Incandescent bulbs have a filament inside an inert atmosphere to prevent combustion. The filament consists of wire and a resistor as shown in Figure 1. When a current flows through the filament the light bulb illuminates. The brightness of the light depends on the resistance of the resistor and the current passing through it. If the filament breaks, current cannot flow across the filament and the light will not come on.

An ideal circuit is one in which resistance only exists in the components of the circuit, and not in the adjoining wires. Figure 2 shows an ideal circuit in which these light bulbs with resistors are placed in series to create a string of lights. In the circuit shown in Figure 3, the bulbs are placed in parallel to create the string. In both circuits, a battery is connected as shown and supplies a voltage to the circuit. In both circuits, R₁ = 125 Ω, R₂ = 150 Ω, R₃ = 300 Ω, and R₄ = 225 Ω.

Figure 1. Schematic of an incandescent lightbulb containing a filament resistor

Figure 2. Series circuit with 4 resistors

Figure 3. Parallel circuit with 4 resistors

An experiment is performed to determine the electric charge passing through a point in the circuit shown in Figure 2. The results are shown in Figure 4.

Figure 4. Experimental results of charge passed through circuit over time
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