5.2 The Ohm's Law Formula

This is the second week of Module 5 in the Navy Basic Electricity and Electronics series. This module is on the relationships between current, voltage, and resistance. The first section was:

5.1 Voltage, Resistance, and Current

1. You can calculate the current in a system without shutting it down and measuring it. You can use "Ohm's Law" if you know the voltage and the resistance. The formula is:

E=IR

The voltage equals the current times the resistance. We can rearrange the formula as well. Current equals the voltage divided by the resistance, and the resistance equals the voltage divided by the current.

2. You can use this formula for an entire circuit. You can also use it for a distinct part of a circuit.

3. There can be more complex situations where the value of one resistor is known but not the value of another. In such cases, knowing algebra will come in handy so that you can set up an equation and then solve for the unknown.

5.1 Voltage, Resistance, and Current

Module 5 of the Navy Basic Electricity and Electronics series starts today. This module is called Relationships of Current, Voltage, and Resistance. The first four modules were:

1. Electrical Current
2. Voltage
3. Resistance
4. Measuring Current and Voltage in Series Circuits

The first section of the new module is titled: "Voltage, Resistance, and Current." "The relationship of voltage, resistance, and current is probably the most important concept you will learn in your study of electricity" (6).

• You cannot really address current directly. You can only change it by increasing the force moving it (relates to voltage) or the resistance opposing it.
• When voltage goes up, current goes up. When voltage goes down, current goes down (assuming that resistance is held constant).
• So the amount of current is "directly proportional" to the amount of voltage.
• When resistance goes up, current goes down. When resistance goes down, current goes up (assuming voltage is constant). 
• EMF (electromotive force, which relates to voltage) and resistance are inherent quantities that depend on the physical components you are using in a circuit. Current is a secondary characteristic--it changes on the basis of the physical components.
• The relationships between voltage, resistance, and current were unfolded by George Simon Ohm (1789-1854). Ohm's Law states, "Current is directly proportional to voltage and inversely proportional to resistance."

4.3 Using a Voltmeter

This is the third and final week of Module 4 in the Navy Basic Electricity and Electronics series. The first two weeks were:

• 4.1 Current in a Series Circuit
• 4.2 Voltage in a Series Circuit

Here are my take-aways from this final section of this module:

• Always connect the multimeter in parallel when you are using it as a voltmeter (the opposite of an ammeter).
• If you do not know the voltage range you are measuring, start with the highest reading and work down.
• Your multimeter will likely have a section for DC (direct current) and a section for AC (alternating current). AC is the kind of electricity that comes out of a wall socket. DC is the kind of current that comes from a battery or self-contained unit.
• Black lead goes to the more negative side of some load bearing or current generating object, red on the more positive side.

4.2 Voltage in a Series Circuit

This is the second week of Module 4 in the Navy Basic Electricity and Electronics series. The first week was:

• 4.1 Current in a Series Circuit

1. While amperage measures anywhere in a circuit, as long as it is connected in series. Voltage can only be measured where there is a difference in potential, and it is measured in parallel.

2. A difference in potential exists either where there is a "voltage rise" across a battery or source of electromotive force... or it can be measured across a "voltage drop," something that offers resistance to that force (e.g., a resistor, a lamp, etc...).

3. The voltage source can be symbolized with an E, or E_s (source voltage), or E_a (applied voltage) or E_T (total voltage).

4. Kirchoff's Voltage Law is that the total voltage drop will always equal the total applied voltage. This is more or less the same as the conservation of energy. The voltage used up by each resistor or load will add up to the total voltage across the battery or voltage source.

5. A second important rule is that in a series circuit, the largest voltage drop will take place over the largest resistance.

2.4 Generating AC Voltage

This is the fourth week of Module 2 of the Navy Basic Electricity and Electronics series. The last few weeks were:

2.1 Electromotive Force
2.2 Magnetism
2.3 Electromagnetic Induction

This section is about how alternating current (such as what comes out of our wall plugs) can be generated.

