Two resistors, of resistance R1 and R2 respectively, are connected in parallel.Derive an expression for the effective resistance of the two resistors in terms of R1 and R2. (12)
12 V
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12 Fig 18.37
‘SECTION B’
HALF QUESTION Typical of Question 12 in Leaving Cert. Exam
Q8 Answer all of the following parts (a), (b), (c), and (d).
(a) At power stations, electricity is transformed to very high voltage levels of up to 400 000 kilovolts. It is then fed into transmission cables carrying the electricity throughout the country. Overhead lines are the standard method of designing and building transmission networks worldwide, outside of built-up areas. Electrical energy is transmitted from a particular power station whose output is 200 kV at a rate of 50 MW to a transformer station, where the voltage is reduced for distribution onwards.The total length of the cables connecting the power station to the transformer station is 25 km.The cables have a diameter of 16 mm and are made from a material of resistivity
4.0 108 Æ m .
Calculate: (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.
(b) Draw a graph to show the relationship between current and voltage (with voltage on the x-axis) for each of the following conductors:
(i) a metal at constant temperature
How would the graph differ if the temperature of the metal increased as the voltage increased? (ii) an ionic solution with active electrodes
How would the graph differ if the concentration of the ionic solution was reduced? How would the graph differ if inactive electrodes were used? (iii) a cathode ray tube.
(c) State Ohm’s law. The graph in Fig 18.38 shows how the current varies with voltage in a gas discharge tube. 330 INVESTIGATING PHYSICS
