Forces Across a Gap on a Winter Camping Trip

Investigate electric and magnetic forces that act without touching.

Answer Key

Investigation A: the charged balloon

Read the trip log and study Table A. Then answer the questions.

Trip Log: A Dry Night at Pine Hollow Cabin

1The Ridgeline Outdoor Club spent a January weekend at a cabin in the mountains. Cold winter air holds very little water vapor, and the air inside the heated cabin was dry too. All evening, campers felt small shocks when they reached for the metal door handle, and their hair crackled and rose toward their fleece jackets as they pulled them off. Club leader Ms. Odette Brannigan said these were signs of static electricity and challenged the group to test it.

2In Investigation A, Farah Nakamura rubbed a balloon on a wool hat for a set number of strokes. She then lowered the balloon slowly over a pile of small tissue-paper bits on a table. Dmitri Valdivia held a ruler upright next to the pile and called out the height of the balloon above the table when the first bit jumped up toward it. They used the same balloon, the same hat and bits of the same size every time, and they wiped the balloon with a damp cloth between trials to remove leftover charge. They ran three trials for each number of strokes, including a set with no rubbing at all. Their results are in Table A.

3In Investigation B, the campers used a magnet from the base of a camping lantern and a small compass. With the magnet far away, the compass needle pointed north. Farah slid the magnet toward the compass from the east side and stopped at each distance shown in Table B, while Dmitri read how many degrees the needle had turned away from north. Then they held the magnet 4 cm away again, this time with a folded wool mitten between the magnet and the compass. The needle turned 45 degrees. With the mitten alone and no magnet, the needle did not turn.

4Before bed, Farah rubbed two balloons on the hat and hung each one from a thread. When the threads were held 10 cm apart, the balloons swung away from each other and never touched.

Table A: Height of Balloon When First Bit Jumped
Strokes on hat	Trial 1 (cm)	Trial 2 (cm)	Trial 3 (cm)	Average (cm)
0	0	0	0	0
5	2	2.5	1.5	2
10	3.5	4	4.5	4
20	5.5	6.5	6	6

Use the averages in Table A.

With 20 strokes, the average height when the first bit jumped was 4 cm greater than with 5 strokes.

What is the independent variable in Investigation A?

A. The air temperature inside the cabin
B. The number of strokes on the wool hat
C. The size of the tissue-paper bits
D. The height where the first bit jumped

Why was it useful to include the trials with 0 strokes?

A. They show an unrubbed balloon lifts no bits, so rubbing matters.
B. They were practice rounds, so their results can be left out.
C. They show that air currents in the cabin pulled the bits up.
D. They show that tissue paper is too heavy for a balloon to lift.

Which result from Investigation A is the best evidence that an electric field surrounds the charged balloon?

A. Paper bits jumped up to the balloon across a gap of a few centimeters.
B. The paper bits had to be pressed onto the balloon before they rose.
C. The balloon lifted bits from the greatest height after 5 strokes.
D. The balloon lifted paper bits even when it had not been rubbed.

Investigation B: the magnet and the compass

Study Table B and use the trip log to answer the questions.

Table B: Compass Needle Turn
Magnet distance (cm)	Needle turn (degrees)
2	80
4	45
6	20
8	10
10	4
12	2

Use Table B.

Moving the magnet from 2 cm to 4 cm away made the needle turn 35 degrees less.

Which statement best describes the pattern in Table B?

A. The needle turned when the magnet touched the case, not before.
B. The needle turned more as the magnet moved farther away.
C. The needle turned by about the same amount at 2 cm and 12 cm.
D. The magnet's pull on the needle got weaker as distance grew.

With a folded mitten between the magnet and the compass at 4 cm, the needle turned 45 degrees. What does this result show?

A. The mitten blocked the magnetic field, so the needle stayed still.
B. The magnetic field passed through the mitten and turned the needle.
C. The wool mitten turned the needle by itself, with no help from the magnet.
D. The mitten made the field twice as strong, so the needle turned farther.

Sort each event from the camping trip by the kind of force involved.

A charged balloon lifts a paper bit from 4 cm away.A camper's boot kicks snow off the cabin step.The lantern magnet turns the compass needle from 6 cm away.A rope tied to a sled pulls it up the trail.Two charged balloons on threads swing away from each other.

Acts across a gap (field force)	Acts by touching (contact force)
A charged balloon lifts a paper bit from 4 cm away. The lantern magnet turns the compass needle from 6 cm away. Two charged balloons on threads swing away from each other.	A camper's boot kicks snow off the cabin step. A rope tied to a sled pulls it up the trail.

The campers took one reading at each distance in Investigation B. Which change would make their results more trustworthy?

A. Use a different magnet at each distance to compare more magnets.
B. Estimate each distance by eye instead of using the ruler.
C. Repeat each distance three times and average the readings.
D. Skip the middle distances and test just 2 cm and 12 cm.

10.

Dmitri says, "A force only works when two things touch, like the rope pulling the sled." Do the trip results support his idea? Write a claim, give evidence from both investigations, and explain your reasoning.

The results do not support Dmitri's idea. In Investigation A, paper bits jumped up to the rubbed balloon from as high as 6.0 cm on average after 20 strokes, so the balloon pulled them across a gap. In Investigation B, the magnet turned the compass needle 45 degrees from 4 cm away, and it still did so with a mitten in between. The two charged balloons also pushed apart without touching. Since these objects pushed or pulled each other without contact, electric and magnetic fields must let them exert forces across a distance, and the force gets weaker as the distance grows.