Aren't You Just a Ray of Negativity?

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SWBAT describe the experiments that led to the discovery of the electron, with particular focus on how like charges interact.

Big Idea

Thomson's cathode ray experiments led to an early model of the atom based on oppositely charged parts.


This lesson helps fill in the background knowledge between MS PS1-1 where students must Describe the atomic composition of molecules and HS PS1-1 which expects students to Predict properties based on patterns of valence electrons.  This standard correlates directly to the Cross Cutting Concept Patterns. There is a gap of knowledge about subatomic particles between the two standards. 

In this lesson, students are still discovering the fundamental particles that make an atom.  Students have begun performing some mini-experiments at this point, and understand what it is like to work under time constraints.  Today I will introduce a stations activity and how we switch in class.  My students will have very little practical experience with electromagnetic forces, so the stations are strongly needed to build a conceptual understanding of forces to explain Thomson's results.  Students will be using the 7th Science and Engineering Practice: engage in arguments from evidence.

Materials needed for the "Playing with Charges" stations:

Station 1- Evidence of magnetic fields  Setup

  • Bar magnet
  • Clear plastic petri dish with iron filings inside.  Sealed with scotch or electrical tape to avoid potential messes.


Station 2- Interaction of magnetic fields  Setup

  • 2 bar magnets


Station 3- Transfer of electrical charge  Setup

  • Inflated balloon
  • Electroscope


Station 4- Interaction of electrical charges  Setup

  • Rabbit Fur
  • Glass rod
  • Inflated Balloon
  • Faucet with aspirator attachment


Station 5- Interaction of electrical charges  Setup

  • Two inflated balloons
  • Aluminum Can
  • Wool Fabric


Station 6- Thomson's First Cathode Ray Experiment


Station 7- Thomson's Second Cathode Ray Experiment


Both video clips are from High Voltage Fun on YouTube, edited with permission.


5 minutes

Today is a day that opens with some fun.  Students have heard the phrase "Opposites Attract", but may not have any concept of the Paula Abdul song.  I open the class asking them to pay attention to the lyrics, which is nearly impossible due to the animated wolf, while watching the original music video.

After viewing the video, I ask if any students know why "Opposites Attract" is a "natural fact" according to the lyrics.  Some students may have an idea, but their lack of prior knowledge in physics will generally make it murky at best.  Next, I explain that I have a lab to help them understand it, and ask a student from each table to come up and get a lab sheet for each person at their table.

Playing With Charges

25 minutes

Once students have copies of the lab sheet, which provides them a place to record observations and propose explanations for each station, I show them where each station is, and inform them they will have 3 minutes per station.  When they hear the whistle (I use an athletic coaches whistle, or a bicycle bell) they are to move to the next station on their paper.  I point each student table to a specific lab station to start at, and tell them to come back and find the place to record their observations and explanations for that particular station. 

While students are working, I circulate the room to encourage-- both their sense of wonder and their ideas -- and to answer procedural questions.  I keep time on my cellphone, and signal rotations.  When students finish the last station, they return to their tables and work on their explanations.

Anticipated Observations:

  • Station 1- Iron follows magnet.  Iron stands up when magnet is near.  Iron arranges itself around the end of the magnet.
  • Station 2- Opposite ends attract.  Like ends repel or push away
  • Station 3- Nothing happens when touching with finger.  Balloon causes two piece of foil to "float" or push away from each other.
  • Station 4- Stream of water moves towards the glass rod and balloon
  • Station 5- Balloons repel each other.  Each balloon attracts the aluminum can
  • Station 6- One end of the magnet attracts the beam, the other end of the magnet repels the beam.
  • Station 7- The beam pushes the paddle wheel through the tube.

Stations Debrief

20 minutes

During the time that students are working on their seventh and final stations, I place a copy of the student directions for each station on each student table.  After the station time ends, I ask students to work as a table to complete any explanations for the stations they were unable to do during the lab, and to answer the final three questions at the bottom of the sheet.  I allow them 8 minutes to complete this work.  While students are working, I cue up both videos on my teacher computer to help us debrief.

Next we discuss their answers to the final three questions.  I focus on the idea of charges being opposite, and that magnets act like charges.  Secondly, I highlight that while opposites attract; likes repel, or push away.  Finally, I push students to apply that knowledge to the idea that in Station 6's video, the magnet attracts the beam on one side and repels it with the other because the beam is charged.

After we discuss that the beam is charged -- I ask, "Is it  a beam of light?"  If they reply "yes", I use a toy paddle wheel and ask what would happen if I shine a flashlight beam on it.  They generally reply that it won't move.  I test their hypothesis, and remind them of the second video (station 7) with a replay.  By showing the beam causing the paddle wheel to move, we prove it is a stream of charged particles, and when the beam reverses, the paddle wheel reverses.

I explain how Thomson discovered what they had found-- when a negative end of a magnet came near the beam, the beam moved away; while it was attracted to the positive end of a magnet.  From this, we end the debrief with the concept that atoms must have a negative portion to their structure, which became known as electrons.