How Things Work 10.1 Static Electricity

How does the strength of the force of charges vary with distance?

The forces between charges weaken with distance.

Electric polarization

- a distribution of electric charge that is non-uniform so that the object has a region of positive charge and a region of negative charge.

Electric charge

- an intrinsic property of matter that gives rise to electrostatic forces between charged particles. Electric charge is a conserved physical quantity. A specific charge can have a positive amount of charge (positive charge) or a negative amount (negative charge). SI unit is the coulomb.

Voltage

- electrostatic potential energy/charge

Electrostatic potential energy

- energy stored in the forces between electric charges.

Electrostatic forces

- forces between stationary electric charges. The force experienced by a charge particle in the presence of other charged particles.

Neutral

- having zero electric charge.

Net electric charge

- the sum of all charges on an object (+ and -). Positive charges increase net charge and negative charges decrease it. Net charge can be negative.

Suppose that you had an electrically charged stick. If you divided the stick in half, each half would have half the original charge. If you split each of these halves, each piece would have a quarter of the original charge. Can you keep on dividing the charge in this manner forever? A: No, because charge is quantized. B: No, because charges are destroyed each time the stick is broken. C: Yes, because there is an infinite amount of charge. D: Yes, because charges are created each time the stick is broken.

A: No, because charge is quantized.

A charged sock can stick to the wall because A: a charge can polarize a neutral object. B: like charges repel. C: like charges attract. D: opposite charges attract.

A: a charge can polarize a neutral object.

You pick up two identical boxes of candy and slide off their plastic wrappers. When you hold those two wrappers near one another, they A: repel because they have like charges. B: attract because they have like charges. C: repel because they have opposite charges. D: attract because they have opposite charges.

A: repel because they have like charges. Explanation: Since you removed identical wrappers from identical boxes, both wrappers must end up with the same type of charge, either positive or negative. Like charges repel, so those two wrappers will repel one another.

Law of Conservation of Charge

Charges within a closed system may be transferred from one object to another, but charge is neither created nor destroyed.

Sparks occur when there is A: a large electric potential energy. B: a large voltage. C: a large charge. D: a lot of electrons.

B: a large voltage.

If you put electric charge in a metal pot that is suspended in the air, where will that charge become located? A: on the inside of the pot B: on the outside of the pot C: both the inside and outside of the pot

B: on the outside of the pot

You touch a charged plastic wrapper to a glass window that was electrically neutral. The total charge on the wrapper and the window is now A: almost twice the charge that was on the wrapper. B: more than twice the charge that was on the wrapper. C: equal to the charge that was on the wrapper. D: twice the charge that was on the wrapper.

C: equal to the charge that was on the wrapper. Explanation: Electric charge is a conserved quantity so the total charge of an electrically isolated system can't change. The wrapper and window are an electrically isolated system, so their total charge is the same before and after they touch.

You determine that a charged plastic wrapper has a negative charge, so it must have A: more atoms than electrons. B: more atoms than protons. C: more electrons than protons. D: more protons than electrons.

C: more electrons than protons. Explanation: The charged constituents of the wrapper are its electrons and protons. Since each electron has -1 elementary unit of electric charge and each proton has +1 elementary unit of electric charge, the only way the wrapper can have a net negative charge is to have more electrons than protons.

When the charged plastic wrapper touches the originally neutral glass window, the wrapper begins to cling to that window. The two objects attract one another because A: the window acquires an opposite charge and attracts the charged wrapper. B: the window acquires a like charge and attracts the charged wrapper. C: the wrapper's charge electrically polarizes the window and the polarized window attracts the charged wrapper. D: the wrapper loses its electric charge to the window and the charged window attracts the electrically neutral wrapper.

C: the wrapper's charge electrically polarizes the window and the polarized window attracts the charged wrapper. Explanation: The glass contains countless positive and negative charges, which shift in response to the wrapper's charge. The wrapper pulls the window's opposite charges slightly closer to it and pushes the window's like charges slightly farther away from it. The result of the window's polarization is a net attraction between the wrapper and the window.

You begin moving the negatively charged wrapper toward the candy box from which you removed it. As you move them closer together, the electric field midway between the wrapper and the box... A: reverses direction B: remains unchanged C: becomes weaker D: becomes stronger

D: becomes stronger Explanation: As the distance separating the negatively charged wrapper and the positively charged box decreases, a positive test charge place between them would experience an increasing electrostatic force. Since the electric field is proportional to that electrostatic force, the electric field grows stronger as the distance between the objects decreases.

As you move the two charged plastic wrappers closer together and the distance between them decreases, the forces they exert on one another A: decrease and are proportional to distance2 (distance squared). B: decrease and are proportional to distance. C: increase and are proportional to 1/distance. D: increase and are proportional to 1/distance2 (1 divided by distance squared).

D: increase and are proportional to 1/distance2 (1 divided by distance squared). Explanation: As recognized by Coulomb's law, the forces between two charges obey an "inverse square law" -- the forces are inversely proportional to the square of the distance separating those two charges. As you move the charged wrappers closer together, their separation decreases and the forces they exert on one another increase.

Protons/Neutrons/Electrons/Nucleus -

Protons - positively charged subatomic particles found in atomic nuclei. Neutrons - the electrically neutral subatomic particles that, together with protons, make up atomic nuclei Electrons - tiny negatively charged particles that make up the outer portions of atoms and that are the main carriers of electricity and heat in animals. Nucleus - Positively charged central component of an atom, containing most of the atoms mass and about which the electrons are arranged.

To charge the electroscope negatively by induction

a positively charged rod (lucite rubbed with silk) is brought near one side of the ball on top of the electroscope. At the same time, touch the ball on the opposite side from the charged rod. The positively charged rod causes a slight polarization on the ball, with electrons being attracted to the rod, leaving more positive charges than negative on the side of the ball opposite the rod. When you touch the ball, electrons flow from your finger to the ball, giving the electroscope a net negative charge. When the charged rod is removed from near the ball, the leaves repel.

How atoms become charged

atoms become charged when they become more positive or more negative by adding or removing electrons

To charge the electroscope by conduction negatively

the end of a charged ebonite rod is touched to the ball on top of the electroscope and electrons flow from the rod to the ball and the foil leaves, leaving a net negative charge. Because the leaves have like charges on them, they repel from each other.

To charge the electroscope positively by conduction

the lucite or glass rod is rubbed with silk and touched to the ball on the electroscope. This time electrons flow out of the electroscope onto the rod leaving a net positive charge on the electroscope foil leaves.