Jeopardy Review

Here's all the questions and answers from today's Jeopardy game!

• Jeopardy Review

Conference for Girls in Science

Hi Everyone,

Here's a link to a great opportunity for girls who are interested in science.

• INSPIRING FUTURE WOMEN IN SCIENCE CONFERENCE

Jan. 19 – Electricity Mini-Test Solutions

Here are the solutions to the recent test.  Remember to use all your tests as study materials for the final exam.

• Electricity Mini-Test Solutions Period 2
• Electricity Mini-Test Solutions Period 5

Exam Review

Congratulations on completing the final test!  We have covered all the content for this course now.

Here's how the rest of the term looks:

• Monday, Jan. 19: Speaker project due
• Monday, Jan. 26: Exam for Period 2
• Thursday, Jan. 29: Exam for Period 5

Here are some example review material:

• Review Topics and Textbook Questions
• Extra Review Questions

Here's more information about the exam:

• Exam Outline
• Formula Sheet 1
• Formula Sheet 2

You should also bring your regular formula sheet that we have been using throughout the term.

Good luck in your studies!

Jan. 12 – Transformers

Watch this:

As you're watching it, remember that the bulbs are NOT plugged into anything!  The current through them is completely induced by the giant coil under them.

Formative assessment: can you predict the direction of the current through this wire if it is moving up through the magnetic field?

Answer: use the right hand rule to find the current goes into the page.

The final topic for this unit and the term is transformers!

Transformers!

Transfer electric energy from one circuit into another.

 - Use Faraday’s Law of Induction

 - Consists of an iron core and two coils of wire from different circuits.

 - A changing current (AC) in on coil creates a changing magnetic field in the core.  This induces a current in the second coil.

This can be used to increase or decrease the potential difference in the second coil “transforming the voltage.”

step-up transformer increases voltage,  V2 > V1

step-down transformer decreases voltage, V2 < V1

Energy cannot be created or destroyed.  

Therefore power is constant.

        P1 = P2
   I1 V1 = I2 V2
V1 / V2 = I2 / I1

where the 1 and 2 refer to the first or second loop.

The number of coils determines what V2 will be.

Let N1 and N2 be the number of coils on each side.

N1 / N2 = V1 / V2

How is this useful?

Ex) A power plant produces 4.0 MW of power at 2.2 x 10^4 V and is transmitted over 950 km.

a) How much power is loss in the transmission wire if the resistance along the power line is 0.054 Ω / km.

Answer:

I = P/V = 182 A

Power loss = I² R

                   = (182 A)²(0.054 Ω/km) (950 km)

                   = 1.70 x 10^6 W

b) If the plant steps-up the voltage to 500 kV, how much power is lost?

Answer:

must solve for I2….  

I2 = (V1 / V2)*I1

I2 = 8.01 A

Power loss = I² R

                   = (8.01 A)²(0.054 Ω/km) (950 km)

                   = 3.29 x10^3 W

Less power is loss if the voltage is stepped up.

Long range power lines have high V and small I.

Once it reaches the consumer, another transformer steps-down the potential to make it useful (120 V).

That's it!

Homework

• Continue working on homework from the Unit Outline.
• Mini-test is Thursday, Jan. 15.
• Continue working on our speaker project (Due Monday, Jan. 19).

Jan. 9 – DC Motors and Induction

Let's finish the question from yesterday.

Ex: Two parallel wires carry a current.

a) If the currents are in the same direction…

     wires attract!

b) If the currents are in opposite directions…

     wires repel!

Electric DC Motors

- consists of a loop of wire in a magnetic field.

- when current flows through the loop, a magnetic force rotates the loop.

- a brush connector allows current to travel in the same direction at all times and freely rotate.

How do you increase the force of rotation?

  - stronger current, I

  - longer length, L

  - stronger field, B

Easiest is to increase L by coiling.

Two-pole motor replaces the loop with a solenoid and uses opposing magnetic fields to generate more force.

  - this force is not constant.

Example of two pole motor.

To have more consistent force, add more poles.  ie. three-pole motor.

This is the small motor that is commonly used in toy cars.

