Chapter 20-21

Question 1

Two kinds of charges:

Answer 1

• positive and negative

Question 2

Fundamental Law of Electric Charge

Answer 2

• Opposite electric charges attract each other
• Similar electric charges repel each other.
• Charged objects attract some neutral objects.

Question 3

• the source of electric charges:

Answer 3

the atom.

Question 4

Structure of the atom:

Answer 4

• A simple view is that the atom has a positively charged nucleus with negatively charged electrons moving about the nucleus. Normally, the number of positive and negative charges are equal so the atom is electrically neutral. Since protons, which are positively charged, are tightly bound together in the nucleus by the strong nuclear force, they do not move. In solids, atoms do not move. So all/ charges in solids are due to the movement of electrons: i…, Atoms gain or lose electrons and become ionized. Positively charged objects have a deficit of electrons and negatively charged objects have an excess of electrons.

Question 5

If a plastic ruler is rubbed with cloth, where do electrons move?

Answer 5

electrons move from the cloth to the ruler because the atoms of the cloth hold electrons less tightly than the atoms of the ruler do, so some leave the cloth and go onto the ruler, resulting in a negatively charged ruler and a positively charged cloth.

Question 6

Why is there much more static electricity on dry days than on humid ones?

Answer 6

• the excess electrons eventually “leak off” the charged object and go back to the object with a deficit via water molecules (which are polar molecules) in the air. Thus there is much more static electricity on dry days than on humid ones, because objects can hold charge much longer.

Question 7

There is a law of conservation of charge.

Answer 7

When you understand that charge is due to the movement of electrons it makes sense. The electrons do not disappear, they simply move. So when you charge an object (e.g. comb with hair), the comb becomes negatively charged and the hair becomes positively charged with exactly the same amount of charge.

Question 8

The symbol for electric charge is

Answer 8

“Q”

Question 9

• Units for electric charge:

Answer 9

Most common one is the coulomb (C).

Question 10

• The smallest known charge is the amount on an electron or a proton.

Answer 10

→ called the elementary charge

Question 11

The symbol for the elementary charge

Answer 11

is “e”. e = 1.602 × 10^-19 C.

Question 12

Charge on electrons and protons

Answer 12

• The charge on an electron is -e; the charge on a proton is +e.

Question 13

Conductors

Answer 13

are solids in which electrons are able to easily travel from one atom to another.
→ have 1, 2, or 3 electrons in the outer shell, so they are quite loosely held
→ includes most metals - best conductors are silver, copper, aluminum, and gold

Question 14

Insulators

Answer 14

are solids in which electrons are not free to move easily from atom to atom.
→ insulators which are elements have 5, 6, 7, or 8 electrons in the outer shell, so they are tightly held
→ most good insulators are compounds rather than individual elements - the outer electrons are bound up in chemical bonds with other atoms
→ examples: plastic, cork, glass, wood, rubber -

Question 15

Semiconductors

Answer 15

are in between. They will not conduct naturally, but with a certain voltage applied, they will conduct.
→ have 4 electrons in the outer shell
→ used to make electronic components such as diodes, transistors, and integrated circuits
→ examples: silicon, germanium, and carbon

Question 16

Electric Force and Electric Field

Answer 16

• to “attract” or “repel” means that charged objects exert force on each other.
• Most forces on objects are due to contact between the objects. But we’ve seen that gravitational and electric forces act at a distance. (As does magnetic force.) We explain how this is possible by describing such forces in terms of “fields”. - We’ve talked about gravitational fields before. Recall: “g” can be viewed as the strength of the gravitational field
  → the force on an object divided by the mass of the object -

Question 17

The concept of an electric field is attributed to Michael Faraday (1800s) he said:

Answer 17

that extending outward from every charge is an electric field → like gravity, it goes in all directions and goes to infinity. when another charge is placed near the first, it feels a force because of this field.

Question 18

How to analyze an electric field:

Answer 18

we determine the force on a small positive test charge. - it is common to use a capital “Q” to represent the main charge producing the electric field and a small “q” to represent this test charge.

Question 19

The force vectors:

Answer 19

Indicates the direction of the field, and their relative length indicates the strength of the field at that point since they show the relative strength of the force on the test charge. T

his idea of viewing the force on a test charge is used to describe the electric field around a charge.

Question 20

Defn. The electric field at a point in space is defined as:

Answer 20

the force exerted on a positive test charge at that point divided by the magnitude of the test charge.

Note: Both the force and the field are vectors! → the direction of the field is the same as that of the force on a positive test charge.

Question 20

A test charge

Answer 20

a charge so small that it does not significantly alter the field being measured, but does feel the effect of the field.

Question 21

• Units for electric field:

Answer 21

N/C

Question 22

Electric Potential Energy and Potential Difference Important notes:

Answer 22

• An object has gravitational potential energy because it is placed in a gravitational field. (If there was no gravitational field, the object wouldn’t have any potential energy.)
• • Definition- Electric Potential, or just potential, is the electric potential energy per unit of charge.

Symbol for potential: V

• Just like with potential energy, the choice of a zero reference for potential is arbitrary, so most often we deal with the Potential Difference between two points A and B.
• Defn. The Potential Difference between points A and B is the difference in the electric potential energy a charge has as it moves from point A to point B divided by the amount of the charge.

Units for potential and potential difference: J/C = V (volts) - after Alessandro Volta, Italian Physicist

• Sometimes, a conductor connected to the ground is considered to be zero potential, and then potential differences, or more commonly, voltages, are measured from there.
• Other times we refer to two other points and refer to the potential difference between the two points