2.1.11 - Electrical Circuits

Discussion: The motion of electric charges creates a current. By definition, the current is the time rate of charge passing a given point. The dimension of current is ampere= coulombs/second.

Any source of electric current is characterized in terms of the electric potential difference which it can maintain. This is often called the electromotive force or emf and it is measured in volts. When the emf is zero, there will be no current if a wire is connected across the output of the source. If the emf is non-zero there will be a current proportional to the emf. The principal characteristic of a cell or battery is that it maintains an electric potential between its terminals.

When an electric potential V appears across an object, a current may occur. Generally, the current will be proportional to the voltage. The ratio of the voltage to the current is the resistance R, R = V/I.

In this experiment, you will use flashlight lamps for resistors. When a current passes though the filament of the lamp it becomes hot. When it becomes very hot it gives off light. Thus the lamps can be used as crude indicators of electric current.

Equipment: A special board onto which three switches, three small lamps, and a battery holder have been mounted, batteries, and connecting wires.

Procedure: Before you do anything else, you should study the arrangement of components and wires and try to trace the possible paths for the current. Be able to identify all parts of the circuit board in terms of the schematic diagram above.

Part 1:Examine the battery holder and insert the dry cells in such a way that the voltage from A to D is maximum. With all switches opened so that no current can pass through them, connect points A and C together with a clip lead. Then connect points D and E together.

Part 2:Close switch S1 and observe what happens. The glowing of the lamp indicates that current is passing through it. Make a table in your notebook similar to the one shown below. Fill it in for all possible combinations of the switches. Explain all of your observations.

Part 3:Eliminate one of the dry cells by moving the connection from point D to the point F between the two batteries. Then repeat Part 2.

Questions:

1) What is a closed circuit?

2) What is an open circuit?

3) What is a short circuit?

4) A switch of the type used here is called a single pole, single throw switch. Another very common switch is the single pole, double throw switch. See the diagram below.

Show how it is possible to use two double throw switches to allow one lamp to be controlled by two switches so that the lamp goes off or on by flipping either switch. Use only one power source. (Hint: Show this by drawing the circuit diagram.)

5) What can you say about voltages and currents for two resistors connected in parallel (as R2 and R3 when S3 is closed)?

6) What can you say about voltages and currents for two resistors connected in series (as R1 and R3 when S1 and S3 are open)?