Give Your Electricity Unit a Charge

Laura Henriques

<laura.henriques@csulb.edu>

Saturday October 20, 2012 4:35pm - 5:35pm San Jose Convention Center, Room 210F

Overview of Today’s Session

Static Electricity - Transferring a charge & Inducing a charge Sticky tape experiment Balloons, electrophorus, electroscopes and other electrostatic devices PHET simulations

Current Electricity – simple circuits

Lighting a Bulb & Rules for lighting a bulb Children's ideas about how electricity works Insulators & conductors Conductivity tester Secret circuits (quiz cards)

Electricity & Magnetism

Sticky Tape Experiment

• Make two strips of tape. Fold over the top so that you have a tab or handle

• Label one piece of tape “B” and the other “T”

B

T

Sticky Tape Experiment

• Make two strips of tape. Fold over the top so that you have a tab or handle

• Label one piece of tape “B” and the other “T”

• Put the “B” tape on your table. Place the “T” tape directly on top of the “B” tape

B

T

What happens when….

• You bring a T near a B?

• You bring a T near another T?

• You bring a B near another B?

What can you infer from this?

Review…structure of the atom

• Key points to consider – Where are the electrons and where are the protons? Which are easier to move?

Pink Proton & Blue Electrons

Transfer of Charge

• Balloons – charge the balloon on our hair or sweater. What kind

of charge does it have? Let’s compare to the T & B tape charges, what does that tell us about the charge on the tape?

– PHET Simulation: John Travoltage, Balloons & Static Electricity

Induced Charges • While a transfer of charge rips electrons from one object to

another, an induced charge is a rearrangement of charge within an object. Inducing a charge does not change the overall charge of the object, just how the charges are arranged.

• Rubbing the balloon on the sweater is a transfer of electrons from sweater to balloon.

• Bringing balloon near the wall induces a charge in the wall. There is no net change to the charge of the wall but there is a section which is positive and a section which is negative.

Induced Charges (cont.)

• When a charge is induced it is the electrons which move (remember protons are permanently placed, electrons are free to move)

• Induced charges are always attractive.

• Try this… charged balloon near an empty soda can charged balloon near soap bubbles charged balloon near a stream/trickle of water

Electrophorus

Building an Inexpensive Electrophorus

+ - - - + - - - + - - + - - + -

Foam after being rubbed

(negatively charged)

Electrons taken from hair

and put onto the foam

base. More electrons than

protons are on the base

making it net negative.

+ - + - + - + - + - + - + -

Foam base before being

rubbed (Neutral charge)

Building an Inexpensive Electrophorus

• Tape a foam cup inside an aluminum pie plate (foam cup acts as insulating handle, pie plate is the conductor)

• Place pie plate onto the charged foam base.

Charging the Electrophorus

We need to touch the inside of the pie plate to take away the excess negative charges from the top of the pie pate. This makes the pie plate net postive.

Electroscopes

Building Inexpensive Electroscopes

• Foam cup

• Straw

• Paperclip & thin foil or foil coated straw & thread

• After you have built your device you will charge it with your electrophorus. What happens?

Moving from static to current electricity

• Charge the electrophorus and then hold your hand near to the dangling pith ball

Can you make a bulb light?

• Materials: battery, bulb and wire

• Challenge: using only these three items, make the bulb light

• Draw what you did and note if it made the bulb light

• Minds of Their Own video clip

Typical Student Misconceptions about Circuits

• Unipolar Model

• Clashing Currents Model

• Attenuation Model

Building Circuits

• After students created rules for making a bulb light you can challenge them to build a circuit with multiple bulbs. This leads to series and parallel circuits.

• PHET online circuit builder

Hotdog Circuits

• Relevant Topic/Unit: Electricity (series & parallel circuits)

• Materials: stripped extension cord (male end intact), skewers, block of wood, hotdogs, stopwatch (or clock with second hand) [OR wood with five nails with electric cord attached] buns and condiments optional!

• Set-up: you will create circuits where the hotdog is the resistor

• Safety Considerations: 120 volts with exposed conducting ends! I like to use a power strip to turn on/off the current. Hotdog will be very hot but the ends of the hotdog which are not part of the circuit will still be cool.

The Board & Nails Version

1-dog Circuit 2-dog Series Circuit 2-dog Parallel Circuit

Teaching Tips for Hotdog Circuits

• We are using the current to cook the hotdog. Higher current results in faster cooking time. (Power = current x voltage)

• Time how long it takes to cook a single hotdog (it is “cooked” when the skin splits)

• Predict how long it will take to cook two hotdogs in series (should be twice as long as 1 dog circuit)

• Predict how long it will take to cook two hotdogs in parallel (should be same as 1 dog circuit)

Showing the Relationship between Electricity & Magnetism

We can use an overhead projector, coils of wire and a compass as a current detector.

Overhead wrapped with coils of wire with a compass underneath allows us to see the effects of introducing a change to the magnetic field.

• Relevant Topic: Electricity & Magnetism

• Materials: Eddy Current or Lenz’ Law Kit (from $20 - $100 depending on size: any scientific retailer or even magic shops! You can also make your own.)

• Set-up: Drop the steel mass through the tube. Then drop the neodymium magnetic mass through the tube.

Lenz’s Law

• The steel mass will free fall as expected. The magnet will not. Show students that there is no magnetic attraction between the pipe and magnet.

• So why does it fall slower? The changing magnetic field due to the falling magnet generates an eddy current in the tube. This current is in the direction that its own magnetic field opposes the change (the falling of the magnet) that is creating it. (Lenz’s Law)

• Make it appear to be magic. Have a student drop the steel and then with slight of hand switch them and you drop the magnet (or vice versa).

• Drop multiple magnets. What will happen? (Even slower until the mass reaches a certain value.)

• Try aluminum tube vs. copper tube. (Aluminum is less conductive.) • Try different thickness of tubes. (Greater thickness, more current

→ slower magnet drop).

Teaching Tips for Lenz’s Law