18.1 Static Electricity and Charge: Conservation of Charge

Two socks are pulled out of the dryer and experience static cling, i.e. they are pulled together by the electric force. Which one of the following would cause this electric force? A. One sock is positively charged and the other is negatively charged. B. Both socks are negatively charged. C. Both socks are positively charged.

A, energy requirement to remove electrons to another atom is less than proton.

Describe the two phenomena observed when a neutral object (made of e- and p+) has an attractive electric force to a charged obj due to polarization. (Charging by Induction)

a charge is created in a nearby object, without direct contact. When the neutral obj is either a Insulator or Conductor Conductor: Charged obj(+) will attract e- from the neutral obj creating an attractive e-force Insulator

If a suspended object A is attracted to object B, can we conclude that object A is charged? A. Yes B. No

B, Object A might have a charge opposite in sign to that of B, but it also might be a neutral conductor. In this latter case, object B causes object A to be polarized, pulling charge of one sign at the near face of A and pushing an equal amount of charge of the opposite sign to the far face, as in Figure CQ15.3. Then the force of attraction exerted by B on the induced charge on the near side of A is slightly larger than the force of repulsion exerted by B on the induced charge on the far side of A. Therefore, the net force on A is toward B.

A person observes that in the winter, his hair stands on end when it is brushed. This is because his hair becomes positively charged. Why does the act of brushing make his hair positively charged? A. Protons are moved from the brush to his hair. B. Electrons are converted to protons by the thermal energy that results from brushing. C. Electrons are moved from his hair to the brush. D. Some of the protons in his hair increase in charge due to the thermal energy that results from brushing. E. Neutrons are split so that there are additional protons in his hair.

C, because less energy is needed to remove e- to another atom than protons.

What happens when a charged insulator is placed near an uncharged metallic object? A. They repel each other. B. They may attract or repel each other, depending on whether the charge on the insulator is positive or negative. C. They attract each other. D. The charged insulator always spontaneously discharges. E. They exert no electrostatic force on each other.

C, because the charged insulator will create a dipole where the free e- will be attracted or repelled by the charged object which creates an attractive force.

Why does my socks cling to my shirt when I remove it from the dryer

The energy from the friction and heat removes e- from one material to another creating an attractive force between materials (one positively charge the other negatively charged) the electric force will attract one to the other.

History of Static Electricity

The first record of its effects dates to ancient Greeks who noted more than 500 years B.C. that polishing amber temporarily enabled it to attract bits of straw. The very word electric derives from the Greek word for amber (electron). Borneo amber was mined in Sabah, Malaysia, from shale-sandstone-mudstone veins. When a piece of amber is rubbed with a piece of silk, the amber gains more electrons, giving it a net negative charge. At the same time, the silk, having lost electrons, becomes positively charged.

Where do these charges come from?

The law of conservation of charge is absolute—it has never been observed to be violated A cloud of electrons surround atoms and take a significantly reduced amount of energy to move to one atom to another.

Why shound we know about the insulator and conductive quality of objects

To determine how well charge transfers between objects

Cautions for Static Electricity

We also have to be cautious of static electricity, especially in dry climates. When we pump gasoline, we are warned to discharge ourselves (after sliding across the seat) on a metal surface before grabbing the gas nozzle. Attendants in hospital operating rooms must wear booties with aluminum foil on the bottoms to avoid creating sparks which may ignite the oxygen being used.

All macro-neutral objects are attracted to charged obj because of the polarization. A. True B. False

A

Some of the most basic characteristics of static electricity include:

- The effects of static electricity are explained by a physical quantity not previously introduced, called electric charge. - There are only two types of charge, one called positive and the other called negative. - Like charges repel, whereas unlike charges attract. - The force between charges decreases with distance.

Examples of Insulators

Rubber Plastic Wood Glass PVC Plexiglass

Describe three common situations that generate static electricity.

Friction(Rubbing neutral obj together) Inducing charge on neutral object

Separation of charge in Atoms

Charges in atoms and molecules can be separated—for example, by rubbing materials together. Some atoms and molecules have a greater affinity for electrons than others and will become negatively charged by close contact in rubbing, leaving the other material positively charged. (See Figure 18.8.) Positive charge can similarly be induced by rubbing. Methods other than rubbing can also separate charges.

SI unit of charge

Coulomb (C) and elementary charge are significantly less than C (10e-19), so you will have to divide charge with elementary charge to receive an answer concerning electrons or protons(e)

electrostatic force—the existence of two types of charge, the observation that like charges repel, unlike charges attract, and the decrease of force with distance—were eventually refined, and expressed as a mathematical formula.

Coulombs Law

Exactly how does the force depend on the amount of charge and the distance between charges?

Equal and opposite in magnitude

Can you create or destroy charge?

No charge is actually created or destroyed when charges are separated as we have been discussing. Rather, existing charges are moved about. In fact, in all situations the total amount of charge is always constant. This universally obeyed law of nature is called the law of conservation of charge. The Net charge will never change

Interactions between Positive and negative charges that we observe same charges repel one another opposing charges attract one another

Positive and negative charges that we observe same charges repel one another opposite charge attracts one another

Is there a smallest unit of charge?

elementary charge(q or Q) = 1.6e-19C {q} is commonly used for charge and the subscript e size 12{e}indicates the charge of a single electron (or proton)

Relationship between gravitational Force and Coulombs Force on Large and small scales

gravitational force is completely negligible on a small scale, where the interactions of individual charged particles are important. On a large scale, such as between the Earth and a person, the reverse is true. Most objects are nearly electrically neutral, and so attractive and repulsive Coulomb forces nearly cancel. Gravitational force on a large scale dominates interactions between large objects because it is always attractive, while Coulomb forces tend to cancel.

A conductor

is a material that allow charges to flow across it.Some of the electrons in metals and similar conductors are not bound to individual atoms or sites in the material. These free electrons can move through the material much as air moves through loose sand. Any substance that has free electrons and allows charge to move relatively freely through it is called a conductor.The moving electrons may collide with fixed atoms and molecules, losing some energy, but they can move in a conductor. Superconductors allow the movement of charge without any loss of energy. Salty water and other similar conducting materials contain free ions that can move through them. An ion is an atom or molecule having a positive or negative (nonzero) total charge. In other words, the total number of electrons is not equal to the total number of protons. Copper Salt water

Definition of Polarization

is the separation of charges in an object that remains neutral