1. Resistors in Series or in Parallel
Unit 14.2

2. Resistors in Series
In a circuit that consists of a single bulb and a battery, the potential difference across the bulb equals the terminal voltage.

3. Resistors in Series
The total current in the circuit can be found using the equation ∆V = IR

4. Resistors in Series
What happens when a second bulb is added to such a circuit, as shown in below?

5. Resistors in Series
When moving through this circuit, charges that pass through one bulb must also move through the second bulb.

6. Resistors in Series
Because all charges in the circuit must follow the same conducting path, these bulbs are said to be connected in series.

7. Resistors in Series
Light-bulb filaments are resistors, thus the figure represents the two bulbs as resistors.

8. Resistors in Series
According to the conservation of charge, charges cannot build up or disappear at a point.

9. Resistors in Series
For this reason, the amount of charge that enters one bulb in a given time interval equals the amount of charge that exits that bulb in the same amount of time.

10. Resistors in Series
Because there is only one path for a charge to follow, the amount of charge entering and exiting the first bulb must equal the amount of charge that enters and exits the second bulb in the same time interval.

11. Resistors in Series
Because the current is the amount of charge moving past a point per unit of time, the current in the first bulb must equal the current in the second bulb.

12. Resistors in Series
This is true for any number of resistors arranged in series. When many resistors are connected in series, the current in each resistor is the same.

13. Resistors in Series
The total current in a series circuit depends on how many resistors are presented and how much resistance each offers.

14. Resistors in Series
Thus, to find the total current, first use the individual resistance values to find the total resistance of the circuit, called the equivalent resistance.

15. Resistors in Series
Then the equivalent resistance can be used to find the current.

16. Resistors in Series
Equivalent resistance equals the total of individual resistances in series Req = R1 + R2 + R3 + …

17. Resistors in Series
What happens to a series circuit when a single bulb burns out? Because the circuit is no longer closed, there is no current in it and all of the bulbs go dark.

18. Resistors in Series
Why, then, would anyone arrange resistors in series? Resistance can be placed in series with a device in order to regulate the current in that device.

19. Resistors in Series
Another advantage to placing resistors in series is that several lesser resistances can be used to add up to a single greater resistance that is unavailable.

20. Resistors in Series
Finally, in some cases, it is important to have a circuit that will have no current if any one of its component parts fails.

21. Resistors in parallel
As discussed above, when a single bulb in a series light set burns out, the entire string of lights goes dark because the circuit is no longer closed.

22. Resistors in parallel
What would happen if there were alternative pathways for the movement of charge, as shown below?

23. Resistors in parallel
A wiring arrangement that provides alternative pathways for the movement of a charge is a parallel arrangement.

24. Resistors in parallel
The bulbs of the decorative light set shown in the schematic diagram in the figure are arranged in parallel with each other.

25. Resistors in parallel
In the diagram, you will notice that in the parallel arrangement, there is more than one path for a current.

26. Resistors in parallel
So even if one bulb goes out in a parallel circuit, the other bulbs will stay on.

27. Resistors in parallel
The equivalent resistance of resistors in parallel can be calculated using a reciprocal relationship. 1 = Req R1 R2 R3