Electric Current
Voltage may be viewed as an “electric pressure” that can produce a flow of charge, or current, within a conductor. The flow is restrained by the resistance it encounters. When the flow takes place along one direction, it is called direct current (DC). When if flows back and forth it is called alternating current (AC). The rate at which energy is transferred by electric current is power.
Flow of Charge
Charge flows when there is potential difference, or difference in potential “voltage”, between the ends of the conductor. The flow of charge will continue until both ends reach a common potential. When there is no potential difference, there is no longer a flow of charge to the conductor. To obtain a sustained flow of charge in a conductor, a difference in potential must be maintained while charge flows from one end to the other.
Electric Current
The flow of electric charge is called electric current. In solid conductors, the electrons carry the charge through the circuit because they are free to move throughout the atomic network. These electrons are called conduction electrons. Protons do not move around because they are bound inside the atomic nuclei and are pretty much locked in place. Electric current is measured in amperes (A). An ampere is a flow of one coulomb of charge per second.
I = q/t
Current carrying wire does not have a net electric charge. While the current is flowing negative electrons swarm through the atomic network that is composed of positively charged atomic nuclei. Under ordinary conditions, the number of electrons in a wire is equal to the number of positive protons in atomic nuclei. When an electrons flow into wire, the number entering one end is the same as the number leaving the other. The net charge of the wire is normally zero at every moment.
Voltage Sources
Charges do not flow unless there is a potential difference. A voltage source provides a potential difference. Dry cells, wet cells, and generators are capable of maintaining a study flow. A battery is just two or more cells connected together. In dry cells and wet cells energy released in a chemical reaction occurring inside the cell is converted into electrical energy. Generators, such as the alternators in automobiles, convert mechanical energy to electrical energy. The potential energy per coulomb of charge available to electrons moving between terminals is the voltage sometimes called the electromotive force (EMF). The voltage provides the electric pressure to move electrons between the terminals in a circuit. When the prongs of a plug are inserted into the average home outlet, an average electric pressure of 120 volts is placed across the circuit connected to the prongs. This means that 120 joules of energy is supplied to each coulomb of charge that is made to flow in the circuit. It is important to remember that voltage causes current, it doesn’t go anywhere. It is the charges that move not the voltage. It is appropriate to say that charges flow through a circuit and that there is a voltage applied across the circuit.
Electric Resistance
Current also depends on the resistance that the conductor offers to the flow of charge, which is called the electric resistance. Resistance of a wire depends on the conductivity of the material used in the wire, as well as the thickness, and the length of the wire.
Thick wires have less resistance than thin wires; long wires have more resistance than short wires. Electric resistance also depends on temperature. Increased temperature means increased resistance. Electric resistance is measured in units called ohms (e).
R = V/I
where V = volts, I = amps, and R = resistance in ohms.
Ohms Law
Current in a circuit is directionally proportional to the voltage across the circuit and is inversely proportionally to the resistance of the circuit. Current equals voltage divided by resistance. The relationship among the units of measurement for these 3 quantities is:
1 amp = 1 volt/ohm
The formula for Ohms Law may be expressed as: V = IR
For a given circuit of constant resistance current voltage are proportional. This means you get twice the current for twice the voltage. The greater the voltage the greater the current. If resistance is doubled for a circuit the current will be half of what it would be otherwise. The greater the resistance the less the current.
The damaging effects of shock are the result of current passing through the body. This current depends on voltage applied and also on the electric resistance of the human body. A current of .005 amps is painful. At .010 amps involuntary muscle contractions occur. Loss of muscle control occurs around .015 amps. A current of greater than .070 amps can be fatal.
DC and AC Current
By alternating the polarity voltage at the generator or other voltage source electrons in a circuit move first in one direction and then in the opposite direction alternating back and forth about relatively fixed positions. AC currents alternate back and forth at frequency of 60 cycles per second. Voltage in the United States has been standardized at 120 volts due to the multitude of power plants built prior to 1900. In Europe, they have adopted 220 volts as their standard due to later construction of power plants since older plants were not operation. AC current is more popular due to the fact that it can be transmitted great distances with easy voltage step hops, which result in lower heat losses in the wires.
