Resistivity vs Resistance: What Is the Difference?
Physics · Current Electricity · NEET
Resistance (R) belongs to a specific object and changes if you cut, stretch, or reshape the wire. Resistivity (rho) belongs to the material itself (like copper or silver) and stays the same no matter the shape or size. They are linked by R = rho L / A. Memory hook: resistivity is the material's "nature"; resistance is that piece of wire's "situation."
At a glance
Belongs to
A specific object/wire
The material itself
Symbol
R
rho
SI unit
ohm (Omega)
ohm-metre (Omega m)
Depends on size/shape (L, A)
Yes
No
Changes on stretching/cutting
Yes
No
Depends on temperature
Yes
Yes
Formula link
R = rho L / A
rho = R A / L
Two copper wires: resistivity rho is identical (same material), but resistance R differs because length L and area A differ. R = rho L / A.
Your doubts, answered
Are resistivity and resistance the same thing?
No. Resistance R is a property of a particular object and depends on its length L and cross-section area A. Resistivity rho is a property of the material only. Two copper wires of different sizes have different R but the same rho. They connect through R = rho L / A.
If I stretch or cut the wire, does its resistivity change?
No. Stretching, cutting, folding, or bending changes L and A, so the resistance R changes. But the material is still the same metal, so its resistivity rho stays exactly the same (at fixed temperature). This is the most common NEET trap.
What are the units of resistance and resistivity?
Resistance R is measured in ohm (Omega). Resistivity rho is measured in ohm-metre (Omega m). If you ever see the unit ohm-metre, the question is about resistivity, not resistance.
Does resistivity depend on length and area at all?
No. Resistivity rho depends only on the material and its temperature. Length and area affect resistance R, not rho. So rho of copper is the same for a thick short bar and a thin long wire.
Why do a copper wire and an iron wire of the same size have different resistance?
Because they are different materials, so they have different resistivity rho. With the same L and A, larger rho gives larger R (from R = rho L / A). Iron has higher rho than copper, so the iron wire has more resistance.
Which is the true measure of how good a conductor is: resistance or resistivity?
Resistivity. To compare materials fairly you must remove the effect of size and shape, and rho does exactly that. Silver has the smallest resistivity, so it is the best conductor, regardless of the wire's dimensions.
⚠️ The NEET trap ✗ Resistivity doubles because the wire got longer, so R uses a new bigger rho. ✓ Resistivity rho does NOT change — the material is unchanged. Only the resistance R changes: stretching to 2L (constant volume) makes A become A/2, so R = rho L/A rises to 4 times. Length and area change R, never rho. 🧠 When you stretch a wire to twice its length, students think its resistivity doubles.
Real NEET questions
NEET 2023
On the basis of electrical conductivity, which one of the following materials has the smallest resistivity?
A · Glass
B · Silicon
C · Germanium
D · Silver ✓
Solution: Resistivity is the inverse of conductivity: rho = 1/sigma. The material with the highest conductivity has the smallest resistivity. Silver is a metal with the highest conductivity among the options, while Si and Ge are semiconductors and glass is an insulator (very high rho). So silver has the smallest resistivity. Note this depends only on the material, not on any wire size — that is the whole point of resistivity.
NEET 2023
Wire A has resistance 81 Omega. Wire B is of the same material, equal length, with a diameter three times that of A. The resistance of wire B is:
A · 729 Omega
B · 243 Omega
C · 81 Omega
D · 9 Omega ✓
Solution: Same material and same length means resistivity rho and L are identical for both wires — rho does NOT change. Only area changes. Using R = rho L / A with A = pi d^2 / 4, we get R proportional to 1/d^2. Tripling the diameter makes A become 9 times larger, so R becomes 1/9 as big. R_B = 81 / 9 = 9 Omega. Resistivity stayed the same; resistance changed because of geometry.
Solved Current Electricity NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Resistivity rho is also known as specific resistance. It is the resistance of a unit cube of the material (1 m x 1 m x 1 m), which is why it depends only on the material and temperature.
What is the formula linking resistance and resistivity?
R = rho L / A, where L is length and A is cross-section area. Rearranged, rho = R A / L. This shows R depends on shape while rho does not.
Does temperature change resistance or resistivity?
Both. Rising temperature increases rho for metals, which in turn increases R. But the size (L and A) can be treated as fixed, so the change in R comes from the change in rho.
Which one is a scalar material constant?
Resistivity rho is the material constant (at a given temperature). Resistance R is not a material constant because it depends on the object's dimensions.
How do I tell from a question whether it is about R or rho?
Check the unit and the wording. Ohm (Omega) and words like this wire or this resistor mean resistance. Ohm-metre (Omega m) and words like material, best conductor, or specific resistance mean resistivity.
