Physics Ch. 22

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Electricity at rest, "static electricity"


wide range of electrical phenomena that (in one form or another) underlie everything around us. Lightning, match strike spark, bonds between atoms, molecules

Why does the gravitational force between Earth and Moon predominate over electrical forces?

The electric force between Earth and Moon cancels out because they have an equal number of positive and negative charges.

Which part of an atom is positively charged, and which part is negatively charged?

The nucleus is positively charged and the electron cloud is negatively charged.

How does the charge of one electron compare to that of another electron? How does it compare with the charge of a proton? How do the masses of protons and electrons compare?

All electrons have the same charge. Electron charge is equal and opposite to the proton charge. A proton has 1800 times the mass of an electron.

What is the most common net charge of an atom?

Neutral Atoms usually have as many electrons as protons, so the atom has a zero net charge.

What is a positive ion? What is a negative ion?

A positive ion is a neutral atom that has lost one or more electrons. A negative ion is a neutral atom that has gained one or more electrons.

What is meant by conservation of charge?

Net charge cannot be created or destroyed.

What is meant by saying that charge is quantized?

All charged objects have a charge that is an integer multiple of the charge of an electron. There is no smaller units of charge (than one electron/proton) observed!

Which particle has exactly one quantum unit of charge?

Proton or electron

How does one coulomb of charge compare with the charge of a single electron?

1 C is associated with 6.25 × 1018 electrons. That’s 6.25 billion billion* electrons!

How is Coulomb’s law similar to Newton’s law of gravitation? How is it different?

Newton’s law of gravitation is attractive, whereas Coulomb’s law is attractive or repulsive. Both are proportional to the inverse square of distance.

Why are metals good conductors of both heat and electricity?

The outer shell electrons in metals are free to move from atom to atom. "loose" outer shell e-

Why are materials such as glass and rubber good insulators?

Electrons are tightly bound to their atoms, making them poor conductors of heat. The outer electrons belong to particular atoms.

How does a semiconductor differ from a conductor or an insulator?

A semiconductor is neither a good conductor nor a good insulator – it has a middle range of resistivity. fair insulator in pure crystalline form, excellent conductor when e- added or removed

What is a transistor composed of, and what are some of its functions?

A transistor is made of thin layers of semiconducting materials sandwiched together. It can be a switch, an oscillator, or an amplifier. Behaves as both conductor and insulator (thus on/off switch, up/down amplifier)

How does the flow of current differ in a superconductor compared with the flow in ordinary conductors?

ordinary conductors have a small resistance to the flow of electric charge, but Superconductors have infinite conductivity (current flows forever)

What happens to electrons in any charging process?

Electrons transfer from one place to another.

What kind of charging occurs when you slide your body across a plastic surface?

Charging by friction occurs. Electrons are transferred when one object rubs against another. *same with socks on carpet!

What kind of charging occurs during thunderstorms?

Ice crystals are charged by friction and the ground is charged by induction. as warm water vapor rises in the air, it brushes against ice crystals high in the air above, producing a charge (friction).

What is the primary purpose of a lightning rod?

To prevent a fire caused by lightning. If sufficient charge does not leak from air to rod, charge will attract to the rod and direct to the ground instead of building.

In terms of net charge, how does an electrically polarized object differ from an electrically charged object?

An electrically polarized object can have zero net charge, while a charged object cannot have zero net charge. Polarized: one side of atom is more +, other side is more -. Charged: entire atom is either + or – charged.

What is an electric dipole?

In an electric dipole, positive and negative charges are separated on opposite sides of an object. Distribution of electric charge is not perfectly even.

Which molecule is an example of a common electric dipole?

water molecules!

Give two examples of common force fields and name the sources of these fields.

Gravity fields made by mass, electric fields made by charge

How is the direction of an electric field defined?

The direction of the field is the direction of the force on a positive test charge.

Why is there no electric field at the center of a charged spherical metal ball?

Because of mutual repulsion, mobile electrons in the conductor will spread out uniformly over the outer surface of the ball, so the force and the field on a test charge at the center is zero because opposing forces balance in every direction.

When charges mutually repel and distribute themselves on the surface of conductors, what becomes of the electric field inside the conductor?

The charges arrange themselves to make the field inside the conductor zero. ex.) in sphere, in car

How much energy is given to each coulomb of charge that flows through a 1.5-V battery?

1.5 joules 1 volt = 1 joule/coulomb

A balloon may easily be charged to several thousand volts. Does that mean it has several thousand joules of energy? Explain.

No. Energy is voltage times charge, so you need a coulomb of charge to get several thousand joules of energy. Difference between electric potential energy and electric potential.

Where is the energy stored in a capacitor?

The energy is stored in the electric field between the plates. (figure 22.26)

Why is charge usually transferred by electrons rather than by protons?

the electrons move around the neucles in which the protons and neutrons are bounded by the strong neuclear force. e-s which are faraway from necucles are free from electrosatic attraction of the necucles moreover due to less mass they feel high acceleration for same force ……. hence only these e-s are able to move and conduct

How can you charge an object negatively with only the help of a positively charged object?

Place a strong positive charge close to, but do not touch the object to be charged. This pushes the negative charges to the other side of the object. Connect a wire(that is connected to the ground) to the object. The negative charges(electrons) will flow out of the object into the ground. Disconnect the wire. The object is positively charged.

The proportionality constant k in Coulomb’s law is huge in ordinary units, whereas the proportionality constant G in Newton’s law of gravitation is tiny. What does this indicate about the relative strengths of these two forces?

Coulomb’s force is much ‘stronger’ than Gravitation. If 2 objects are repelled from each other due to electrical repulsion, they would have to be VERY massive (in relation to the small electric potential causing the repulsion) in order for the Gravitational force to cancel out the Coulomb force.

Imagine a proton at rest a certain distance from a negatively charged plate. It is released and collides with the plate. Then imagine the similar case of an electron at rest the same distance away from a positively charged plate. In which case will the moving particle have the greater speed when the collision occurs? Why?

Because a proton has A LOT more mass than an electron. Newton’s law F=ma indicates acceleration is related to the mass of the object having the force exerted on it. Since the force is the same in both cases, the acceleration must be lower for the proton because it’s mass is higher. I’ll show an example where the numbers are no where near real, but it will show the relationship: Proton: F=100 m=20 therefore, a = 5 Electron F=100 m=5 therefore a=20 Notice how (m) times (a) must equal the same value in both cases? That’s what’s causing the electron to move so much faster, because it’s so much less massive.

Why is it safe to remain inside a car during a lightning storm?

It’s because you are pretty much completely enclosed by a conducting shell formed by the car body. It’s called a "Faraday cage." The electric field inside a conducting shell is zero no matter what goes on outside. When there are holes in the shell (as for the car windows) then the field isn’t exactly zero but it is greatly reduced from the outside value. This is related to the shielding effect of the metal full of holes in a microwave oven door. The charge spreads out over the roof surface. As long as you do not touch the roof! There is still a small charge that will hit the car.

Why is it dangerous to touch the terminals of a high-voltage capacitor even after the charging circuit is turned off?

A capacitor is used to store charge. It does this by the means of an electrochemical reaction. After the charging circuit is turned off, this charge – if not discharged by any means – is still present in the capacitor. High-Voltage capacitors can amass sufficient current for it to be lethal at said voltage, and they can deliver all of it at the slightest contact.

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