Voltage Sources and Electric Current

Georgia Perimeter College

Objectives

  1. Describe “electric current” in terms of the rate of flow of charge along an electric field, and its corresponding unit of measure, the “ampere”.

  2. Describe a simple electric circuit (dc circuit and ac circuit) in terms of a voltage source (such as a battery), connecting wires, and an electrical device, and some real-world applications

  3. Distinguish between conventional (positive) current and electron current

  4. Determine the resistance of a conductor and its relationship to the electric current in a circuit using Ohm’s Law, and its corresponding unit of measure, the “ohm”

  5. Describe the properties of an object that determine its electric resistance

  6. Describe how to use an ammeter to measure the electric currents and a voltmeter to measure the voltages for a group of resistors in series, parallel, or series-parallel combination

This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S8P2. Students will be familiar with the forms and transformations of energy.
    c. Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, sound) and their characteristics.
  • S8P5. Students will recognize characteristics of gravity, electricity, and magnetism as major kinds of forces acting in nature.
    b. Demonstrate the advantages and disadvantages of series and parallel circuits and how they transfer energy.
    c. Investigate and explain that electric currents and magnets can exert force on each other.

 

This section addresses, in whole or in part, the following Benchmarks for Science Literacy:
  • Energy can change from one form to another, although in the process some energy is always converted to heat. Some systems transform energy with less loss of heat than others.

  • Electrical energy can be produced from a variety of energy sources and can be transformed into almost any other form of energy. Moreover, electricity is used to distribute energy quickly and conveniently to distant locations.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.
  • Electrical circuits provide a means of transferring electrical energy when heat, light, sound, and chemical changes are produced.
  • In most chemical and nuclear reactions, energy is transferred into or out of a system. Heat, light, mechanical motion, or electricity might all be involved in such transfers.

Voltage Sources and Electric Current

The majority of electrical devices and applications involve electric charges in motion, as different from static electricity.

In solid conductors, especially metals, the outer electrons in the atoms are relatively free to flow from point to point in the conductor.

In ionic liquid conductors (and ionic solutions), both positive and negative ions can move.

The electric potential difference or voltage causes the flow of charge. With zero voltage (i.e. when there is no difference in electric potential) across the ends of the conductor, there is no flow of charge.

The rate of flow of charges per unit time through a material is defined as electric current. The electric current is a measure of how many charges flow past a section of the conductor during a period of time (e.g. per second interval).

Current flows through connecting wires (conductors) and other electrical devices in a closed path or loop known as an "electric circuit".

A potential difference (voltage) is created by a separation of charge across the ends of the conductor. So, for a sustained electric current, a voltage source must maintain the separation of charges, thereby creating the potential difference that pushes charges through a conductor.

The more common voltage sources are batteries and electric power generators.

After reading this content summary, review the relevant chapter and the relevant sections (Chapter 9) in your textbook to learn more about the highlighted topics (in bold face, underline, or italics). Click on the hyperlinked words to review the related information online.

Answer the Review Questions for each section after covering the relevant material. Be sure to complete all online practice exercises and activities to check your understanding before proceeding to the next topic.

Vocabulary

battery

terminal

grounded

electric current

q

I

ampere, amp

electron current

conventional current

AC

DC

alternating current

direct current

ohm

resistance

conductivity

Ohm’s Law

Voltage Sources

Every electric circuit must have a voltage source, which acts as the source of energy for maintaining a voltage that causes current in a complete circuit. To maintain a steady current, a voltage source does work to "pump" (positive) charges to a higher electric potential. These charges in turn expend the potential energy through the circuit as they flow back to the lower electric potential.

A battery, as a voltage source, uses chemical energy (during a chemical reaction inside the battery) to separate and store positive charges at the positively charged terminal and also store negative charges at the negatively charged terminal of the battery.




 

Image courtesy of the Energy Information Administration

The work done to separate the charges is stored as electric potential energy.

The negative terminal of a battery is usually grounded or assigned an electric potential of 0.0 V (i.e., zero volt).

Therefore, the voltage of other parts of the circuit is measured relative to the 0 V at the negative terminal.

The schematic symbol of a battery, with the possible maximum voltage is shown below. Note the labels: (+) at the positive terminal, and (-) at the negative terminal.

It must be emphasized that a battery maintains a fixed voltage, but does not provide a constant current. The amount of current depends on the maximum possible voltage of the battery and the electric resistance of the devices connected in the circuit.

The positive terminal of a certain automobile battery has a potential of 12 V and the negative terminal is grounded or at 0 V. The potential difference (or voltage) between the battery terminals is then, V(+) - V(-) = 12 V - 0 V = 12 V. A battery cable connected across the terminals of this battery will  have a terminal voltage of 12 V.

The electric generator, as a voltage source, uses electromagnetic means to separate and maintain opposite charges at the terminals of the generator.

Review Questions

1.      What causes electric current in terms of electric potential and electric potential difference?

 

Electric Current

Electric current (I) is defined as the rate at which charge (q) flows through a cross section of a conductor in a unit time (t) or




 

so,  



 

The units of current is coulomb/second. 1 C/s = 1 ampere (A) or amp

Describing Electric Current

In electrical circuits of metallic conductors (e.g. a wire), the charge flow is due to the motion of the loosely-attached electrons, known as conduction electrons.

