1

25070- H3-ws58 (ans)

NAME2blame : ______________________

Resistance 1) Resistance is the opposition to what in a conductor. CURRENT FLOW 2) The symbol for Resistance is

R

3) Resistance R is measured in OHMS 4) Draw the Greek letter Omega -

Ω

5) What are the factors that affect the actual value of resistance of a conductor. These are:  the (L) LENGTH of the conductor  the (csa) CROSS-SECTIONAL AREA of the conductor  the (00) TEMPERATURE of the conductor  the (p) RESISTIVITY of the conductor material 6) Resistance, or opposition to current flow, is due to collisions between ELECTRONS and static particles as the (e) ELECTRONS flow through the conductor. 7) Does changing the shape of the conductor affect the number of collisions that occur between electrons and static particles?

YES

8) If the length of the conductor is increased, what will happen to the number of electron collisions? THE COLLISIONS WILL INCREASE 9) Why is there an increase? THERE IS AN INCREASE OF PARTICLES 10)

This increase in collisions is known in electrical terms as?

RESISTANCE 11)

This increase is directly PROPORTIONAL

12)

If the length is doubled then the resistance is also DOUBLED as

there will be TWICE as many electron collisions. So: 13)

R (ohms)  L (metres) If the cross-sectional area of the conductor is increased, the effective

density of the material will DECREASE 14)

Why is this?

THERE IS NOW MORE SPACE FOR THE ELECTRONS TO MOVE IN

2

15)

This will DECREASE the number of collisions, and so reduce the

resistance of the conductor. 16)

This change is inversely proportional. If the cross-sectional area is

doubled, then the resistance is HALVED 17)

What is the term used for a material that is measured as being the

resistance between the opposing faces of a one metre cube of the material at a specific temperature? RESISTIVITY 18)

Does this value differ for each type of conductor material? YES / NO

19)

What has the symbol ρ (the Greek letter Rho) and has a unit of ohm-

metres (Ωm)? RESISTIVITY 20)

Fill in on the chart of resistivity values, the missing conductor

materials. Resistivity (Ωm)

Material

17  10-8

Steel

112  10-8

NICHROME

2·44  10-8

Gold

10·09  10-8

Platinum

1·78  10-8

Hard-drawn copper

7·5  10-8

21)

Brass

48  10-8

Manganin

2·83  10-8

ALUMINIUM

1·72  10-8

Soft-drawn copper

1·63  10-8

SILVER

Does steel have a high or a low resistivity when compared to copper?

HIGH 22)

What can be said about the resistance of copper compared to steel?

THE RESISTANCE OF COPPER IS LOWER

3

23)

Complete the formula: ρL R

25)

=

A

ohms.

What do we need to know to determine the resistance of a particular

conductor? i)

LENGTH (M)

ii)

know its CSA (mm2) AND CONVERT TO M2 by x 10-6

iii)

RESISTIVITY (ΩM)

iv)

TEMPERATURE (00) Find the resistance of 75 metres of 2·5mm2 soft-drawn copper

26)

conductor? (old-p32)

Solution:

length = 75 metres 

Rho =

R

Csa = 2·5 mm2 = 2.5  10-6 m2

Ωm

  a 1  72  10 8  75 2  5  10 6

R 

 R  0  516 27)

What is the length of a 16 mm2 hard-drawn copper conductor whose

resistance is 0·5 ohms? (old-p33) Solution: Resistance = 0·5 ohms : Csa = 16 mm2 = 16  10-6 m2 : Rho = 1.78 x10-8 R = p x L and L = R x a a

L = 0.5 x 16 x 10-6 1.78 x 10-8

p

=

449.44 m

4

A 100 metre length of 25mm2 conductor has a resistance of 0·1132

28)

ohms. What material is it made from? (old-p34)

Solution:

Resistance = 0·1132 ohms: CSA = 25 mm2 = 25  10-6 m2 ; length = 100 m

p=Rxa

0.1132 x 25 x 10-6

L

and

p = 2.83 x 10-8 Ωm aluminium

100

TEMPERATURE COEFFICIENT OF RESISTANCE H2-ws59-1/4 PLAY VIDEO 1) What happens to the resistance of all pure metals with an increase of temperature? INCREASES 2) In insulating materials, what happens to the resistance with an increase of temperature? DECREASES 3) What effect does a temperature change have on the resistance of semiconductors? DECREASES 4) What is produced when current flows through electric machines? HEAT 5) What resistance would a machine have to the flow of current when heated up as compared to when the machine was cold and when just started up? AN INCREASE IN TEMPERATURE CAUSES THE RESISTANCE OF A CONDUCTOR TO INCREASE (H2-ws59-2/4) 6) The rate at which temperature affects resistance is different for all substances and is referred to as the? TEMPERATURE COEFFICIENT OF RESISTANCE 7) The Temperature Coefficient of Resistance is symbolised as? THE CHANGE IN RESISTANCE PER OHM, PER DEGREE CHANGE IN TEMPERATURE (H2-ws59-3/4) 8) Calculating the resistance of a material at a different temperature when the resistance at 0° is known is carried out as follows using the formula: Rt = R0 (1 + 0t) Where

