A resistance cross-section made of conductive medium is shown in the figure. The resistivity of conductive medium is ρ, and it is made into hemispherical shell shape with inner radius a and outer radius B respectively (the shadow part in the figure). The spherical electrode with radius a and negligible resistance is embedded into the spherical center of conductive medium to become a lead-out electrode, and a layer of metal film with negligible resistance is plated on the outer spherical shell of conductive medium to become another lead-out electrode An electrode. Let the resistance value of the resistance be r. only one of the four expressions of R given below is reasonable. You may not be able to solve R, but you can judge the rationality of the following expression through certain physical analysis. According to your judgment, the reasonable expression of R should be () A. R＝ρ(b+a)2πabB. R＝ρ(b−a)2πabC. R＝ρab2π(b−a)D. R＝ρab2π(b+a)

A resistance cross-section made of conductive medium is shown in the figure. The resistivity of conductive medium is ρ, and it is made into hemispherical shell shape with inner radius a and outer radius B respectively (the shadow part in the figure). The spherical electrode with radius a and negligible resistance is embedded into the spherical center of conductive medium to become a lead-out electrode, and a layer of metal film with negligible resistance is plated on the outer spherical shell of conductive medium to become another lead-out electrode An electrode. Let the resistance value of the resistance be r. only one of the four expressions of R given below is reasonable. You may not be able to solve R, but you can judge the rationality of the following expression through certain physical analysis. According to your judgment, the reasonable expression of R should be () A. R＝ρ(b+a)2πabB. R＝ρ(b−a)2πabC. R＝ρab2π(b−a)D. R＝ρab2π(b+a)

A. The unit on the left side of the equation is Ω, the unit on the right side is Ω, and the unit is reasonable. Substituting B = a to get R ≠ 0, because the resistance is a very thin layer, and the resistance should be very small, this equation is unreasonable. So a is wrong

According to Ohm's law, the three physical quantities of current, voltage and resistance refer to the three physical quantities of conductor (or part of circuit) at the moment

Ohm's Law: for a certain section of circuit, the current passing through it is directly proportional to the voltage applied at its two ends and inversely proportional to its resistance. At the same time, the current and voltage of the same section of circuit at the same time