In the absence of any voltage across the ends of the wire, the motion of these conduction electrons inside the conductor is random, often colliding with other electrons and the relatively-stationary positive ions of the metal, and there is no net movement in any direction.

When a voltage is applied across the ends of the wire in a circuit, an electric field is created everywhere in the circuit that is directed, along the connecting wire, from the positive terminal to the negative terminal.

Two ways of describing electric current in a circuit are:

  1. Electron current

  2. Conventional current

Electron Current describes the drift of the negative charges from the negative to the positive terminal of the battery.


Conventional current describes current as the flow of positive charges from the positive to the negative terminal of a battery. Therefore, the direction of the electron current is opposite to the direction of conventional current.

 

1. The battery (or voltage source) maintains a potential difference (voltage) across the ends of the wire.

2. This voltage establishes an electric field (E) inside the wire.

3. The electric field points in the direction of decreasing electric potential and drives (conventional) current (I) through the wire, from the positive terminal to the negative terminal.

4. The current is in the direction of decreasing potential (V), i.e. from the positive terminal at a higher potential toward the negative terminal.

5. Electron current (negative charges) flow along the circuit wire from the negatively charged terminal toward the positive terminal, opposite to the direction of the conventional current.

 
However, since the flow of negative charges from the negative terminal is mathematically equivalent to the flow of positive charges from the positive terminal, there is no significant difference which description of current is used. Nonetheless, consistent with the concept of the applying a (positive) test charge to determine the direction of the electric field, the conventional current model is universally applied.

Direct current (dc) and Alternating current (ac)

Voltage sources such as chemical batteries, solar cells, and fuel cells produce direct current (dc) in which the charges flow in one direction.

 

Image courtesy of U.S. Environmental Protection Agency

Electric utilities, household electrical circuits, and the electrical industry use the alternating current (ac), usually produced by an electric generator. The electrons in an alternating current move back and forth due to the reversing directions of the electric field inside the wire. 

 

Portable electrical generator

 


Direct Current
 


Alternating Current


Dry-Cell batteries

Flashlight batteries

Alkaline batteries

Rechargeable batteries
NiCad (nickel-cadmium) batteries
Lead-Acid automobile batteries or  "wet-cell" batteries
Solar cells

Fuel cells

 


Household electrical circuits
 Electric utilities
 Electric generators


 

Review Questions

1.      Explain the difference between electron current and conventional current?

2.      What is the difference between ac and dc currents?

3.      How does increasing voltage affect current?

 

Electric Resistance

If a voltage is applied across the ends of the object and charge flows, you can measure the current.

The resistance of an object describes the opposition to the flow of charges through the object and the amount of current depends on the electric resistance of the object. For the same voltage, greater the resistance resulted in smaller current.

Four factors affect the electric resistance of an object:

1. The inherent property of the material

Different materials have different resistance. Electrons flow through conductors in varying extent for different conductors. Electrons are slowed by the interactions with the atoms of the material resulting in varying amounts of electric current. Most conductors have smaller resistances and thus allow more current. Insulators have high resistances and block the current. Materials with low electric resistance allow large currents and are described as having a high electrical conductivity. Conversely high resistance implies smaller currents and low conductivity.

2. Temperature

Considering the random motion of the conduction electrons in the atoms of a wire, the electrons lose energy gained from the electric field to the stationary positive metal ions during collisions. With applied voltage across the wire, the increased vibration energy of these metal ions results in an increase in temperature of the wire, which causes a higher electric resistance.  Lowering the temperature of the wire can reduce the electric resistance.

3.  Length

The resistance of a conductor depends directly on its length. The longer the wire, the greater the resistance.

4. Diameter (cross-sectional area)

The resistance of a conductor depends inversely on the thickness of the wire. The thicker the wire (i.e. with a larger diameter), the smaller the resistance.

Measuring Electric Resistance

Electric resistance is the ratio of the voltage (V) applied between the ends of the object to the resulting current (I) or



 

Sometimes written V=IR.

The unit of resistance is the ohm = volt/amp. The schematic symbol for resistance (R) is shown below:

Ohm's Law (as expressed above) defines the relationship between voltage, current, and resistance. For a given circuit of constant resistance, the greater the voltage, the greater the current. If the conductor offers increased resistance, then more voltage will be required to maintain the same amount of current through the circuit.

Electrical resistance devices (i.e. resistors) that have a constant resistance that is independent of the voltage, are said to obey Ohm's law. Resistors that do not obey Ohm's law are described as non-ohmic resistors. For such non-ohmic resistors, such as the filament of a light bulb, the resistance of the filament is not constant and depends on the voltage. When the bulb is switched on, the filament heats up as current passes through it. The resistance of the filament increases with increase in temperature and the current becomes smaller. A cold filament conducts the greatest current. This explains why a light bulb usually burns out at the instant the bulb is switched on.

Resistors having high resistances are necessary to prevent too much current in electrical circuits of devices such as televisions, computers.

Review Question

1.      How is resistance related to the current in a circuit?

 

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Content provided by Mr. Martin O. Okafor, Georgia Perimeter College

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 10, 2007
Modified May 18, 2007