Electric Circuits

Georgia Perimeter College

Objectives

  1. Demonstrate an understanding of the difference between parallel and series circuits.

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.  

This section addresses, in whole or in part, the following Benchmarks for Science Literacy:
  • Magnetic forces are very closely related to electric forces and can be thought of as different aspects of a single electromagnetic force. Moving electric charges produce magnetic forces and moving magnets produce electric forces. The interplay of electric and magnetic forces is the basis for electric motors, generators, and many other modern technologies, including the production of electromagnetic waves.

  • 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.
  • Make safe electrical connections with various plugs, sockets, and terminals.
  • Troubleshoot common mechanical and electrical systems, checking for possible causes of malfunction, and decide on that basis whether to make a change or get advice from an expert before proceeding.
     

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.
  • In some materials, such as metals, electrons flow easily, whereas in insulating materials such as glass they can hardly flow at all. Semiconducting materials have intermediate behavior. At low temperatures some materials become superconductors and offer no resistance to the flow of electrons.
  • Electricity in circuits can produce light, heat, sound, and magnetic effects. Electrical circuits require a complete loop through which an electrical current can pass.

 

Electric Circuits and Electric Power

 

Circuits from the Energy Information Administration

Electricity travels in closed loops, or circuits (from the word circle). It must have a complete path before the electrons can move. If a circuit is open, the electrons cannot flow. When we flip on a light switch, we close a circuit. The electricity flows from the electric wire through the light and back into the wire. When we flip the switch off, we open the circuit. No electricity flows to the light. When we turn a light switch on, electricity flows through a tiny wire in the bulb. The wire gets very hot. It makes the gas in the bulb glow. When the bulb burns out, the tiny wire has broken. The path through the bulb is gone. When we turn on the TV, electricity flows through wires inside the set, producing pictures and sound. Sometimes electricity runs motors—in washers or mixers. Electricity does a lot of work for us. We use it many times each day.

 

http://www.eia.doe.gov/kids/energyfacts/sources/images/LightBulbs_small.jpg

An electric circuit is a closed path along which electrons can flow, without any gaps.

All electric circuits generally have:

(1) a voltage source, such as a battery or generator,
(2) connecting wires, and
(3) an electric device (such as a light bulb or electric motor) that has resistance R.

The voltage source in the circuit provides the energy needed to move the electrons against the electric field to a higher potential. The work done by the voltage source is equal to the work done by the electrical devices in the circuit, including the energy lost to the resistance. Usually, an electric circuit includes an electric switch that is used to interrupt (when the switch is open) or complete the continuous path of electron flow (when the switch is closed). In other words, to control the electric circuit, a switch is used to cut off the energy (switch off) or to allow energy to flow (switch on).

It must be emphasized that, as a voltage source, 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.

Parts (or components) of an electric circuit can be arranged in series or in parallel.

In a series circuit, the parts of the circuit (such as the battery, a switch, and the electric device) are connected one after another (in series) in a single closed loop. You can trace the pathway that the current takes around the circuit with your finger, passing through the devices in series, as shown below. Thus, the electrons that flow in this circuit leave the negative terminal of the battery, pass through the resistance of each electric device, and return to the positive terminal of the battery.

In a parallel circuit, all the devices share a common connection to the high-potential terminal of the voltage source and also a common connection to the low-potential terminal of the voltage source. In parallel circuits, different devices are on separate "parallel" branches. Therefore, there are different paths for currents such that a break in the flow of charges in one path (as shown in the diagram below by the light bulb that is not lit due to the open switch) does not interrupt the flow along other paths.

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. The learner should answer the Guiding 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.

Series Circuits

In a series circuit, there is only one path for the same current that passes through the resistances of the electric devices (light bulbs). If one device (e.g. bulb) in series burns out, the circuit is broken and there is no other path for the flow of charges. The consequence is that the other devices no longer work.

