Core Unit III: Electricity
B. Current and Potential Difference

1. Current

Key Concepts

The magnitude of the charge on a single proton or electron is called the elementary charge, e.

e = 1.602 x 10^-19 where C = 1 coulomb.

The fundamental law of electric charges states that:

Opposite electric charges attract each other.
Similar electric charges repel each other.
Charged objects attract some neutral objects.

The relationship between the quantity of charge (Q) on an object and the number of elementary charges (N)is:

(Q = Ne) where (e) is the elementary charge, 1.602 X 10^-19 C

An electric circuit is a closed loop conducting path consisting of a source of electrical energy, a conductor, and a load which utilizes the electrical energy.

An electric current is the rate of flow of charge passing through a cross-sectional area in a conductor. It is considered to be a flow of positive charges.

The SI fundamental unit for current is the ampere (A). 1 A = 1 C/s

where (I) is the current, (Q) is the quantity of charge, and (t) is the time.

An ammeter is used to measure current in an electric circuit. It must be connected in series with the circuit. It must have a low internal resistance.

A galvanometer can be used to measure the direction and the magnitude of small electric currents.

Electric circuits are represented by using neatly drawn schematic diagrams. Block diagrams and pictorial diagrams are also used.

A schematic diagram is a plan or a design which represents components and their relationship to one another by symbols.

By convention, certain symbols are used on schematic diagrams to represent different elements.

Schematic diagrams show how the elements in an electric circuit are connected.

Schematic diagrams are useful for troubleshooting or repairing electric circuits. They are also used to design new circuits.

Direct current (DC) involves the continuous flow of electrons in the same direction.

Alternating current (AC) involves a periodic reversal in the direction of electron flow.

Learning Outcomes

1. Define the following terms: elementary charge, electric circuit, electric current, ammeter, schematic diagram, direct current, alternating current.

2. State the main ideas in the fundamental law of electric charges.

3. State the SI unit for electric charge.

4. Apply the relationship between the quantity of charge (Q) on an object and the number of elementary charges (N) to solve problems.

5. State the SI fundamental unit for current.

6. Solve problems relating to electric current.

7. Explain current in terms of electron flow.

8. State the name of a device that can be used to measure electric current in a circuit.

9. Identify some symbols used on schematic diagrams.

10. Draw a schematic diagram of an electric circuit.

11. Explain how schematic diagrams can be used.

12. Explain the difference between direct and alternating current.

Teaching Suggestions, Activities and Demonstrations

1. Demonstrate the correct use of an ammeter to measure the current in an electric circuit.

2. Attach a climbing arc (Jacob's Ladder) to a high voltage power supply. Observe the movement of the arc up the tube. Close the tube to produce oxides of nitrogen. Aspirate the gas from the tube and dissolve in water. Test the pH. Compare this with the pH of tap water.

   This activity shows how lightning helps to produce nitrates which stimulate plant growth. The ozone produced in the air might also be noticeable. Tie this activity to Biology.

   Other interesting demonstrations can be performed if the equipment is available. Tesla coils, Van de Graff generators, and Wimshurst machines are useful devices to have in the Physics 30 program.

3. Rub a fluorescent tube with a piece of transparent sandwich wrapping material. It should glow in the dark when it is rubbed. A fluorescent tube placed in a microwave oven will begin to glow.

   Place one of the contacts of the fluorescent tube near a Tesla coil. It will glow. Some of the students may wonder why you do not "get a shock" when you are holding the fluorescent tube close to the Tesla coil. Spectrum tubes can be used as well.

   Place a 60 W incandescent light bulb near a Tesla coil. Have students observe the discharge that takes place inside the light bulb.

4. If universal gravitation has been covered, Coulomb's Force could be introduced. Students may be able to recognize the analogy between the two.

   

   where F is the force between the two charges, q₁ and q₂, separated by the distance r.

   Coulomb's constant, (k) = 8.987 Nm²/C²