The current in battery-operated devices such as pocket calculator is DC. An AC-DC converter can be used to convert DC to AC operation. The converter used a diode, which is a tiny electronic device that acts as a 1-way valve to allow electron flow in only 1 direction. To maintain continuous current while smoothing the bumps a capacitor is used. Since a capacitor acts as a storage reservoir for charges, it takes time to add or remove electrons from the pleats of the capacitor, a capacitor therefore produces a retarding effect on changes on current flow, there by smoothing out the pulse output from the AC current.
At room temperature the electrons inside a metal wire have an average speed of a few million kilometers per hour due to their thermal motion. This does not produce a current because the motion is random. There is no net flow in any on direction. It is a pulsating electric field that can travel through a circuit at nearly the speed of light. Electrons continue their random motions in all directions while simultaneously being nudge along the wire by the electric field. Before they gain appreciable speed the conduction electrons bump into anchored metallic ions in their paths and transfer some of their kinetic energy to them. This is why current carrying wire becomes hot. These conclusions interrupt the motion of the electrons so that their actual drift speed or net speed through the wire due to the field is extremely low. In a typical DC current such as in a automobile, electrons have a net average drift speed of about .01 CM per second. It would take about 3 hours for an electron to travel through 1 meter of wire. In an AC current the conduction electrons don’t make any net progress in any direction. They drift a tiny fraction of a CM in one direction and then they move back. It is the pattern of oscillating motion that is carried across wires at nearly the speed of light. The electrons already in the wires vibrate to the rhythm of the traveling pattern.
When you plug a lamp into a AC outlet energy flows from the outlet into the lamp not electrons. Energy is carried by the electric field and causes a vibratory motion of electrons that already exist in the lamp element. Most of the electrical energy appears as heat while some of it takes the form of light. Utility companies do not sell electrons, they sell energy. You supply the electrons. Hence, when you receive an AC electric shock, the electrons making up the current in your body originated in your body and did not come from the wire or through your body and into the ground, energy does. The energy simply causes free electrons in your body to vibrate in unison. Small vibrations tingle where as large vibrations can be fatal.
Power
The rate that energy is used is power. It is expressed by:
P = IV = V²/R = I²R = E/t
The power dissipated in a resistor is proportional to the square of the current that passes through it and the resistance. The energy is changed from electrical to thermal energy. Power varies directly with the resistance. If all the electric energy is converted into thermal energy of the resistor, the increase in thermal energy is
E = I²R
Power companies do not sell electrons or power; they sell energy. The basic unit of energy used to determine the amount of your power bill is the kilowatt hour (kWh) which is the rate of energy use (P) multiplied by time (one hour)
E = Pt
then by multiplying the number of kilowatt hours by the cost per kilowatt, the amount owed on your electric bill is determined.
Review Questions:
1. The current through a microwave connected to a 120 V source is 8.0A. What is the power rating of the microwave?
2. A current of 1.2 A flows through a light bulb when it is connected across a 120 V source. What is the power rating of the bulb?
3. A flashlight bulb is connected across a 3.0 V difference in potential. The current through the lamp is 1.5 A. (a) What is the power rating of the lamp? (b) How much electric energy does the lamp convert in 11 minutes?
4. A voltage of 75 V is placed across a 15 ohm resistor. What is the current through the resistor?
5. A resistance of 60 ohms has a current of 0.4 A through it when it is connected to the terminals of a battery. what is the voltage of the battery?
6. A 12-V battery is connected to a device and 24 mA of current flows through it. If the device obeys Ohm's Las, how much current will flow when a 24-V battery is used?
7. A lamp draws a 66 mA current when connected to a 6.0 V battery. When a 9.0 V battery is used, the lamp draws 74 mA. (a) Does the lamp obey Ohm's Law? (b) How much power does the lamp dissipate at 6.0 V? (c) What is the current through the lamp as it is turned on if it is connected to a potential difference of 120 V?
8. The current through a lamp connected across 120 V is 0.4 A when the lamp is on. (a) What is its resistance when on? (b) When the lamp is cold, its resistance is one fifth as large as when the lamp is hot. What is the cold resistance? (c) What is the current through the lamp as it is turned on if it is connected to a potential difference of 120 V?
9. How much energy does a 60 W bulb use in half an hour? If the light bulb is 12% efficient, how much thermal energy does it generate during the half hour?
10. Nevada Power charges $0.072 per kilowatt hour for energy. If you leave a 75 W light bulb on from 10:00 pm and turn it off at 6:00 am, how much will it cost you? It is connected to a 120 V potential difference.
