Electric Charge and Electrostatics Lecture

A set of question-and-answer flashcards covering conservation of charge, the triboelectric series, conductors, insulators, semiconductors, superconductors, charging by conduction and induction, electron transfer during rubbing, and operation of a gold-leaf electroscope.

What fundamental principle says that no electron is destroyed or created during charge transfer?

The law of conservation of charge.

According to the law of conservation of charge, what happens to the total charge of an isolated system during any process?

It remains constant; charge is merely redistributed.

What does the triboelectric series tell us about materials?

It lists materials in an order that predicts which will become positively or negatively charged, and how strongly, when rubbed together.

When human hair is rubbed against rubber, which object becomes positively charged?

Human hair becomes positively charged.

When nylon is rubbed against a PVC pipe, which object becomes negatively charged?

The PVC pipe becomes negatively charged (nylon becomes positive).

When leather is rubbed against Styrofoam, which object acquires a negative charge?

Styrofoam acquires the negative charge (leather becomes positive).

What property of metals makes them good conductors of electricity?

They contain many free electrons that can move easily through the material.

Give three common examples of electrical insulators.

Rubber, plastic, and glass (other examples include mica, paper, and air).

What is a semiconductor?

A material that acts as an insulator in pure form but conducts electricity when doped with tiny amounts of other elements.

What is a superconductor?

A material that offers practically no electrical resistance below a critical temperature.

Who discovered superconductivity, in what year, and with what material?

Heike Kamerlingh Onnes in 1911 by cooling mercury to about 4 K.

In charging by conduction, do the two objects touch each other?

Yes. The charged object makes direct contact with the neutral object.

After charging by conduction, what sign of charge does the formerly neutral object take on relative to the charging body?

It takes on the same sign of charge as the charging body.

How does charging by induction differ from charging by conduction?

Induction involves no contact; a charged body is merely brought near a neutral body, causing charge separation and, with grounding, the neutral body gains the opposite charge.

After induction and grounding, is the induced charge on the neutral object the same as or opposite to that of the inducing body?

Opposite.

When glass becomes positively charged after rubbing with felt, in which direction do electrons move?

Electrons move from the glass to the felt.

If glass gains a charge of +5 units during rubbing, what charge does the felt gain?

–5 units (equal in magnitude, opposite in sign).

Does rubbing two objects together create electric charge?

No. Charge is only transferred between the objects; total charge is conserved.

After rubbing, what type of force exists between a positively charged glass rod and the negatively charged felt?

They attract each other due to opposite charges.

When a neutral metal sphere is touched by a positively charged rod, what particles actually move and in which direction?

Electrons move from the sphere to the rod.

After contact with a positively charged rod, what charge does the sphere carry and why?

The sphere becomes positively charged because it lost electrons.

Does charging a sphere by touching it with a charged rod violate the conservation of charge?

No. The total charge of the rod–sphere system remains the same; it is just redistributed.

What instrument is commonly used to detect and identify electric charge on a body?

An electroscope (commonly a gold-leaf electroscope).

In a gold-leaf electroscope, what visible sign indicates that the electroscope is charged?

Divergence (separation) of the two gold leaves.

If a positively charged rod is brought near a positively charged electroscope, what happens to the separation of the leaves?

The leaf divergence increases because like charges repel.

If an uncharged object is brought near a charged electroscope, what generally happens to the leaf divergence?

The leaf divergence decreases, indicating partial neutralization or redistribution of charge.