Grueneich 2011
Theatre Tech
Electricity Notes
Electrical Current is defined as the flow of electrons from one point to another. The unit of measurement for this electron flow is the ampere.
Electrical current: the flow or movement of electrons through a conductor
Ampere: the unit of measurement of electrical current
Potential:the difference in electrical charge between two bodies; measured in volts
Volts:the unit of measurement of electrical potential
Source: The origin of electrical potential, such as a battery or 120-volt wall outlet.
Load:A device that converts electrical energy into another form of energy: a lamp converts electrical energy to light and heat; an electrical motor converts electricity to mechanical energy
Circuit:A conductive path through which electricity flows
Every electrical system must have three parts: a source, a load, and a circuit. The source is a mechanism that provides a difference in potential or, voltage. The load is a device that uses the electricity to perform some function. The circuit is a pathway that the current follows as it flows from the negative to the positive terminal of the source.
Ohm’s Law
German physicist, Georg Simon Ohm, discovered in the nineteenth century that some very basic rules apply to the functioning of electricity in a circuit. These relationships have been formalized as Ohm’s Law, and they are the primary mathematical expression used in determining electron action within circuit. Ohm’s Law states: as voltage increases, current increases; as resistance increases, current decreases.
Resistance: the opposition to electron flow within a conductor, measured in ohms; the amount of the resistance is dependent on the chemical makeup of the material through which the electricity if flowing
The relationship of Ohm’s Law can be mathematically expressed as:
I =
where I = current in amperes, E = voltage in volts, and R = resistance in ohms. The basic formula can be rearranged into two other forms. Each of these can be used to find the value of the other components of the relationship.
E = IR
R =
The Power Formula
Another formula, which is a derivation of Ohm’s Law, is much more useful when dealing with higher voltage electricity. It is called the power formula. This formula is used when it is necessary to determine how much power will be consumed by an electrical circuit.
The amount of electrical energy converted, or consumed, is measured in watts. Usually the wattage figure is written on a label located somewhere on the device. Almost all household lamps have both the voltage and wattage on the top of the bulb. Stage lighting lamps have this information printed on either the metal lamp base or the top of the lamp.
The power formula is usually referred to colloquially as either the “pie” or the “West Virginia” formula:
P = IEW = VA
wherewhere
P = power in wattsW = power in watts
I = current in amperes V = voltage in volts
E = voltage in voltsA = current in amperes
The power formula can be rearranged as Ohm’s Law can:
P = IEW = VA
E =A =
I =V =
Example: You want to put a desk lamp on a table, but the power cord won’t reach from the table to the wall outlet. You go to the hardware store to buy an extension cord, and the only information attached to the power cord indicates that it will safely carry 6 amperes of current. You know that the voltage in your home is 117 volts (standard household voltage in the U.S.). The lamp you plan to use is rated at 150 watts. To determine if the extension cord is safe to use, you will need to find out how many amperes of current the 150-watt lamp will create. To find the answer just plug the known information (V = 117, W = 150 ) into the appropriate variation of the power formula – the variation that has the unknown variable (A) located on the left side of the equal sign.
A =
A =
A = 1.28 amps
The lamp creates a current of 1.28 amperes, so the extension cord, which can carry 6 amperes, will be safe to use
Practical Information
The output load voltage of dimming systems in the United States is 117-120 volts alternating current (VAC)
Fuse: A device to protect a circuit from an overload; has a soft metal strip that melts, breaking circuit continuity
Circuit breaker: A device to protect a circuit from an overload; has a magnetic device that trips open, breaking circuit continuity
American Wire Gauge Current Capacity Chart
Gauge of Wire1012141618
Capacity in Amps252015 6 3
The problems illustrate how the power formula can be used to calculate the safe electrical load limits of typical stage lighting situations
Problem No. 1 The output voltage of a dimmer is 120 VAC. The dimmer can handle 20 amperes of current. What is the maximum safe load that can be placed on this dimmer?
watts = volts times amperes (W = VA)
W = 120 x 20
W = 2,400 watts
The dimmer can safely carry any load up to, but not exceeding, 2,400 watts.
Problem No. 2 The system voltage is 120 VAC. The dimmer can carry 2,400 watts (2.4 kilowatts, or KW). The 14-guage cable connecting the dimmer to the lighting instruments can carry 15 amperes. How many 500-watt lighting instruments can be safely loaded onto the dimmer?
W = VA
W = 120 x 15
W = 1,800 watts
The cable can carry a maximum load of 1,800 watts. To determine the number of 500-watt lighting instruments that can be carried by the cable, divide 1,800 by 500.
1,800
500
Theoretically, the cable can safely carry 3.6 instruments. Practically, it can safely carry three instruments. Even though the dimmer can safely carry four instruments, the single cable connecting the dimmer to the instruments can handle only the current flow generated by three 500-watt stage lighting instruments
Electrical Curent
There are two types of electrical current, direct and alternating.
Direct Currentin direct current (DC) the electron flow is in one direction only. The flow of the current is always from the negative terminal of a batter to its positive terminal. All batteries are examples of direct current sources.
Alternating CurrentThe overwhelming majority of electrical power generated by power stations is alternating current (AC). The electron flow in AC is the same as in DC with an exception – the current flow periodically changes direction. In this United States, alternating current changes polarity at the rate of sixty cycles per second (60 Hz). This means that the electricity changes polarity (direction) every 1/120th of a second.
If the wires connected to the terminals of a battery were reversed, the lamp would still emit light, but the current flow would have changed directions. Alternating current works like this example, except that the direction of current flow changes direction every 1/120th of a second
The advantages of AC over DC are that AC is easier and cheaper to generate and that there is less voltage loss when the electricity is transmitted over a great distance