Dec. 19 – Circuits

Last time we talked about Ohm's Law, V = I R.  Which one of these graphs best represents Ohm's Law?

The answer is (a).  If you graph voltage vs. current, and you get a linear graph, that means Ohm's law is obeyed.

Ohmic - the material obeys Ohm’s Law (V vs I graph is linear).

Non-Ohmic - the material does not obey Ohm’s Law (V vs. I graph is curved).

We will always assume Ohmic resistors.

Circuits

Circuits are closed paths for charges to flow.

An analogy of circuits as a roller coaster ride.

Here's what circuits look like in real life.  Every line on the board is a wire.

Conventional current flows from + to –.  Electron flow is opposite ( – to + ).

AC - Alternating Current, direction constantly changes, no + or –.

DC - Direct current, charges only move in one direction.

In this course, we will only be working with DC circuits.

Types of Circuits

Series

 - components are connected with one path

 - total resistance is the SUM of each resistance.

• The current is the same for all resistors connected in series.

Parallel

 - components are connected with multiple paths for charges to flow.

 - total resistance is the INVERSE SUM 

• The voltage is the same for all resistors connected in parallel.

Using these rules, we can replace a set of resistors with an equivalent resistor

 Mixed Circuits

 - contains combinations of components in series or parallel

 - total resistance must be calculated one section at a time

Kirchhoff's Laws

Kirchhoff's Current Law

Also known as the junction rule.

 - All current entering a location must exit that location.

 - Current in equals Current out.

            I in   =  I out

Kirchhoff’s Voltage Law

Also known as the loop rule.

 - The potential difference around a loop must be zero.

           V1 + V2 + V3 + … = 0 

Handout

Here are some challenging circuit questions that are more difficult than the ones in your text book.

• Complex Circuits Practice

Dec. 18 – Speaker Project

Today we assigned the ISP!  You and your group will have to build a speaker!

Handout

• Speaker Project Marking Scheme

Here's a video to help you get started.  You do not need to follow this video!  Use whatever resources you want and whatever design works for you and the materials you have.

Dec. 17 – Unit 5: Electricity and Magnetism

Schedule for the next few classes...

• Today: New Unit
• Tomorrow: ISP assigned, work period, bring laptops/tablets.
• Friday: Lab Due.
• Jan. 5: Welcome back.
• Jan. 6: Quiz on Circuits

Unit 5: Electricity and Magnetism

What is charge?

  - can be positive, negative, or neutral.

  - a source of electric field (force) and also reacts to that force. (ie. charges exert forces on each other).

  - the smallest charge is 1 electron (or proton) = 1.602 x 10 ^-19 C

  - C is Coulombs, units of charge.

  - Unlike gravity, charges attract or repel.

Electric Potential

Electric forces can do work.  Therefore, there is a form of electric energy.  We call it electric potential energy.

The electric potential energy per Coulomb is called “voltage”.  Also called electric potential, or potential difference.

V = ∆E/q

Units are "volts", 1 V = 1 J / 1 C

Like gravitational potential, zero can be at any point, only differences matter.  Often written as ∆V.

Current

The movement of charges.

I = Q/∆t

Units are "amperes", 1 A = 1 C / 1 s

Ohm’s Law

Potential difference makes charge flow, but the flow is restricted by resistance.

Example: 

A flashlight requires 4 AA batteries (1.5 V each).  If the bulb has 500 Ω of resistance, what current goes through the bulb?

     I = V ÷ R  = (1.5 A x 4) ÷ 500 Ω = 0.012 A = 12 mA

b) If the same bulb is plugged into a wall socket (120 V), what current is drawn?

     I = V ÷ R  = 120 V ÷ 500 Ω = 0.24 A = 240 mA

Most houses have 15 A “breakers” for safety.

Example:

A TASER can deliver up to 50 000 V through the air.  Once it contacts you, the voltage drops to 1200 V.  A wall socket provides 120 V.  Which is more dangerous?

The resistance of a human depends on the point of entry, moisture, tissue, etc.

Dry skin, R = 100 000 Ω

Broken skin, R = 1000 Ω

Use broken skin and a wall socket.

I = V/R              I = 120 mA

Taser.

I = V/R             I = 1.2 A

Looks like a Taser is more dangerous… why doesn’t it kill you?  Remember, current is charge/second, if you have limited charge, you have limited current.

A taser has limited charge in the battery.

Taser claims to deliver 0.002 A up to 0.03 A.

Wall socket has unlimited supply of charge.

Handout

• Unit Outline

Dec. 15 – Review and Study Period

Hi Everyone,

I hope your studying for your test is going well.  Many people have asked me for extra questions, so here are a few to tie you over until the test tomorrow:

• Standing Wave Problems

Make sure to get some rest before the test as well.

Good luck!

Dec. 12 – Thinking Question

Here's the thinking question from last year:

• Thinking Question
• Thinking Question Solutions

Good luck in your studying!

Dec. 11 – Lab

Today we did the lab!  Remember, the lab is due on Dec. 19.

Here are some additional materials you might find useful, solutions to former worksheets.

• Superposition Exercises Solutions
• Formative Quiz Solutions

Tomorrow we will do the previous thinking question and a work period.

Dec. 10 – Doppler Shift, Lab and Quiz

Today we played with an Airzooka!

Then we finished up the Doppler shift and discussed the next few days.

Here are the notes:

Handouts

• Speed of Sound Lab
• Formative Quiz

You should now be able to do all the textbook homework from the Unit Outline.

Reminder: the unit test is next Tuesday!