Rt = resistance at the temperature that is required t°C R0 = resistance at 0°C t = temperature at 00c  = temperature coefficient of resistance at 0°C

5

9) Determine the resistance of a copper coil at a temperature of 65°C if the resistance at 0°C was 150  given that 0 = 0·00427. (Old-p37) Rt

10)

=

R0 (1 + 0t) ohms

=

150 (1 + (0·00427 x 65))

=

191.63 ohms

Calculating resistances at different temperatures. If the resistance at

a temperature other than 0°C is known instead however, the formula cannot be used directly. To do this, we must first calculate the temperature at 00c and then calculate the resistance at the new temp.

11)

Identify the formula. (old-p38)

( two temperatures other than 00 or 200 c) Calculate the resistance of a copper coil at a temperature of 70°C if the resistance of the coil at 35°C is 120. First find the resistance at 0°C. R t  R 0 (1  α 0 t) R0 

Rt (1  α 0 t)

R0 

120 (1  (0  00427 35))

 R 0  104.4 ohms at 0

2nd) Then find the resistance at 70°C.

R 2  R 0 (1  α 0 t)  R 2  R 0 (1  α 0 t)  R 2  104  4(1  0  00427  70)  R 2  135.6ohms

6

12)

All of the materials that show an increase in resistance with an

increase in temperature are referred to as having a.

(H2-ws59-4/4)

POSITIVE TEMPERATURE COEFFICIENT 13)

The abbreviation for the above is? PTC

14)

Some materials have a decrease in resistance as the temperature

rises. These are referred to as having a? NEGATIVE TEMPERATURE COEFFICIENT 15)

What two devices are used as resistance temperature detectors?

WIRE WOUND AND THIN FILM DEVICES 16)

Applications of resistance temperature detectors are found in

electrical equipment such as? OVENS, AIR-CONDITIONERS, REFRIGERATORS, AIRFLOW SENSORS, PRECISION THERMOMETERS 17)

What type of thermistor is used to protect an electric motor from

overheating? 18)

PTC THERMISTORS

Where must the protective device be fitted on the electric motor for

the effective operation of the thermistor to take place? IN THE CONNECTION END, AT LEAST 8mm BELOW THE SURFACE OF THE WINDINGS 19)

The AS/NZS3000:2010 referring to the resistivity of soils and earth

electrode, does not refer to a minimum soil resistance, but refers to a minimum?

ELECTRODE DEPTH

20)

What causes heat to be produced in a cable? CURRENT FLOW

21)

What effect does this heat have on the power that can be supplied by

a cable? The hotter the cable gets THE MORE RESISTANCE IN THE CABLE RESULTING IN LESS POWER BEING SUPPLIED. 22)

The insulation on a cable acts as an electrical insulator, what else is

the cable insulated from? HEAT 23)

The insulation on the cable is marked as V90, what is meant by this

marking? THE TEMPERATURE THAT THE CABLE WILL SAFELY WITHSTAND and in this case it is 900 C

7

24) INSULATION RESISTANCE 25070-H2-ws85

the longer the cable the

more _LEAKAGE_ will occur from the cable, therefore a shorter cable will have less leakage and a higher resistance value. 25) If the length of a cable is doubled the insulation resistance will? DOUBLE/HALVE !

_HALVE_ because there is more leakage.

26) The breakdown of the insulation is a major cause of? _EQUIPMENT FAILURE_ 27) An insulation test is looking for a response from the insulation that communicates? __________________________________________ 28) Insulation resistance can vary between zero and infinity, which is best? __INFINITY__ 29)

What working voltage is injected into the circuit when an insulation

test is done? __TWICE THE WORKING VOLTAGE__ (mice) 30)

What voltage would a 230V circuit be tested with? __500V__

31)

Is the voltage used for the insulation test ac or dc ? _DC_ (straight)

32)

What effect does depositing of contaminants from the environment

have on the circuit? There is a chemical attack on the insulation. 33) Answer the following six questions assuming the resistivity of copper is 1.78 x 10-8 ohms per metre and Aluminium is 2.84 x 10-8 ohms per metre. A1)

Determine the resistance of 120m of copper cable whose cross sectional area is 1.5mm2.

A2)

Calculate the resistance of 50m of copper cable 4mm2.

A3) Find the cross sectional area of a copper cable which is 90m long and has a resistance of 0.267 ohms. A4) Find the cross sectional area of a copper cable 42m long which carries a current of 36 A with a voltage drop of 2.69 volts. A5) Resistance wire has a resistivity of 50 x 10-8ohms per meter. Find the length to make a heating element with a resistance of 20 ohms. The wire is 0.75 mm2. A6) Calculate the total voltage drop in 75m of twin 16mm2 copper cable, when it carries 25 A. Calculate the voltage drop if Aluminium was used. R= and also V = I x R