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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
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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
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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
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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
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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