Water that comes from the tap is a good conductor of electricity. In the chapter preview, you can see a photograph that shows a stream of water being attracted to a charged comb.
a. Explain why the water stream is attracted to the comb.
b. Water droplets that break off from the stream are strongly charged. If the comb has a positive charge, what is the charge of the droplets?
The correct answer and explanation is :
a. Why the water stream is attracted to the comb:
Water is a good conductor of electricity because it contains dissolved ions, such as salts, minerals, and impurities, which allow electrical charges to move freely through it. When a charged object, like the comb in the photograph, is placed near a stream of water, it influences the water due to electrostatic forces.
The comb’s charge creates an electric field that polarizes the water molecules. Water molecules are polar, meaning they have a positively charged end (hydrogen atoms) and a negatively charged end (oxygen atom). When the charged comb is brought near the water stream, the negative or positive charges of the comb exert an attractive or repulsive force on the polar water molecules.
If the comb is negatively charged, it will attract the positive ends of the water molecules (hydrogen), and if the comb is positively charged, it will attract the negative ends of the water molecules (oxygen). As a result, the water stream bends towards the comb, visibly demonstrating the interaction between the charged comb and the water’s polar molecules.
Additionally, because water is conductive, charges can move through the water stream, creating a weak electric current that enhances the attraction between the stream and the comb.
b. What is the charge of the droplets if the comb has a positive charge?
If the comb has a positive charge, the droplets of water that break off from the stream will carry a negative charge.
Here’s why: When the charged comb induces polarization in the water molecules, the negative part (oxygen) of the water molecules will be attracted to the positive comb, and the positive part (hydrogen) will be repelled. As the stream of water breaks into droplets, the surface of each droplet tends to lose the positive charge from the water molecules, and the excess negative charge accumulates. Therefore, the droplets that break off from the stream carry a negative charge.
This phenomenon is similar to how charges are transferred in other situations involving conductive materials; the charged comb induces an imbalance of charge, resulting in droplets with an opposite charge to that of the comb. Hence, if the comb is positively charged, the water droplets will be negatively charged.