Download 2002 Q8 - Loreto Balbriggan

Dept. of Education and Science – Physics Higher Level - 2002.
Question 8.
Define (i) power, (ii) resistivity.
Describe an experiment that demonstrates the heating effect of an electric current.
The ESB supplies electrical energy at a rate of 2 MW to an industrial park from a local power station, whose output voltage
is 10 kV. The total length of the cables connecting the industrial park to the power station is 15 km. The cables have a
diameter of 10 mm and are made from a material of resistivity 5.0 × 10–8 . m.
(i)
the total resistance of the cables
(ii)
the current flowing in the cables;
(iii)
the rate at which energy is “lost” in the cables.
Suggest a method of reducing the energy “lost” in the cables.
__________________________________________________
Define (i) power, (ii) resistivity.
Power is the rate at which work is done or at which energy is converted. Its is measured in Watts (W) where 1 W = 1Js -1 .
The resistivity of a substance is the resistance of a piece of unit length and unit cross-section area of the substance.
ρ=
RA . It is measured in Ωm.
l
Describe an experiment that demonstrates the heating effect of an electric current.
+
12 V
-
Steel wool
strand
Starting from V = 0, increase the voltage across the strand of steel wool. As the current increases the strand will start to
warm and eventually glow and then snap.
Calculate
(i)
R=
(ii)
the total resistance of the cables;
ρl 5 .0 x10 −8 x15 ,000
=
= 9. 5Ω
A
πx( 5.0 x10 −3 ) 2
the current flowing in the cables;
W = VI ⇒ I =
(iii)
W 2 x10 6
=
= 200 A
V
1x10 4
the rate at which energy is “lost” in the cables.
P = RI 2 = 9 .5 x200 2 = 3. 8 x10 5W
Suggest a method of reducing the energy “lost” in the cables.
Increasing the transmission voltage or using thicker cables would reduce energy loss.