-6
r = 1.954 × 10-3m
问题 11:最初,电线的长度是 L。电线的长度从 L 变为 2L,保持横截面积不变。找到阻力的初始与最终比率?
解决方案:
Initially,
The Length of wire = Lm
The area of cross-section of wire = Am2
Resistance of wire = R2Ω
Finally,
The Length of wire = 2Lm
The area of cross-section of wire = Am2
Resistance of wire = R2Ω
The ratio of initial to final is,
问题 12:电阻率的 SI 单位是什么?
回答:
The S.I. unit of resistivity is row (ρ).