A carbon resistor is 4 mm long and has a constant cross section of 0.2 mm2. The conductivity of carbon at room temperature is 3 × 104 per ohm·m. In a circuit its potential at one end of the resistor is 12 volts relative to ground, and at the other end the potential is 16 volts. Calculate the resistance and the current .

The resistance is found to be 6Ω

The current is found to be 0.66 A

Explanation:

The resistance of a conductor in terms of its dimensions is given as:

R = ρL/A

where,

R = resistance = ?

ρ = resistivity = 3 x 10⁴ Ω.m

L = Length = 4 mm = 0.004 m

A = Cross-sectional area = 0.2 mm² = 0.2 x 10⁻⁶ m²

Therefore,

R = (3 x 10⁴ Ω.m)(0.004 m)/(0.2 x 10⁻⁶ m²)

R = 6 Ω

Now, the potential difference between both ends of the resistor is:

ΔV = 16 V - 12 V = 4 V

Now, from Ohm's Law:

V = IR

I = V/R

I = 4 V/ 6 Ω

I = 0.66 A

current and voltage can both be different in different parts of a circuit so it matters where you place the ammeter and voltmete