User Manual

If you wrap two wires from different circuits around different ends of an iron bar then a current flowing through the wire from

the first circuit will magnetically create a current in the wire from the second circuit! If the second coil has twice as many

turns (more magnetic linkage) as the first coil then the second coil will have twice the voltage but half the current as the

first coil. A device like this is called a transformer. Your PK-101 includes one. It consists of a 400mH coil (called the

primary) and a 2mH coil (called the secondary) wrapped around an iron bar. Both coils have middle tap points allowing

use of half the coil’s inductance. In Experiment 26 we used the 400mH coil by itself but usually it will be used to drive a

speaker, which needs a high current with low voltage. The symbol for a transformer is shown on the right:

The magnetic field created in an iron bar by an electric current in the coil around it can be harnessed if the bar is allowed

to rotate - it is a motor. It could be used to drive the wheels of a car, for example. The reverse is also true, if a magnet

within a coil is rotating then an electric current is created in the coil - a generator. These two statements may not seem

important to you at first but they are actually the foundation of our present society. Nearly all of the electricity used in our

world is produced at enormous generators driven by steam or water pressure. Wires are used to efficiently transport this

energy to homes and businesses where it is used. Motors convert the electricity back into mechanical form to drive

machinery and appliances.

It must be remembered that all of the inductance proper

ties discussed here for coils and transformers only apply to AC

(alternating current). For DC, inductors act as wires with no special properties and transformers are just two separate,

unconnected wires.

TEST YOUR KNOWLEDGE #2

1. A diode has very high resistance when it is __________-biased.

2. Diodes whose turn-on energy is so high that light is generated are known as __________.

3. The transistor is best thought of as a current __________.

4. An __________ circuit is one that might have many resistors, diodes, capacitors and transistors on a single piece of

silicon.

5. A transistor is __________ when the circuit resistances, not the transistor itself, are limiting the transistor’s collector

current.

6. Inductors have low resistance to __________ current and high resistance to __________ current.

7. Adding inductors in parallel __________ the inductance while adding inductors in series __________ the inductance.

8. Electrons not only have electric properties but also __________ properties.

9. Wrapping a coil around an iron bar __________ the inductance.

10. If the second coil in a transformer has half as many turns as the first coil, then the second coil will have __________

as much alternating current as the first coil.

(Answers are on page 3)

Symbol for TRANSFORMER

TRANSFORMER