By law of conservation of energy, the energy provided by the battery is equal to the sum of the energies transferred to the resistance of each of the devices in the circuit. Therefore, when the circuit has more than one resistance device, the total resistance (or the series equivalent resistance) is equal to the sum of the resistances of each device in the circuit. As more light bulbs (devices) are added to the series circuit, the overall resistance increases and the current decreases. So, with more bulbs in the series circuit, there is reduced current and the bulbs become dimmer.

By applying Ohm's law, the total current in the series circuit is numerically equal to the voltage supplied by the source divided by the total (series) resistance in the circuit.

The energy lost when current passes through a resistance results in a voltage drop. This drop in the voltage is larger for larger resistances. In a series circuit, the sum of the voltage drops across the resistance of each device is equal to the total voltage supplied by the source (e.g. battery). This relationship is known as Kirchhoff's voltage rule.

Review Questions

1.      What is the difference between electric potential and electric potential energy?

2.      How do electric positive and negative charges move between two points of different electric potentials?

3.      What is the electric potential difference (or voltage) between two points if 12.0 joules of energy are required to move 1.0 coulomb of charge between those two points?

4.      How much energy is needed to move electric charges between two points along an equipotential surface?

Activity/Assignments

a.      Click on this simulation: DC series circuits and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

Parallel Circuits

In a parallel circuit, at least one branch of the circuit creates different paths (or branches) for charge flow. The voltage is the same across each branch because the ends of a device in each branch are connected the same two points of the circuit. So, each branch has its own path from one terminal of the voltage source to the other terminal.

By the law of conservation of charge, the total current in a parallel circuit is equal to the sum of all the branch currents. This relationship is the Kirchhoff's current rule. Since the voltage across each branch is the same, the current in each branch of the parallel circuit depends on how much resistance is in that branch. The branch with the lowest resistance draws the most current.

Increasing the number of parallel branches provides additional paths for current. The additional current passes along the new parallel branch without altering the current in the original branches. This results in the increase of the total current because the overall  resistance (or parallel equivalent resistance) has decreased. So, the parallel equivalent resistance is smaller than the resistance of any one of the branches. With more branches to a parallel circuit, the brightness of the light bulbs does not change since the current in each branch remains the same at the same voltage.

Review Questions

1.      Can you explain what happens to the bulbs in a circuit, with three bulbs, when one of the bulbs burn out for (a) a series circuit, and (b) a parallel circuit?

Activity/Assignments

a.      Click on this simulation: DC parallel circuits and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

e.       

Measuring Electric Current and Electric Voltage

 Measuring Current

An ammeter is used for measuring current in a circuit. Since the electrons flow through any device in a circuit, an ammeter must be placed in series with the device whose current is to be measured. In order to measure the current in a series circuit with a total resistance R and the voltage source, as shown, an ammeter will be inserted between the battery and the resistor so that the current in the circuit I must pass through the ammeter.

Refer to the circuit diagram below.

With one or more resistors present, as in a complex circuit, the current through a particular resistor R can be measured by placing an ammeter directly in series with the resistor. This implies that we have to break the circuit connections at one end of the resistor in order to connect the ammeter to the resistor. Thus, the other end of the ammeter reconnects the rest of the circuit so that the same current I passes through the ammeter and that resistor.

 

Measuring Voltage

A voltmeter is used to measure voltage. Since the voltmeter measures the difference in potential across a device, from one side to the other, the voltmeter must be connected in parallel with that particular device. Refer to the diagram below.

 

With one or more resistors present, as in a complex circuit, the voltage drop across a particular resistor R can be measured by placing a voltmeter directly in parallel with that resistor. The voltmeter will be connected across both ends of the resistor, without breaking the connections in the circuit.

 

Activity/Assignments

a.      Click on this simulation: Electrical measurements with Ammeter and Voltmeter and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

 

 

 

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

Some content provided by the Energy Information Administration.

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

Page created March 10, 2007