Review

conductor - material in which free electrons can move throughout the material

insulator - material in which all electrons are bound to nuclei in atoms; in an atom, the electron cloud can shift to create an atomic or molecular dipole.

Representation of a neutral insulator

Representation of a polarized insulator

+- +-+- +-+- +-+- +-+- +-+- +-

Interaction of a Neutral Conductor and Charged

Insulator

-+

+

++ +

+

++

+ +

+ ++- -

-

-----

- -

- --

+

+

+

+

---

- --- --

-

---

-

-

-

Interaction of a charged insulator and a neutral

conductor• Free electrons are

repelled from rod.• Right side + charged &

left side - charged.• Net attractive force

between rod and conductor.

-+

+

++ +

+

++

+ +

+ ++

- -

-

----

--

- ---

+

+

+

+

---

- --- --

-

---

-

-

-

More negativelycharged area

More positivelycharged area

Interaction of a neutral insulator and a charged insulator

+-+-

+-

+-

+-+-

+-

+-+-

+-

+-+-

+-

+-

+-

+-+-

+-

-

+

+

+

+

---

- --- --

-

---

-

-

-

Induced Charge in an Insulator

• Electron clouds in atoms are repelled by rod.

• Electrons are not free to move throughout the material, but polarization occurs to create atomic dipoles.

• Surface of insulator becomes + charged.

• Net attractive force between rod and insulator.

+-

+- +-

+-

+-

+-+- -

+

+

+

+

---

- --- --

-

---

-

-

-

Poll

Poll

Poll

Poll

Charging by induction

A & B are identical metal blocks. What is the final charge of block B?

1. +6 nC

2. +3 nC

3. 0

4. -3 nC

5. -6 nC

1 & 2 are identical metal blocks. What happens?

1. protons move from A to B

2. electrons move from B to A

3. both protons and electrons move

4. no charges move

Two aluminum blocks, A and B, are initially neutral. They have insulating handles, which are not shown. At a time after t4, what is the net charge of A?

1. positive

2. negative

3. neutral

Surface Charge

In a conductor, excess charge will always be on the surface. This is called surface charge.

The amount of surface charge per unit area is called surface charge density.

Battery

A battery acts like an “electron pump” that creates excess electrons on the - terminal and a deficit of electrons on the + terminal.

Current

Electron current is the rate of flow of electrons. When electrons flow through a thin conductor (i.e. a filament), it can become very hot and perhaps so hot that it gives off light (i.e. a light bulb). Current will only flow if there is a closed path from the - terminal to the + terminal of the battery. A bulb can be used as a current indicator that tells you that electrons are flowing (but does not tell you the direction).

If you instead consider the flow of + charge, then it flows in the opposite direction of electron current and is called conventional current (or just current for short).

Direction of electron current in a simple circuit of a battery and bulb

Direction of conventional current in a simple circuit of a battery and bulb

Capacitor

A capacitor is made of two, very close, parallel plates (i.e. conductors). They do NOT touch; therefore, electrons cannot travel across the gap.

Battery, bulb, and capacitor

At t=0, the clips are connected.

Battery, bulb, and capacitor

At t=0, the clips are connected (with the battery removed).

How does the bulb affect how long it takes to charge the capacitor?

Change the bulb to a different type. Why does it affect how long it takes to charge the capacitor?

The total charge on the capacitor plates after a “long time” (called maximum charge) is the same no matter which bulb is being used (after all, the filament is just a conductor that allows electrons to flow through it). So, the bulb affects the rate that electrons flow. Though one bulb, electrons flow fast (i.e. high current). Through the other bulb, electrons flow slow (i.e. low current). This quality of the bulb is called resistance.