Nov. 14 – Calculating Resistance

Here's a fun video that shows how you can create circuits and resistors using pencil and paper!

Videos:

• Paper circuit
  http://www.youtube.com/watch?v=BwKQ9Idq9FM
  
• Listening to variable resistance
  http://www.youtube.com/watch?v=LsvqEN3XVX4

Next we talked about how resistance is calculated.

Ohm's Law

Learning Goals: Understand Ohm's Law.

Success Criteria: You can solve problems involving Ohm's Law.

The total resistance in a circuit will affect the current.

• If the resistance increases, the current decreases.
• This is called Ohm's Law.

In equations, this is written as,

Here are a couple of examples:

Example

If a light bulb has 3.0 V of potential difference across it and a current of 4.0 A going through it.  What is its resistance?

G: ∆V = 3.0 V, I = 4.0 A

R: What is the resistance, R?

A: R = V / I

S: R = 3.0V / 4.0 A

        = 0.75 Ω

S: The light bulb has a resistance of 0.75 Ω.

Example

An average human body has a resistance of 1875 Ω from one hand to the other if the skin is dry.  If this person touches a 100 V power supply, how much current goes through him?

G: R = 1875 Ω, ∆V = 100 V

R: I = ?

A: I = V / R

S: I = 100 V / 1875 Ω

      = 0.053 A

S: A current of 53 mA will go through him.  He will be immobilized!

Next we did a review of graphing and calculating slope.  Tomorrow, you will be doing a lab where you measure the current and potential difference across different resistors.

Handouts: 

• Graphing Ohm's Law Practice
• Ohm's Law Lab

Homework:

• Complete the graphing practice.
• Read the Ohm's Law Lab for tomorrow.