Electricity - Basic Navy Training Courses
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¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
During the late 1700's and early 1800's, three great electrical discoveries
were made. An Italian, named Volta, discovered haw to. produce an emf from a primary
cell. He gave his name to. the measuring unit of electromotive force - the VOLT.
Ampere, a Frenchman, measured current flaw and gave his name to. the measuring unit
of current - the AMPERE. A German, named Ohm, measured the resistance of circuits
and conductors and gave his name to the resistance measuring unit - the OHM. Ohm
did more than experiment with resistance -he connected his awn discoveries with
those of Volta and Ampere. The result was OHM'S LAW. Make sure you understand each
one of the three quantities in electricity - they make up Ohm's Law. On the following
page is a table of these important quantities, their symbols, their units, their
abbreviations, and their effects an a circuit.
||Unit of Measure
||Abbreviation of Unit
||Effects in a circuit
|EMF or potential or voltage
||Force which makes current flow through a circuit
||The friction or opposition to the flow of current offered
by the conductors and electrical devices in a circuit
||a. or amp.
||The flow of electrons through a circuit
WHAT IS OHM'S LAW?
You know that increasing the potential ,will IN-CREASE the current. Likewise,
increasing the resistance will DECREASE the current. Ohm's law is this relationship
of emf, current, and resistance expressed in mathematical terms.
It says -
That is, the current, I (in amps) equals the emf, E (in volts) divided by the
resistance R, (in ohms).
Figure 31 shows a simple electrical circuit - generator, load, and connecting
wires. In this case, the load is a lamp, but ANY electrical appliance is a LOAD.
Notice the ammeter connected to read' the current and the voltmeter connected to
read the emf of the generator. If the resistance of this circuit is 2 ohms and the
emf read on the voltmeter is 12 volts, then -
Figure 31. - Simple circuit, voltage constant.
The ammeter will read 6 amperes. Which means that the load draws 6 amperes.
Figure 32. - Effect of voltage on current, R is constant.
Imagine that a battery of cells is used, instead o£ a generator, as an emf source.
The circuit. would look like figure 32.
Each cell produces 2 volts. Notice that connecting the line at-A, B, C, or D,
CHANGES the number of cells included in the' circuit. This will give you voltages
of 2, 4, 6, and 8 volts. The table at the. left of the diagram gives the current
flowing for each voltage. For each value of voltage, the current is calculated by
Ohm's law. Remember, current is DIRECTLY proportional to voltage.
Figure 33. - Effect of resistance on current, E is
In any electrical circuit, IF YOU HOLD THE RESISTANCE CONSTANT AND INCREASE
THE VOLTAGE, THE CURRENT INCREASES IN PROPORTION TO THE IN-CREASE IN VOLTAGE. IF
YOU HOLD THE RESISTANCE CONSTANT AND DECREASE THE VOLTAGE, THE CURRENT DECREASES
IN PROPORTION TO. THE DECREASE IN VOLTAGE.
Take a look at figure 33. In this circuit, the voltage remains constant, but
the resistance of the load is 2,4, 6, or 8 ohms depending on which tap, A, B, C,
or D is connected. In the table at the left of the figure, the current flow is given
for each connection. Notice that current is INVERSELY proportional to resistance.
In any electrical circuit, IF YOU HOLD THE VOLTAGE CONSTANT AND INCREASE THE
RESISTANCE, THE CURRENT DECREASES IN PROPORTION TO THE INCREASE IN RESISTANCE. IF
YOU HOLD THE VOLTAGE CONSTANT AND DECREASE THE RESISTANCE, THE CURRENT INCREASES
IN PROPORTION TO THE DECREASE IN RESISTANCE.
Reviewing the tables in figures 32 and 33, you will find that any number in
the current column I, can be obtained by dividing the voltage by the resistance.
Any number in the resistance column R, can be found by dividing the voltage
by the current.
Furthermore, any number in the voltage column, E, can be obtained by multiplying
the current and the resistance.
From your knowledge of mathematics, you recognize that these three equations
are variations of one formula.
I=E/R, R=E/I, and E=IR
If you know any two of these quantities in a circuit, or in any part of a circuit,
you can calculate the other quantity by applying the proper equation.
1. A vacuum tube filament has a resistance of 12 ohms when connected to a 6-volt
battery. What is the current in the filament?
2. An ignition coil draws 8 amperes at 6 volts. What is the resistance of the
3. A starter motor has a resistance of 0.04 ohm and draws 150 amperes at starting.
What is the voltage applied to this motor?
REMEMBER THESE POINTS -
1. The strength of the electrical CURRENT, or amperage, depends on the RESISTANCE
of the circuit AND the VOLTAGE applied to the circuit. Ohm's law will tell you how
much current is flowing.
2. The RESISTANCE does not depend on either current or voltage. The character
of the conducting path - wires and load - determine the resistance. You DO NOT change
resistance by changing current or voltage. Ohm's law will tell you how much resistance
is contained in the circuit.
3. The emf of a circuit does NOT depend on either CURRENT or RESISTANCE. The
emf is determined entirely by the generator or battery supplying the circuit. Ohm's
law will tell you how much voltage is required for a given current through a given
Chapter 6 Quiz