Electricity - Factors Affecting Resistance of Conducting Wires PDF

Figure 11.5 Electric circuit to study the factors on which the resistance of conducting wires depends n Now, plug the key. Note the current in the ammeter. n Replace the nichrome wire by another nichrome wire of same thickness but twice the length, that is 2l [marked (2) in the Fig. 1...

Figure 11.5 Electric circuit to study the factors on which the resistance of conducting wires depends n Now, plug the key. Note the current in the ammeter. n Replace the nichrome wire by another nichrome wire of same thickness but twice the length, that is 2l [marked (2) in the Fig. 11.5]. n Note the ammeter reading. n Now replace the wire by a thicker nichrome wire, of the same length l [marked (3)]. A thicker wire has a larger cross-sectional area. Again note down the current through the circuit. n Instead of taking a nichrome wire, connect a copper wire [marked (4) in Fig. 11.5] in the circuit. Let the wire be of the same length and same area of cross-section as that of the first nichrome wire [marked (1)]. Note the value of the current. n Notice the difference in the current in all cases. n Does the current depend on the length of the conductor? n Does the current depend on the area of cross-section of the wire used? It is observed that the ammeter reading decreases to one-half when the length of the wire is doubled. The ammeter reading is increased when a thicker wire of the same material and of the same length is used in the circuit. A change in ammeter reading is observed when a wire of different material of the same length and the same area of cross-section is used. On applying Ohm’s law [Eqs. (11.5) – (11.7)], we observe that the resistance of the conductor depends (i) on its length, (ii) on its area of cross-section, and (iii) on the nature of its material. Precise measurements have shown that resistance of a uniform metallic conductor is directly proportional to its length (l ) and inversely proportional to the area of cross-section (A). That is, R∝l (11.8) and R ∝ 1/A (11.9) Combining Eqs. (11.8) and (11.9) we get l R∝ A l or, R=ρ (11.10) A where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor. The SI unit of resistivity is Ω m. It is a characteristic property of the material. The metals and alloys 178 Science 2024-25