STEM OLYMPIAD QUIZ

Which vector shows the direction of the electric force on a positive sodium ion, Na⁺?

• Vector A

• Vector B

• Vector C

• There's no electric force on the ion.

Answer explanation

Direction B is correct — the ion will be accelerated in the direction of the external field.

Which vector shows the direction of the electric force on the negatively charged ion?

• Vector A

• Vector B

• Vector C

• There's no electric force on the charge.

Answer explanation

Electric field points in the direction in which the positive charges move (so negative charges move in the opposite direction).

Because of this asymmetry, the two forces will tend to separate the charges.

Suppose we take a jar of ions and suddenly place it in an external electric field. What will be the immediate reaction of all the ions in the jar?

• The positive and negative ions will separate.

• The ions will collectively move in the direction of the electric field.

• Nothing will change because the electric field doesn't penetrate the glass jar.

Answer explanation

Electric field tells us which direction positive charges move, while negative charges move in the opposite direction.

The two forces will tend to separate the charges:

What can we say about the direction of the electric field produced by this buildup of charges at the sides of the jar, compared to the direction of the external field?

• It'll be in the opposite direction.

• It'll be in the same direction.

• We can't predict the direction.

Answer explanation

Electric fields point away from positive charges, and toward negative charges. As a result, the positive charges will build up where the external electric field is pointing, and the negative charges on the opposite side.

This will produce an electric field in the opposite direction to the external field.

If electrons are free to move throughout the frame, on which surface of the frame do electrons congregate before lightning strikes?

• The bottom surface

• The top surface

• They spread out over the entire surface.

Answer explanation

The electrons move in the opposite direction as the field. Since the field is pointing upwards, the electrons move downwards through the walls of the car toward the bottom surface.

This leaves an imbalance of electrons with a positive charge on the top surface: