Core Unit III: Electricity
B. Current and Potential Difference

2. Electric Potential Difference

Key Concepts

Electric lines of force represent the direction that a positive test charge would move in an electric field. By convention, they originate at positively charged objects and terminate at negatively charged objects.

A charge in an electric field experiences an electric force.

Work is done by the electric field if the electric force acting on the charge causes it to move from one point to another. These two points differ in their electric potential.

The magnitude of the work done on the charge by the electric field is a measure of the difference in potential.

The electric potential difference (V) is the work done per unit charge as a charge is moved be- tween two points in an electric field.

The volt (V) is the unit used to measure electric potential difference. (Students frequently confuse the symbol V used to express the unit (volt) with the term used in the expression for electric potential difference. It should be emphasized that sometimes the same symbol can be used in different ways to express different things. Other examples can be used to illustrate this.)

When solving problems, students should be able to develop any of the following relationships. (Other possibilities exist, but these are some of the more commonly used ones.)

   1 volt = 1 joule/coulomb

   1 volt = 1 (newton ^. metre)/coulomb

   1 volt = 1 joule/(ampere ^. second)

   1 volt = 1 (newton ^. metre)/(ampere ^. second)

   1 volt = 1 (kg ^. m²/s²)/(A ^. s)

An electric potential difference must exist for current to flow in an electric circuit.

As charge moves from one point to another in an electric circuit, energy is released. This results in a decrease in electric potential. The decrease in electric potential implies that there is an electric potential difference between the two points. (The term electric potential difference is preferable to "voltage.")

The potential of the earth is arbitrarily said to be zero.

An object connected directly to the ground can be described as being earthed.

A ground may be a common plane of zero voltage compared with the rest of the circuit. This is sometimes called a chassis ground.

The potential at any point in an electric field can be either positive or negative with respect to the earth, depending on the nature of the charge.

A voltmeter is used to measure the electric potential difference between two points in an electric circuit. It is connected in parallel across a load in the circuit. It has a very high internal resistance.

In a direct current circuit, both an ammeter and a voltmeter must be connected so that a given terminal on the meter must be able to be traced back to the same terminal on the source of the electric potential difference.

On multi-range meters, the highest setting should be used first, and lower settings should be used progressively. (Newer electronic meters use slightly different methods.) Connecting the meters incorrectly could damage them.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: electric field, positive test charge, electric lines of force, chassis ground, electric potential difference.

2. State the convention used to represent electric lines of force in an electric field.

3. Explain what happens to a charge in an electric field.

4. Explain that work is done on a charge in an electric field if the electric force causes the charge to move from one point to another.

5. Explain that the magnitude of the work done in moving the charge is a measure of the difference in potential between two points.

6. State and apply correctly the units used to measure electric potential difference.

7. Recognize that some symbols used in physics can occasionally represent different things, when they are used in different contexts.

8. Solve problems involving electric potential difference.

9. Illustrate relationships that exist between various fundamental and derived units in physics.

10. Recognize that a decrease in energy when a charge moves through a circuit results in a drop in electric potential.

11. Explain that there is a zero potential difference between a ground and the rest of the circuit.

12. Distinguish between a positive and a negative electric potential difference.

13. Identify the instrument which is used to measure electric potential difference in an electric circuit.

14. Show the correct method for connecting a voltmeter in an electric circuit.

15. Explain how the terminals of both a voltmeter and an ammeter must be connected in an electric circuit.

16. Develop different relationships when solving problems relating to electric potential difference.

Teaching Suggestions, Activities and Demonstrations

1. Demonstrate that current will not flow in an electric circuit unless an electric potential difference exists.

2. Design and perform an activity to investigate electric potential difference.

3. Demonstrate the proper care and use of equipment. Also demonstrate correct safety procedures when working with electrical equipment. Emphasize the proper safety precautions that need to be taken into consideration when working with electricity.

4. In Europe A.C. electrical energy is provided at 240 V and 50 Hz. Have students research the reasons why a different standard exists in Europe from the one currently available in North America. What are some of the pros and cons of the European system? What types of problems are encountered by travellers when they go to places which use different electrical standards? What kinds of things need to be done if one were taking electrical equipment to Europe?

5. Currently, research is being undertaken to determine if any adverse health effects result from living close to high voltage electrical transmission lines. Search the available literature to determine what conclusions scientists have been able to reach on this issue.

6. An electrochemical cell can be used to illustrate that chemical energy can be converted into electrical energy.

   Set up an electrochemical cell. Tie this activity in to the Oxidation-Reduction unit in Chemistry

   (For information on some chemicals and electrodes to use, and how to connect the cells, consult a chemistry text. One method is to attach a piece of copper wire to one end of the flashbulb, and a strip of magnesium ribbon to the other end. Hold the flashbulb with a pair of tongs and insert the metals into 3 M HCl. Connect the electrodes to a flashbulb with copper wire. When the electrodes make contact with the chemicals, the flashbulb ignites.)

   Having established that electricity flows through this circuit, remove the used flashbulb and connect a voltmeter and ammeter. Take measurements. See if the voltage or current flow depend on the surface area of the electrodes making contact with the chemical solutions, or on the concentration or temperature of the solutions. Investigate this qualitatively.