Equivalent Resistance

Happy Tuesday!

Make sure you grab a worksheet from the book shelf!

Plan for today

Question: How do we use Ohm’s law if there is more than one resistor?

We will…

1.

Go through these slides

2.

Calculate equivalent resistance

3.

Practice using both types of circuits

First: Which circuit is which?

Draw

Write a "P" for parallel circuit and "S" for a series circuit.

Let’s focus on the simple series circuit.

Ohm’s Law tells us how to solve
when you only have one resistor.

What about now?

Simple series circuits:

JUST ADD.

Math Response

There are three resistors in a series circuit. R1 = 5 ohms, R2 = 6 ohms and R3 = 7 ohms. What is the total resistance? Only submit the number

Type answer here

Correct Answer: 18

Deg^°

Rad

What about the not-so-simple parallel circuit?

Some rules:

1.

Substitute your values

2.

Add the fractions
a.GET A COMMON DENOMINATOR

b.Or, convert into decimals

3.

Find the reciprocal
a.Flip the fraction

b.1 / (decimal)

FIRST: Quick fraction practice

Multiple Choice

1/4 + 1/8

What is the least common denominator?

• 4

• 8

• 24

• 32

Multiple Choice

1/8 + 1/16

• 1/8

• 3/16

• 5/16

• 1/24

NEXT: Applying the Equation

1. Plug in values

2. Add using a common denominator
   

3. Flip your answer

Given: R1 = 2, R2 = 8

Unknown: total R

Equation:

Substitute:

Solve:

GUESS method

There are 2 resistors in a parallel circuit. The first has a resistance of 2 ohms and the second has a resistance of 8 ohms. What is the total resistance?

Please follow along on the board

Math Response

To make sure you're paying attention to this: what was the total resistance for the last question? Keep it as a fraction.

Type answer here

Correct Answer: \frac{8}{5}

Deg^°

Rad

Multiple Choice

In a parallel circuit, R1 = 2, R2 = 4 and R3 = 8. What is the total resistance?

(you may want to work it out on paper)

• 1/2

• 1/8

• 7/8

• 8/7

Wait…what if you have both?

This is why we use equivalent resistance…

We can combine multiple resistors into one!

What started very complicated…is now a simple series circuit.

1. Solve parallel circuits first

2. Add any remaining resistors up

Look at #3 on the worksheet

​This is a combination circuit (it has both!)

Copy down the work shown on the board.

Now look at number 5

Remember:

1. Solve any parallel circuits first

2. Add up the remaining

Practice Worksheet

Start with #1, 2 and 6

Ask yourself: Is this series or parallel?

#4 is combination circuit

1. Start with any parallel circuits first

2. THEN add up the resistance values.