Everything Science Grade 12

13.3 CHAPTER 13. ELECTRODYNAMICS

4. Explain how a DC generator works. Drawa diagram to support your answer.
   Also, describe how a DCgenerator differs from anAC generator.


5. Explain why a current-carrying coil placed ina magnetic field (but not parallel to
   the field) will turn. Refer to the force exerted onmoving charges by a magnetic
   field and the torque on the coil.


6. Explain the basic principle of an electric motor. Draw a diagram to support your
   answer.


7. Give examples of theuse of AC and DC generators.


8. Give examples of theuses of motors.

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13.3 Alternating Current ESCFW

Most students learningabout electricity beginwith what is known as direct current

(DC), which is electricity flowing in a constantdirection. DC is the kind of electricity

made by a battery, withdefinite positive and negative terminals.

However, we have seenthat the electricity produced by some generators alternates

and is therefore knownas alternating current (AC). The main advantage to AC is that

the voltage can be changed using transformers. That means that the voltage can be

stepped up at power stations to a very high voltage so that electrical energy can be

transmitted along powerlines at low current andtherefore experience lowenergy loss

due to heating. The voltage can then be stepped down for use in buildings and street

lights.

FACT

In South Africa alternat-
ing current is generated
at a frequency of 50 Hz. The circuit symbol for alternating current is:

∼

Graphs of voltage against time and current against time for an AC circuit are shown in

Figure 13.9

In an ideal DC circuit the current and voltage are constant. In an AC circuit the current

and voltage vary with time. The value of the current or voltage at any specific time is