Field Lines

Hook 5 min

1. Show the Opposite pair: two "m 3" discs on the gravity board with every line pointing in, and a +3 and a −3 on the electric board. "The objects are the same size. Why are the pictures different?"
2. Take ideas, then: "Charge has two signs. Mass has one. Today you find what else is the same and what's different."

Explore 17 min

1. Pairs press Single, then Halve distance and Double distance. "What happens to both readouts?" (Twice as far, a quarter of the field.)
2. Pairs press Like pair. The probe at the midpoint reads 0.00 on both boards. They press Release probe and watch.
3. Pairs turn on Arrow grid and Strength shading and find where the fields cancel. "Is there such a spot on the Opposite pair electric board?"
4. Pairs switch to Spread and drag the distance to 2, then 3. The same 36 lines cover 4, then 9 tiles. They turn on Show one over r to compare.

Make sense 10 min

1. Write both laws side by side: F = Gm₁m₂ / r² and F = kq₁q₂ / r². "What's the same? What's different?"
2. "Why does doubling the distance cut the lines per tile to a quarter, not a half?" (They spread over an area that grows as r².)
3. "The gravity board always looks like an electric board where every charge is negative. Why can gravity never push?"

Apply 13 min

1. Pairs switch to Real pairs. Compare the hydrogen atom (electric about 2.27 × 10³⁹ times stronger) with Earth and Moon (gravity wins).
2. Pairs use Double and Halve distance and notice the ratio line never changes. "Why?"
3. Pairs calculate the force between two 1 kg balls 1 m apart, each with 1 μC, and check the tool.
4. Discuss: "The electric force is about 10³⁹ times gravity in an atom, yet gravity holds the Moon in orbit. Why?" (Matter is almost neutral, so charges cancel; mass only adds up.)

Exit check 5 min

On a card: "Two charges are 2 m apart. They move to 6 m apart. What happens to the force?" (It becomes 1/9.) Then: "Name one way gravitational and electric forces are alike and one way they differ."