2.         Conductance, Insulators and Resistance

A.        A conductor in electricity is a material that allows electrons to flow through it easily. Metals, in general, are good conductors. Why?

They have loose valence electrons. It is as if their atoms(yellow) bathe in a sea of electrons(white). So it is very easy for an incoming electron(electricity) to displace these electrons.

The property of conductance (which measures how easily electrons are allowed to flow through),G, can be quantified. It is measured in Siemens (S).

B.         An insulator is a material that is a poor conductor of electricity. Examples include plastic, wood, ceramic and glass.

What makes them poor conductors?

Their electrons are held on more tightly.

C.        Resistance is the inverse of conductance. It measures how difficult it is for electrons to flow through a material. In plain English, an insulator, like ceramic, has high resistance and poor conductance. A metal has low resistance and high conductance.

R = 1/G.

Resistance is measured in ohms, W.

Something with high resistance wastes the energy of electrons and generates heat.

Example 1:       If the conductance of a material is 0.20 S, what is its resistance?

R = 1/ 0.20 S = 5 W

Example 2:       If the resistance of a ceramic resistor is 50 W, what is its conductance?

G = 1/R = 1/ 50 = 0.02 S

D.        Factors Affecting Resistance

(1)        The nature of the material

As mentioned metals have low resistance; insulators have high resistance.

Among metals some excellent conductors(in order of offering the least resistance) are

1.         silver

2.         copper(most practical because of its lower cost)

3.         gold

4.         aluminum

Metals like nichrome(nickel + chromium) and mercury are much poorer conductors.

(2)        The thickness of the wire

In general thicker wires offer less resistance. The greater area provides more room for electrons to move through.

(3)        The length of the wire

A wire that is longer than necessary adds resistance. The shorter the wire, the better the conductance.

(4)        The temperature        Higher temperatures cause more random motion and more resistance. Lower temperatures increase conductance.

Example 1        How would you lower the resistance of the following wire?

Al  at 38 oC

• Use a better conducting metal
• Lower the temperature
• Use a shorter wire
• Use a wire with a larger diameter

Example 2        Two wires, both made of brand new copper, did not have the same conductance, even though the wires were of identical length, thickness and at the same temperature. What could have been different, leading to different measurements, assuming that the same quality equipment was used to measure conductance?

One of the copper wires made have had impurities. CuO is a common impurity that can increase resistance.