Here's what the above motor looks like inside.  You can clearly see the three poles.

INDUCTION

Discovered by Michael Faraday.

Faraday’s Law of electromagnetic induction:

A changing magnetic field in the region of a closed-loop conductor will induce an electric current.

- When a wire moves through a magnetic field, a current can be induced.

- There is a potential difference across the wire.

- This potential difference is called an EMF (electromotive force).

- EMF is not a force!  It is a potential difference measured in Volts.

We can use this principle to generate electricity.

  - a motor turns electric energy into kinetic.

  - a generator turns kinetic energy into electric

Handout

• Induction Examples

Lenz’s Law

Gives the direction of an induced current.  

An induced current and emf are in a such a direction as to oppose the change producing them.

Here are some examples.  Try to figure out which way the current flows before scrolling down to see the answers.

Solutions:

Jan 8 – Motor Principle

Watch this:

Yesterday we started talking about solenoids.  Today we continued with a few more notes:

How do you increase the strength of a magnetic field in a solenoid?

  - increase the current

  - increase the number of loops

  - decrease the diameter of the loops

  - changing the core of the solenoid 

    (put in a ferromagnetic material)

Applications of Solenoids

  - speakers

  - bells

  - hoists

  - locks

  - MRI

Here's how powerful an MRI's magnetic field can get:

Motor Principle

When a current-carrying conductor is in an external (perpendicular) magnetic field, it experiences a perpendicular force.

This force is given by the right hand rule.

  - fingers point in direction of B.

  - thumb points in the direction of I.

  - palm points in the direction of F.

Here are two views of the right-hand rule.

Here's what it might look like in real life:

The magnitude of this force is:

F  =  BIL

B = magnitude of magnetic field (Teslas)

I = current (Amperes)

L = length of conductor (meters)

F = force (Newtons)

Using this principle we can build motors!

Here's the simplest motors you can build:

Challenge: Two parallel wires carry current out of the page.  Which direction is the force?

I will give you the answer tomorrow!

Practice: Find the direction of the force.

Answers:

Homework:

Keep up with the Unit Outline and try the back of the sheet I handed out yesterday.

• Motor Principle Problems

Jan. 7 – Magnetism

Here's the schedule for the remainder of the term:

• Last Lesson: Monday, Jan. 12
• Mini Test: Thursday, Jan. 15
• ISP Due: Monday, Jan. 19
• Exam: Monday, Jan. 26

Magnetism

Magnetism is an intrinsic property of some particles, like charge or mass.

Magnets exert forces on other magnets.

Like poles repel  N-N  or S-S

Unlike poles attract   N-S  or S-N

Permanent Magnets

Have a North and South pole.

We draw magnetic field lines to show the direction that a compass would point: away from the N and towards S.

 - field lines never cross.

 - lines get further apart as they leave the magnet.

Poles only exist in pairs.  No such thing as a “monopole”.  If you break a magnet in half, you get two magnets.

The Earth has a magnetic field that protects us from charged particles coming from the sun.

When these particles hit the Earth's atmosphere (at the poles) they create auroras.

Magnetic fields are measured in Teslas.

Ferromagnetic Materials

Some materials, like iron, are made up of magnetic domains.  Many domains point in different directions cancel each other out.

An external magnetic field can line up the domains and make the material behave like a permanent magnet.  (temperature also affects domains)

Electromagnets

When current passes through a conductor, magnetic fields form.  We use the right-hand-rule to predict the direction of the field.

Ex: Current on a wire.

Thumb points in direction of I.

Fingers show direction of B (magnetic field).

At this point, it is useful to use these symbols to draw vectors in 3D:

Ex: Wires coiled into loops can create a magnetic field that looks exactly like a permanent bar magnet.  This is called a solenoid.

Right-hand-rule for solenoids.

 - wrap your fingers in the direction of the current.  Your thumb will point to the N.

Electromagnets are useful for many applications.

Handout

• Right hand rule for solenoids

Jan. 5 – Power in Circuits

Welcome back from the break!

Remember, there's a quiz tomorrow!

Here's the new notes from today:

And here are the solutions to the review worksheet:

• Solutions to Circuits Review