Charges that push and pull by squares

Science · Energy · ages 16-17

Name ______________________ Date ____________

Coulomb's law says force falls as one over distance squared.
Circle one: True False
Two positive charges face each other. Which way do they push?
- Apart, along the joining line
- Together, along the joining line
- At right angles to the joining line
What is the electric field at a point?
- Charge per volt on the plates
- Work per kilogram from infinity
- Force per coulomb on a small positive test charge
A point charge Q is negative. Which way does its field point?
- Away from Q
- Toward Q
- Around Q in circles
Two 1 microcoulomb charges sit 1 m apart. Take k as 9 times 10 to the 9. F equals k q1 q2 over r squared. Give F in newtons.
Answer: ______________
Compared with gravity, how big is the electric force between the electron and proton in hydrogen?
- Far greater
- Slightly smaller
- Exactly equal
A pupil draws field lines crossing between two charges. What is wrong?
- Crossing lines are allowed near conductors
- Lines can only start on negative charge
- Field lines never cross
The electric and gravitational forces between two protons are compared. What does the ratio show?
- Gravity dwarfs electricity
- They are about equal
- Electricity dwarfs gravity

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Doubling r quarters the force, the inverse square signature.
Apart, along the joining line · Like signs repel along the line that joins them.
Force per coulomb on a small positive test charge · E describes what a positive test charge would feel there.
Toward Q · Fields point toward negative Q and away from positive Q.
0.009 · 9 times 10 to the 9 times 10 to the minus 12 gives 0.009 N.
Far greater · At atomic scales the electric pull dwarfs the gravitational one.
Field lines never cross · One point cannot have two force directions, so lines never cross.
Electricity dwarfs gravity · Charge effects outweigh mass effects by an enormous factor.