1. There are two kinds of current: direct current and alternating current. Direct current (DC) is the kind of current you might get from a battery. Its electromotive force (EMF) does not change in polarity but is constant. Its voltage remains at about the same value and moves in the same direction constantly.

AC symbol

By contrast, alternating current changes directions and polarity at regular intervals. It's symbol in a schematic is:

2. If we were to put a loop of wire such as appears in the following diagram (Fig 1) into a magnetic field and were to rotate it, it would generate a current that would alternate in direction and polarity. You can see this using the left hand rule for generators that was introduced in the previous section.

Fig 1

When the wires are parallel to the magnetic field, no current will be generated (0 voltage). When they are moving exactly perpendicular to the magnetic field, a maximum voltage will be generated in one or the other direction.

3. There are two basic parts to a generator (or alternator): the part that moves and the part that doesn't move. The moving part is called the rotor. The part that doesn't move is called the stator.

4. Basically, if we graph the voltage produced, we are going to get a sine wave that alternates from zero to a maximum positive value back to zero to a maximum negative value than back to zero before the cycle repeats itself again (see Fig 2 below).

So here are some relevant concepts:

• cycle - One cycle of the sine wave is a complete rotation of the loop in Fig. 1
• frequency - the number of cycles per second
• Hertz (Hz) - the unit for cycles per second (1 Hz = 1 cycle per second)
• amplitude - The value of the voltage (or amps) at a given time (how high or low the sine graph is at a given point in time)
• peak to peak amplitude - the difference between the maximum and minimum values of the amplitude

  

  Fig 2

• positive alternation - the positive part of the sine wave
• negative alternation - the negative part of the sine wave

5. The frequency of the voltage from a generator depends on the speed of rotation of the loop in Fig. 1. If you add additional magnetic poles, that can also increase the frequency (e.g., a "four pole" generator).

When a coil rotates one complete cycle, it is said to have made one revolution. In degrees, this is a 360 degree turn (written 360°).

7. One final concept in this section is "effective AC" (RMS value). [1] Because AC is constantly changing values, the average amount of work done by such current is not the same as its peak value. The effective AC value is the amount of voltage you would get from an equivalent DC source. So if the effective value of an AC current is 10 volts, then you can get the same amount of work from that AC source as you could get from a 10 volt (DC) battery.

[1] RMS stands for "root mean square," 70.7% of the peak value.

2.1 Electromotive Force (EMF)

This week starts Module 2 of the Navy Basic Electricity and Electronics series. Last week I finished summarizing:

Module 1: Electrical Current

The second module begins with Electromotive Force (EMF) produced by chemical action. This book was written in the early 1970s so it's fun to see how out of date the batteries it has in mind are. Of course the way chemistry works hasn't changed. Here are the bullet points from the first section of this Module:

• Electromotive force (EMF) is the force that moves electrons through a circuit, created by some source like a battery.
• Voltage is related but slightly different. EMF is a force. Voltage is the difference of potential between the positive and negative poles of the battery or source of EMF.
• The module references "dry cell" batteries. The figures are funny because they go back to when both the positive and negative posts of a battery were on the top, the positive in the center and a negative terminal on the outer rim of the top.
• In that scheme, there was a center rod in a battery that was made out of something like carbon. 
• Then the outer "electrode" on the inside of the container might be made out of something like zinc. 
• Then a chemical paste in the middle was an "electrolyte" that facilitated an accumulation of electrons on the zinc electrode, leaving a positive charge on the carbon rod.
• Today, of course, the most common batteries are lithium batteries.
• So within the battery, there is the potential for an electron flow from the positive to the negative terminals of the battery.
• Outside the battery, there is the potential for an electron flow from the negative terminal around a circuit and back to the positive terminal.
• A "volt" is the unit of measurement for potential difference. The same metric prefix applies for volts as for amps (milli-, micro-, kilo-, mega-)
• Batteries can be connected together in series to add up the total amount of voltage (negative to positive, negative to positive, etc).
• When not connected properly (in series opposition as opposed to series aiding), they cancel each other out.
• When connected in parallel, the life of the battery is increased (often done in subs, at least in days gone by).

Next Week: 2.2 Magnetism