# Electrostatics, Magnetism & Circuits Flashcards

## From electric field to complex circuit calculations, use these cards to master the topic of Eletromagnetism and Circuits as tested in most introductory undergrad physics courses and even on the AP Physics exam.

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.

Define:

**conductance** of a material

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

Conductance is generally higher in metals and lower in nonmetals.

Define:

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.

Define:

**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.

Define:

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.

Define:

**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**

Potential energy, expressed in science as U

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.

Define:

**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.

Define:

**absolute potential** of a point in space

The **absolute potential** of a point in space is the voltage necessary to bring a test charge from infinity to that point, a given distance away from a fixed charge.

By convention, the test charge is assumed to be a positive unit charge of 1 Coulomb. This value can be positive or negative depending on the main charge.

Given the absolute potential for a fixed positive charge at a known position, what would be the absolute potential for a negative charge of the same magnitude at the same position?

The negative of the absolute potential value that was given for the positive charge.

Absolute potential will be positive or negative depending on the sign of the charge.

What concept describes the set of positions of equal energy that a positive test charge can be placed, when put next to a stationary charge?

**Equipotential lines.**

All charges have equipotential lines that form closed, non-overlapping loops around them.

A solitary 5C charge is fixed in space and a test charge is placed at 1cm and then 2cm away. Can these two positions be on the same equipotential line?

No. These are not positions where the potential electrostatic energy is constant.

From: energy = Kq_{1}q_{2} / r , the potential energy is inversely proportional to the distance between the fixed charge and the test charge. Doubling the distance will halve the energy, hence these positions cannot be equal energy and cannot be equipotential.

Define:

an **electric dipole**

An **electric dipole** occurs as the result of any distinct separation of positive and negative charge in an object or system.

On the AP Physics exam, the simplest case is always assumed: a linear object with one positive and one negative end.

Describe the motion of a dipole placed into an electric field.

The dipole will rotate so that the positive end is aligned with the electric field and the negative end is exactly opposite to the field.

Assuming that the dipole is overall neutral, there will not be any translation in the field.

Define:

**electric current**, *I*

**Electric current**, *I*, is the directional flow of charge through a conducting medium.

Though the moving charge in a circuit is typically electrons, the conventional direction of current always follows positive charge flow.

What is the formula and SI unit for current?

Current: *I* = ∆Q / ∆t

Where:

∆Q = change in charge (coulombs)

∆t = change in time (seconds)

The SI unit of charge is the **ampere, A,** and represents 1 C/s.

If electrons are moving along a wire from left to right, where is conventional current moving?

Conventional current is moving from right to left.

Conventional current always refers to the flow of positive charge, and will always be opposite to the flow of electrons.

By what proportion will the current change if the amount of charge transferred doubles and the time halves?

The current will be 4x larger.

*I _{0}* = Q / t

given new Q’ = 2Q

and new t’ = (1/2)t

then:

*I’*= Q’ / t’

= 2Q / (1/2)t = 4(Q / t)

= 4

*I*

_{0}Define:

a **battery**

An **electrical battery** (or electrochemical cell) is a device that produces a flow of electrons from anode to cathode.

The necessary electrical energy is created by undergoing redox chemical reactions in the cell.