Peoples Physics Concepts

http://www.ck12.org Chapter 13. Electric Circuits: Batteries and Resistors

13.3 Resistors in Series

• Analyze and solve problems involving circuits with resistors in series.

Students will learn how to analyze and solve problems involving circuits with resistors in series.

Key Equations

Rtotal=R 1 +R 2 +R 3 +...

Guidance

Resistors in Series:All resistors are connected end to end. There is only one river, so they all receive the same
current. But since there is a voltage drop across each resistor, they may all have different voltages across them. The
more resistors in series the more rocks in the river, so the less current that flows.

Example 1

A circuit is wired up with two resistors in series.

Both resistors are in the same ’river’, so both have the same current flowing through them. Neither resistor has a

direct connection to the power supply so neither has 20V across it. But the combined voltages across the individual

resistors add up to 20V.

Question:What is the total resistance of the circuit?

Answer:Thetotal resistance isRtotal=R 1 +R 2 = 90 Ω+ 10 Ω= 100 Ω

Question:What is the total current coming out of the power supply?

Answer:Use Ohm’s Law(V=IR)but solve for current(I=V/R).

Itotal=

Vtotal

Rtotal

=

20 V

100 Ω

= 0. 20 A

Question:How much power does the power supply dissipate?

Answer:P=IV, so the total power equals the total voltage multiplied by the total current. Thus,

Ptotal=ItotalVtotal= ( 0. 20 A)( 20 V) = 4. 0 W. So the Power Supply is outputting 4W (i.e. 4 Joules of energy per

second).