Explain the task to each other again in your own words.

State what you understood the task to be and what is still unclear to you.

We’re supposed to find out where the force that deflects the stream of water comes from and how it works.

The starting point for the deflection is the rubbed plastic ruler.

Recall what you have learned about this in physics class.

When you rub a plastic object with a cloth, charge separation occurs.

The electrostatically charged ruler generates an electric field around itself.

You’ve already learned that water molecules have dipolar properties.

What’s the first thing that happens when a dipole enters an electric field?

An electric dipole aligns itself within a field so that the opposite charge lies closer to the center of charge.

Not all water molecules are aligned in this way, but a large percentage are.

Think about what forces occur between the aligned dipoles and the rubbed ruler.

Make a sketch and assume that the ruler is positively charged.

There are two forces between the electrostatically charged ruler and the water dipoles

  • attraction (opposite charges)
and
  • repulsion (like charges).

If both attractive and repelling forces are at work between the ruler and the water molecules, how is it that the water is deflected?

Recall what you know about forces between different charges and what they are contingent on!

Deflection can occur only if the attractive forces are greater than the repelling forces.

The negative end of the dipole is a little farther away from the negatively charged ruler than is the positive end.

  • The shorter the distance between two charges, the stronger the acting force.
  • The longer the distance, the weaker the acting force.
This applies equally to attractive and repelling forces.

Now you have all the information you need and you can answer the question of why the stream of water is deflected.

Explain your answer using the sketch.

The positively charged ruler generates an electric field. Some of the water dipoles align themselves in this field.

The negatively charged ends are closer to the positively charged ruler than are the positively charged ends, and the forces between the charges become stronger as the distance decreases.

The result: The attraction outweighs the repulsion and the stream of water is deflected toward the ruler.

You can watch the experiment in the video.