25070- H2ws58
NAME2blame : _________________________
Resistance
Resistance is the opposition to what in a conductor. ________________
1) The symbol for Resistance is _______
2) Resistance R is measured in _______
3) Draw the Greek letter Omega - ____.
4) What are the factors that affect the actual value of resistance of a
conductor. These are:
 the (L)______________________________________ of the conductor
 the (csa)_____________________________________ of the conductor
 the (00)______________________________________ of the conductor
 the (p) _______________________________ of the conductor material
5) Resistance, or opposition to current flow, is due to collisions between
electrons and static particles as the (e)____________ flow through the
conductor.
6) Does changing the shape of the conductor affect the number of collisions
that occur between electrons and static particles?
YES / NO
7) If the length of the conductor is increased, what will happen to the number
of electron collisions? __________________________________________
8) Why is there an increase? _____________________________________
___________________________________________________________
9) This increase in collisions is known in electrical terms as (Ω)___________
10)
If the length is doubled then the resistance is also
(x2)_____________ as there will be ____________ as many electron
collisions. So:
11)
R (ohms)  L (metres)
If the cross-sectional area of the conductor is increased, the effective
density of the material will ________________
12)
Why is this? (space) ______________________________________
___________________________________________________________
13)
This will (decrease – increase) the number of collisions, and so
reduce the resistance of the conductor. (scratch out the incorrect word)
14)
This change is inversely proportional. If the cross-sectional area is
doubled, then the resistance is ____________.
Resistivity H2-ws58
15)
What is the term used of 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? ____________________
16)
Does this value differ for each type of conductor material? YES / NO
17)
What has the symbol ρ (the Greek letter Rho) and has a unit of ohm-
metres (Ωm)? ________________________
18)
Fill in on the chart of resistivity values, the missing conductor
material. (58-4/4)
Resistivity (Ωm)
Material
17  10-8
Steel
112  10-8
2·44  10-8
Gold
10·09  10-8
Platinum
1·78  10-8
Hard-drawn copper
7·5  10-8
Brass
48  10-8
Manganin
2·83  10-8
19)
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?
_____________
20)
What can be said about the resistance of copper compared to steel?
_____________________________________________________
21)
The resistance of a conductor material measured between the
opposite faces of a unit cube of the material is known as?
The _________________ of the material
22)
Complete the formula:
ρ
R
25)
=
ohms.
What do we need to know to determine the resistance of a particular
conductor?
i)
_________________ (M)
ii)
know its _________ (mm2)
iii)
__________________________________ (ΩM)
iv)
_________________________________ (00)
v)
_________________________________ (DENIM)
26)
Find the resistance of 75 metres of 2·5mm2 soft-drawn copper
conductor. (59-1/4)
Solution:
length =
Csa = 2·5 mm2

Rho =
R
metres
-6
2
 10 m (area is always measured in __________)
Ωm
 
a
_____ 10 8  ___
R 
_______ 10 6
 R  ______
27)
What is the length of a 16 mm2 hard-drawn copper conductor whose
resistance is 0·5 ohms?
Solution: Resistance = 0·5 ohms :
Csa = 16 mm2 = 16  10-6 m2 : Rho = 1.78 x10-8
28)
A 100 metre length of 25mm2 conductor has a resistance of 0·1132
ohms. What material is it made from?
Solution:
Resistance = 0·1132 ohms
CSA = 25 mm2 = 25  10-6 m2
length = 100 m
Temperature coefficient of resistance H2-ws59 (watch video vrr1)
1) What happens to the resistance of all pure metals with an increase of
temperature? _______________
2) In insulating materials, what happens to the resistance with an increase of
temperature? _____________________
3) What effect does a temperature change have on semiconductors?
____________________________________________________________
4) What is produced when current flows through electric machines?
____________
5) What resistance would a machine have to the flow of current when heated
up, as compared to, when the machine was cold as when just started up?
____________________________________________________________
6) The rate at which temperature affects resistance is different for all
substances and is referred to as the?
____________________________________________()
7) The Temperature Coefficient of Resistance is symbolised as ___________
____________________________________________________________
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 = ________________________________________t°C
R0 =_______________________________________ 0°C
t = ____________________________________ of R1 in °C
 = ______________________________________ 0°C
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.
Rt
=
R0 (1 + 0t) ohms
=
150 (1 + (0·00427 x _______))
=
________________ ohms
10) 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 ( 0)
_______ and then calculate the resistance at the new temperature.
11) Calculate the resistance of a copper coil at a temperature of 70°C if the
resistance of the coil at 35°C is 120.a.
R t  R 0 (1  α 0 t)
R0 
R0 
(1  α 0 t)
_____
(1  (0  00427 35))
 R 0  _____
First find the resistance at 0°C.
13) b. 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 ____)
 R 2  ________
12) All of the materials that show an increase in resistance with an increase in
temperature are referred to as having a?
(ptc)_________________________________________________________
13) The abbreviation for the above is? ___________
14) Some materials have a decrease in resistance as the temperature rises.
These are referred to as having a? _______________________________
15) What two devices are used as resistance temperature detectors?
____________________________________________________________
____________________________________________________________
16) Applications of resistance temperature detectors are found in electrical
equipment such as? __________________________________________
___________________________________________________________
17) What type of thermistor is used to protect an electric motor from
overheating? _______________
18) Where must the protective device be fitted on the electric motor for the
effective operation of the thermistor to take place? ___________________
____________________________________________________________
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? ___________________
20) What causes heat to be produced in a cable? _____________________
___________________________________________________________
21) What effect does this heat have on the power that can be supplied by a
cable? The hotter the cable gets _______________________________
__________________________________________________________
22) The insulation on a cable acts as an electrical insulator, what else is the
cable insulated from? _______________________
23) The insulation on the cable is marked as V90, what is meant by this
marking? ___________________________________________________
____________________________________________________________
____________________________________________________________
24) INSULATION RESISTANCE 25070-H2-85
the longer the cable the
more ________________ 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 !
_______________ because there is more leakage.
26) The breakdown of the insulation is a major cause of?
________________________________
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?
________________________________
29)
What working voltage is injected into the circuit when an insulation
test is done? _____________________________________________ (mice)
30)
What voltage would a 230V circuit be tested with? _______________
31)
Is the voltage used for the insulation test ac or dc ? ________ (straight)
32)
What effect does depositing of contaminants from the environment
have on the circuit? ______________________________________________
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