Electric Currents & Ohm’s Law – Units of Resistivity example 3

Electric Currents & Ohm's Law - Units of Resistivity example 3

Question : (a) A rectangular carbon block has dimensions 1.0 cm × 1.0 cm × 50 cm.

(i) What is the resistance measured between the two square ends ?

(ii) between two opposing rectangular faces / Resistivity of carbon at 20°C is 

3.5 × 105 Ω-m.

(b) A current of 5 A exists in a 10-Ω resistance for 4 minutes 

(i) how many coulombs and

(ii) how many electrons pass through any section of the resistor in this time ? Charge of the electron

= 1.6 × 10−19 C.

Solution :

(a) A rectangular carbon block has dimensions 1.0 cm × 1.0 cm × 50 cm.

(i) What is the resistance measured between the two square ends ? (ii) between two opposing rectangular

faces / Resistivity of carbon at 20°C is 3.5 × 10−5 Ω-m.

(b) A current of 5 A exists in a 10-Ω resistance for 4 minutes (i) how many coulombs and

(ii) how many electrons pass through any section of the resistor in this time ? Charge of the electron = 1.6 × 10−19 C.

 

(b) (i) Q = It = 5 × (4 × 60) = 1200 C

 

(ii) n = Q/e = 1200 / 1.6 x 10-19   

= 75 x 1020

 

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