What unit measures charge, and what signs can be given to it?

The **Coulomb, C,** is the SI unit of charge and can have values that are positive (for current or protons) or negative (for electrons).

Since the AP Physics exam is concerned primarily with current (+) moving, Coulombs will primarily be positive values.

What are the rules for attraction of charges?

**Opposite charges** (+,-) are **attracted **towards each other.

The force of attraction varies proportionally with magnitude of charges.

What are the rules for repulsion of charges?

**Like charges** (+,+ or -,-) are **repulsed** away from each other.

The force of repulsion varies proportionally with magnitude of charges.

What must be true of the total charge in a closed system?

Total charge in a closed system will stay constant. This is the **law of conservation of charge.**

Describe the total charge before and after in the following closed system: 5 protons and 4 electrons collide with sufficient energy to create 4 neutrons and 1 proton.

Charge remains constant at +1.

Charge initially is: +5 + (-4) = +1.

In a closed system charge must be conserved. The final charge confirms this: 4(0) + (+1) = +1.

**conductance **of a material

**Conductance** is the ability of a material to transfer charge.

Conductance is generally higher in metals and lower in nonmetals.

a **conductor**

A **conductor **contains many electrical charges within the medium, and they are relatively moveable.

Common conductors are usually metals with high atomic weight, such as silver, copper, and gold.

Calcium contains 3x more moveable electric charges than Iron. Which is a better conductor?

Calcium has 3x the conductance of iron.

The more movable charges a material has, the higher its relative conductance.

**insulation**

**Insulation **is the degree to which a material is unable to transfer charge.

Insulation is generally higher in polymers, amorphous crystals, or ionic solids. Insulation is low in all other substances.

an **insulator**

An **insulator **has few free electrical charges within the medium, and those are difficult to move.

Common insulators are as glass, quartz, rubber and teflon.

Rubber has 10,000x more moveable electric charges than paraffin. Which is a better insulator?

Paraffin has 10,000x higher resistance (insulation) than rubber.

The fewer moveable charges a material has, the greater its relative resistance (insulation).

What is the relationship for the force between two charged particles separated at a given distance?

**Coulomb's Law**, defined as:

**F = Kq _{1}q_{2} / r^{2}**

Where:

K = Coulombic Constant in Nm^{2}/C^{2}

q_{1} and q_{2} = charges in C

r = distance between the charges in m

What does a positive value of F indicate, in Coulomb's Law for electrostatic force?

A **positive **value of F indicates that the charges are the same sign, and will experience a force of **repulsion**.

What does a negative value of F indicate, in Coulomb's Law for electrostatic force?

A **negative **value of F indicates that the charges are opposite in sign, and will experience a force of **attraction**.

What will be the change in force, if two positive charges separated by a distance r are now moved to a distance 2r apart?

The charges will now experience 1/4 original force.

From: F = Kq_{1}q_{2} / r^{2} the force is inversely proportional to the square of distance. Doubling distance will reduce magnitude of force to 1/4 its original value.

What will be the change in force, if a negative and positive charge at a distance r are replaced by equivalent but now both negative charges?

Force will change from negative (attraction) to positive (repulsion).

Magnitude will be the same, since the strength of the charges remains the same.

What concept and direction describes the path of motion that a positive test charge will travel, when put next to a stationary positive charge?

**Electrostatic field lines. **

By convention, all positive charges have field lines with arrows pointing outwards, since a positive test charge will be repulsed.

What concept and direction describes the path of motion that a positive test charge will travel, when put next to a stationary negative charge?

**Electrostatic field lines. **

By convention, all negative charges have field lines with arrows pointing inwards, since a positive test charge will be attracted.

Describe the motion of a positive test charge, if it's placed exactly at the midpoint between two equal positive stationary charges.

It will remain immobile.

The two positive charges repel the test positive charge equally, hence cancelling each other's force vector. There is no net force on the test charge.

What direction will a positive charge move, when introduced into a uniform electric field pointing directly upwards?

Directly upwards.

A positive charge always follows the electric field.

What direction will a negative charge move, when introduced into a uniform electric field pointing directly upwards?

Directly downwards.

A negative charge always goes exactly opposite to the field lines.

What direction will a non-charged particle move, when introduced into a uniform electric field pointing directly upwards?

No motion. A non-charged particle (neutral) will not be accelerated in any direction by an electric field.

What direction will a proton move, when introduced into a uniform electric field pointing towards the right?

To the right.

Positive charges always move in the direction of field lines.

What direction will an electron move, when introduced into a uniform electric field pointing towards the left?

To the right.

Negative charges move in the exact opposite direction of field lines.

**electric potential**

**Electric potential** (or **electric potential energy**) is the energy of position for charges, relative to each other.

This is the same as the electrostatic force between charges times the distance between them. Electric potential may be positive or negative, depending on the sign of the charges.

What is the relationship between charges and distance that gives electric potential?

**U = Kq _{1}q_{2} / r**

Where:

K = Coulombic constant in Nm^{2}/C^{2}

q_{1} and q_{2} = charges in C

r = distance between the charges in m

If the distance between two positive charges doubles, what will happen to the electic potential between them?

Electric potential will halve.

From: U = Kq_{1}q_{2} / r

U is inversely proportional to r, hence doubling r will halve U.

**potential difference** between two points

**Potential difference**, also called **voltage**, is the potential energy per charge that is lost or gained when a test charge is moved between two positions.

By convention, the moving charge is assumed to be a positive unit test charge of 1 Coulomb.

What is the relationship between charge and distance that gives potential difference (voltage)?

**V = K*q / r**

Where:

K = Coulombic constant in Nm^{2}/C^{2}

q = static charge in C

r = distance between the charge and a positive unit test charge in m

What change in voltage is necessary to hold a unit charge in position, if the main charge is suddenly doubled?

Voltage must double.

From: V = K*q / r

Voltage and main charge are directly proportional, hence doubling one will double the other.