7.1.3 Electric and Magnetic Forces
Phenomenon – Compass and Magnet
DAY 3 - EXPLORE MAGNETIC FORCE
Learning Objective: Some forces such as static electricity and magnetism act on objects without touching .
Learning Objective: Earth produces a magnetic force, and a compass needle is attracted by this force.
Science & Engineering Practices: Asking Questions and defining Problems
Obtaining, Evaluating, and Communicating Information
Creating a Model
Engaging in Argument from Evidence
Crosscutting Concepts:Stability and Change
Cause and Effect
Systems
Formative Assessment: Write this on the board and have students create the question on their exit ticket:
“A compass works by ______force. This force comes from Earth and acts
______(without touching.)”
Materials Needed:
2 solid (non-bendy) plastic drinking straws
60 cm length of string – cut into two pieces
3 large grapes
5 cm cellophane tapefor teacher demonstration
2 cm cubic neodymium magnet (or other large rare-earth magnet)
Ring stand or other stand to hang the straw “torsion bar” from
1 30 cm x 30 cm square of aluminum foil, crushed into aball
1 large iron nail
Compass for each group of 3-4 students
Bar magnet (or cow magnet) for each group of 3-4 students
1 Magnetism Observation Worksheet for each student
1 Earth globe
Compass app on teacher phone or project from computer onto screen
Activities:A- Have students label the date at the top of the next available page in their notebooks.
B- Ask, “We have demonstrated for the past couple days about a force that acts at a distance. What force is that?” (static electricity or electrical force) “Can anyone think of any other forces that can affect objects without having them touch? Turn to your neighbor and talk for 15 seconds about what you think. (guide students toward magnetic and gravitational forces)
Teacher Demonstration – watch this instructional video from Exploratorium before you go live with students OR show this instead of demonstrating
a- Assemble the “torsional pendulum” ahead of time:
- Tape one of the string to the middle of the straw.
- Tie or tape the other end of the string to the stand, ensuring that the straw can rotate freely
- Slide the grapes onto each end of the straw. (There will be one extra grape)
- Adjust the position of the string and grapes so the ends are balanced, but the straw does not have to be completely level.
- Repeat the procedure with the second straw and two of the aluminum foil balls.
b- Show the students the magnet and remind them of how in elementary school they sorted things according to if they were attracted by a magnet or repelled.
c- Demonstrate that the iron nail is very attracted to the magnet and will jump off the table to attach itself to the magnet. Point out that this magnetic force act at a distance – the two objects do not have to touch.
d- Conduct a similar demonstration with the grapes and with the aluminum foil balls, showing that they are not attracted by the magnet.
e- Now hang the grape torsion bar from the ring stand. Hold the neodymium magnet close to one grape (but not touching) and demonstrate how the bar slowly twists because the grape is being repelled by the magnet.
(You don’t have to explain this deep to the students, but here is the explanation for you:
f- Ask the students to explain what they just observed. Have them turn and talk with their group for 30 seconds.Accept any logical explanations – this exercise is just to get them thinking about how a magnet works, and jar their perception slightly.
g- You may want to give them a VERY brief explanation about strong magnets being attracted to the slight difference in charge of a water molecule, with hydrogens on one end and oxygen atoms on the other end. When a magnet is brought close to a grape, it makes those little water molecules spin, and this spinning makes the whole grape move!
Student Hands-On Exploration
Part 1A- Hand out a compass to each group and give each student an observation guide worksheet.
B- Instruct students to determine how the compass works by asking questions, then conducting mini-experiments to answer their questions. For example, a group of students might ask, “Does the compass have to be held flat to work? The “Cause” might be to point the compass at the ceiling, hold it horizontally, and point it at the floor. The “Effect” might be, the needle only moves when it is held horizontally. The “Inference” might be, the compass has to be able to spin in the liquid without touching the top or bottom of the case. If it touches the top of the bottom, the compass does not work.
C- Students should write down their questions, and have observations to back up their inferences. They may discuss their findings with their group. (Many students may already know how a compass works – if so, they can teach the rest of the group and then verify their knowledge with mini-experiments.)
D- Have students conduct at least 3 mini-experiments to determine how a compass works. Call on 2-3 groups to share their questions, investigations, and inferences with the class. Students should complete the first 3 rows of the worksheet with their own questions and investigations.
Part 2E- Now have students predict what will happen if a magnet is placed close to the compass. Record a few hypotheses on the board.
F- Give a bar magnet (or cow magnet) to each group and have them explore the effects of the magnet on the compass. Have students record their observations on their worksheets.
G- Ask students to attempt to explain what they see. Call on several students to share their ideas and point out that this is also a cause-and-effect relationship. What happens with the magnet is the cause and the compass behavior is the effect. Guide students toward reporting that there is a magnetic force acting on the compass needle even though the magnet is not touching the compass. Make sure that every student understands that the compass reacts to Earth’s magnetic fields and the red end of the compass needle (which is itself a magnet) is always attracted to the magnetic pole of Earth.
H- Ask, “Why do you think the compass reacted the way it did to the bar magnet? Guide the responses to the idea that the attraction from the bar magnet was stronger than the attraction from Earth.
I- Remind students that according to Newton’s 1st Law, the needle will stay at rest unless an unbalanced force comes along, so if the needle moves, there must be a new force involved – magnetic force.
J- Ask, “Did you notice that now the NORTH end of the compass needle is attracted to the NORTH end of the magnet? What the Heck?!”
K- Expand the students’ knowledge a little deeper and show this animated 5 minute video which explains some history about compasses and how magnets work.
L- After the video, explain, “Original compasses were made from a material called lodestone, magnetic stones which were first discovered in Greece. If you had a bunch of lodestones suspended in the air you'd notice something interesting. All the lodestones point the same way. The Greeks called the end of the lodestone that pointed to the north pole "north." In reality, the “north” end of the lodestone was attracted to the magnetic south pole of Earth. What we call the geographic “North Pole” is actually the south pole end of the magnet which is Earth.
M- Have students draw a model in their notes showing that the magnetic north end of Earth is actually the South Pole.
N- Hold up the globe and a compass (or use a compass app on your phone) to illustrate, “A compass points to Earth’s magnetic south pole at almost every place on Earth.”
O- Ask, “If you were in an airplane, and you flew North from here, up through Canada, then through the Arctic Circle, what would happen when you passed over the top of the “North Pole” and continued flying down the other side of Earth into Siberia? Allow students to think a moment, then ask them to respond. They should respond that the red painted arrow of the compass would point to the geographic north pole, then as soon as the pole was crossed, the needle on the compass would flip the exact opposite direction because the pole would be behind your plane.
J- Students who finish before the others may conduct a mini experiment to determine if the force is stronger or weaker or unchanged according to distance away from the compass. We will explore this more in a couple days.
K- Have students cut out the worksheet and tape it into their notebooks on today’s page.
L- Have students complete today’s exit ticket:
A compass works by __magnetic__ force. This force comes from Earth and acts _at - a - distance _ (without touching.)
Student Worksheet 7.3.3
Magnetism Observation Worksheet
Question / Cause(What will you do?)
Independent Variable / Effect
(Compass Reaction)
Dependent Variable / Inference
Does the compass have to be held horizontally to work? / Point the compass at the ceiling, hold it flat, point the compass at the floor
Does a bar magnet have more force than Earth’s North Pole?
Does magnetic force change according to how far away the magnet is?