Soln to p79 Exercises

 

 

1.         A substance used in the manufacture of wire that will transport electrical energy should have two of the following properties :

 

1.         Good resistance to corrosion

2.         Poor resistance to corrosion

3.         Good ductility

4.         Poor ductility

Which two properties are they?

 

1 and 3

 

2.         Porcelain is used to support electrical wires on poles. Which two properties of porcelain make it desirable for this use?

 

1. Is a good insulator

2. Is non-ductile

3. Does not rust

4. Breaks easily

 

1 and 3

 

 

3.         Name 4 factors that influence the resistance of a metal conductor.

 

a.                   Length of the wire

b.                   Its cross-sectional area

c.                   Its chemical makeup

d.                   temperature

           

 

4.         There are six electrical wires made of the same substance and having the same length: three have a diameter of 1.5 mm while the other three have a diameter of 3.0 mm.

 

They are placed either end to end to increase the length of the wire or parallel to one another to increase the surface area of the wire.

 

Which three-wire arrangement offers the least resistance to the flow of electric current?

 

The 3.0 mm wires should be placed parallel to each other.

 

 

5.         A device consists of a power supply, a fan and two connectors.  You are to insert a piece of wire between these connectors.

 

Four nichrome wires are illustrated below.  You insert each of these wires in turn.  They are the same temperature, but of different sizes.  These wires are not drawn to scale.

 

With which wire will the fan rotate the fastest? (see next page for rest of multiple choice)

 

 

A)

 

 

 

B)

 

 

C)

 

 

D)

 

 

 

6.         Note the following substances:

 

1. Rubber

2. Plastic

3. Aluminum

4. Copper

5. Steel

 

Which of these substances are insulators?

 

1 and 2

 

 

7.         Which of the following substances best conducts electricity?

 

 

A)

 

Copper

 

C)

 

Nichrome

 

B)

 

Water

 

D)

 

Plastic

 

 

 

8.         Which of the following would increase the electrical conductivity of a circuit?

 

1- A thicker wire

2- A longer wire

3- A decrease in the temperature of the wire

4- The use of porcelain wire

 

 

A)

 

1 and 2

 

C)

 

2 and 4

 

B)

 

1 and 3

 

D)

 

3 and 4

9.         What could be done to the temperature of a circuit in order to improve conductance?

 

Lower temperature

 

10.       Why does a wider wire conduct electricity better than a narrower one?

 

Less crowding of electrons allows better flow.

 

11.       a.         Of the following, which two metals are the best conductors?

 

                        Fe        Cu        Ag        Au

 

            b.         Which of the above 4 is the worst?        Fe

 

12.       In general, why do metals conduct electricity (Think of atomic structure)

 

When metal atoms exist side by side in a solid structure, their loose electrons flow in a “sea of electrons”, which surrounds all of the metal atoms. From this sea, it is very easy for incoming electrons (from an external circuit, for example) to displace some of these electrons and cause them to flow into the rest of the circuit.

 

13.       What is the unit of resistance? Give both word and the symbol.

 

Ohm = W

 

14.       What is the unit of conductance? Again, give both word and the symbol.

                                               

Siemen = S

 

15.       Convert the following. (Show your work)

 

a.         50 S = 1/50 = 0.02_W.

b.         0.100 W = 1/0.100 = 10.0 S.

c.         If G = 1/R, then R =1/G.

d.         R = 12 V/0.5 A. (don't worry about the units until we study Ohm's law)

 G = 0.5/12 = 0.042 S