Page 60 - Teach Yourself Electricity and Electronics

40 Electrical units

In general, the greater the electric current through a wire, the greater the flux den-
sity near the wire. A coiled wire will produce a greater flux density than a single, straight
wire. And, the more turns in the coil, the stronger the magnetic field will be.
Sometimes, magnetic field strength is specified in terms of ampere-turns (At).
This is actually a unit of magnetomotive force. A one-turn wire loop, carrying 1 A of
current, produces a field of 1 At. Doubling the number of turns, or the current, will dou-
ble the number of ampere-turns. Therefore, if you have 10 A flowing in a 10-turn coil,
the magnetomotive force is 10 10, or 100 At. Or, if you have 100 mA flowing in a
100-turn coil, the magnetomotive force is 0.1 100, or, again, 10 At. (Remember that
100 mA 0.1 A.)
Another unit of magnetomotive force is the gilbert. This unit is equal to 0.796 At.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct answers.
The answers are listed in the back of this book.

1. A positive electric pole:
A. Has a deficiency of electrons.
B. Has fewer electrons than the negative pole.
C. Has an excess of electrons.
D. Has more electrons than the negative pole

2. An EMF of one volt:
A. Cannot drive much current through a circuit.
B. Represents a low resistance.
C. Can sometimes produce a large current.
D. Drops to zero in a short time.
3. A potentially lethal electric current is on the order of:
A. 0.01 mA.
B. 0.1 mA.
C. 1 mA.
D. 0.1 A.
4. A current of 25 A is most likely drawn by:
A. A flashlight bulb.
B. A typical household.
C. A power plant.
D. A clock radio.
5. A piece of wire has a conductance of 20 siemens. Its resistance is:
A. 20 Ω.