Week 7 · Unit 2 Force Fields, opener; Lesson 1 Electric Forces (A): static electricity, two kinds of charge, the electrostatic series, uses of static charge, electric field and field lines · 3 lessons
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Electric fields and the lightning rod
Science Prep 1 Term 1, Unit 2: pp. 32–38
GoalDescribe an electric field and its lines, and explain how a lightning rod and good storm habits keep us safe.
New words
electric field · the region around a charge where its electric force actsfield lines · imaginary lines that show the direction of the fieldlightning rod · a pointed metal rod joined to a plate buried in the ground
Explanation
A charge has an electric field around it. The electric field is the region where its force acts, even without touching. We cannot see it, so we draw imaginary field lines. They start on positive charges and end on negative charges, and they never cross each other. A lightning rod is a pointed metal rod on top of a building. It is joined by a thick metal wire to a metal plate buried in the ground. If lightning strikes, the rod leads the charge safely down to the earth instead of through the building. For your own safety in a thunderstorm: stay indoors, never shelter under a tree, and do not do experiments outside.
a positive charge (+)→field lines leave it and go out→they end on a negative charge (−)
Worked examples
Rule
True?
start on positive charges
yes
end on negative charges
yes
two lines can cross
no
Check these rules for field lines. They start on +, they end on −, and they never cross. If a drawing shows two lines crossing, it is wrong.
cloud→pointed rod→metal wire→plate in the ground
Follow the path of lightning on a building with a rod. The charge passes from the cloud to the rod, down the wire, and into the metal plate in the ground. The building itself is not on the path.
A thunderstorm starts while Karim is playing outside. The safe choice is to go indoors at once. A tree is a tall object, so sheltering under it is not safe.
Class activity
Draw and label. Draw a positive charge and a negative charge on a page. Draw arrows from the + to the −, following the rules for field lines. Then draw a house with a lightning rod and a wire going to a plate in the ground. Under it, write three safety rules for a thunderstorm.
Practice
1
What is an electric field?
The region around a charge where its electric force acts.
2
Where do electric field lines start and end?
They start on positive charges and end on negative charges.
3
Why is a lightning rod joined to a metal plate in the ground?
