Conceptual Physics

13.2 Use the oscilloscope in the simulation in the interactive problem in this section to determine (a) Imax, (b) ǻVmax and (c) the

frequency f of the second signal.

(a) A

(b) V

(c) Hz

Section 15 - Transformers

15.1 A transformer with an input current of 0.500 A has 6000 primary turns. How many secondary turns must the transformer have

if an output current of 2.00 A is desired?

turns

15.2 A machine that makes transformers can only wind the coils with a single specific number density. The number of turns is

therefore controlled by the length of the coils. A transformer is designed to transform a 120 V input potential difference to a

9.0 V output. If the input coil is 2.8 cm in length, what is length of the output coil?

m

15.3 The long distance power transmission lines reaching a certain power substation operate at 375,000 V. This potential

difference has to be "stepped down" for local distribution through city power lines at 14,300 V. Substations use very large, oil-

cooled transformers (a photograph of one is in the text) to accomplish this conversion. What is the ratio of primary to

secondary windings in a transformer in such a substation?

15.4 The electricity supplied through a city power

line is transmitted at 14,300 V. On each city

block there are one or more neighborhood

transformers on utility poles that transform the

potential difference to 120 V, as used in

homes and businesses (240 V outlets are

supplied by two 120 V lines). What is the ratio

of primary to secondary windings in a

neighborhood transformer like the one shown

in the photograph?

15.5 (a) The emf produced by a typical hydroelectric generator, at the generator, is 20,000 V. The electricity produced by the

generator is fed through a step-up transformer before being transmitted across the country on "high tension" lines like those

shown in the textbook. If the primary winding of the transformer has 1550 loops, what must be the multiplicity of the secondary

winding to produce a transmission emf of 100,000 V? Of 225,000 V? Of 345,000 V? (b) A current of 4500 A flows from the

generator to the step-up transformer. What current flows through the transmission lines at each one of the three operating

emfs stated in part a? (c) If the total resistance of the power lines between the generator and a city it supplies with electricity

is 112 ȍ, use the results of part b to calculate the power dissipated in the lines at each of the three operating emfs. (d) In

commercial or public power systems, why is electric power transmitted at extremely high emfs and (relatively) low currents,

rather than the other way around?

(a) for 100,000 V for 225,000 V for 345,000 V

(b) A for 100,000 V A for 225,000 V A for 345,000 V

(c) W for 100,000 V W for 225,000 V W for 345,000 V

(d)

Section 17 - Interactive problem: configuring a transformer

17.1 Use the information given in the interactive problem in this section to determine the maximum potential difference in the

primary coil. Test your answer using the simulation.

V

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