Section 3: Electricity and Magnetism

Electric current

The flow of electric charge through a conductor, usually carried by electrons and measured in amperes (A).

Electric charge

A property of matter that causes electric forces; carried by particles such as electrons and protons.

Conductor

A material that allows electric charges to flow easily, such as metals like copper.

Ampere (A)

The unit used to measure electric current.

Direct current (DC)

Electric current that flows in one direction only, such as the current from a battery.

Alternating current (AC)

Electric current that periodically changes direction, such as the current from wall outlets.

Voltage

The potential difference that provides the energy needed to move electric charges through a circuit; measured in volts (V).

Potential difference

Another term for voltage; the difference in electric potential between two points.

Resistance

A property of a material that opposes the flow of electric current; measured in ohms (Ω).

Ohm (Ω)

The unit used to measure electrical resistance.

Ohm’s Law

A relationship stating that voltage equals current multiplied by resistance (V = IR).

Ohmic material

A material with constant resistance that follows Ohm’s law.

Nonohmic material

A material whose resistance changes with voltage or current and does not follow Ohm’s law.

Resistor

A circuit component designed to provide a specific amount of resistance and limit current flow.

Resistivity

An intrinsic property of a material that determines how strongly it resists electric current; measured in ohm-meters (Ω·m).

Factors affecting resistance

Material type, length of the conductor, cross-sectional area, and temperature.

Effect of length on resistance

Increasing the length of a wire increases resistance.

Effect of cross-sectional area on resistance

Increasing the cross-sectional area decreases resistance.

Insulator

A material with very high resistivity that resists the flow of electric current.

Series circuit

A circuit where components are connected in a single path, causing the same current to flow through all components.

Parallel circuit

A circuit where components are connected in multiple paths, allowing the same voltage across each branch.

Voltage in a series circuit

Divided among the components and adds up to the total voltage supplied.

Voltage in a parallel circuit

The same across all components connected in parallel.

Current in a series circuit

The same through all components.

Current in a parallel circuit

Divides among different branches and adds up to the total current.

Equivalent resistance

The total resistance of a circuit that can replace multiple resistors with one resistor.

Equivalent resistance in series

Found by adding all individual resistances together.

Equivalent resistance in parallel

Found using the reciprocal formula; always less than the smallest individual resistance.

Closed circuit

A complete loop that allows electric current to flow.

Open circuit

A broken or incomplete path that prevents current from flowing.

Circuit breaker

A safety device that automatically stops current flow when it becomes too high.

Capacitor

A device that stores electrical energy in an electric field; measured in farads (F).

Capacitance

A measure of how much electric charge a capacitor can store.

Inductor

A coil of wire that stores energy in a magnetic field and opposes changes in current.

Back electromotive force (back-emf)

A voltage produced by an inductor that resists changes in current.

RL circuit

A circuit that contains resistors and inductors and causes current to change gradually.

Main function of a resistor

To limit current and protect circuit components.

Main function of a capacitor

To store and release electrical energy and smooth voltage changes.

Main function of an inductor

To resist changes in current and store energy in a magnetic field.

Electric Charge

A property of matter that causes objects to experience electric forces; can be positive or negative.

Positive Charge

The condition of an object that has lost electrons.

Negative Charge

The condition of an object that has gained electrons.

Electron

A negatively charged subatomic particle that can move between objects.

Neutral Object

An object with equal amounts of positive and negative charge.

Electrostatic Force

The attractive or repulsive force between charged objects.

Charging by Contact (Conduction)

Charging an object by touching it with a charged object, allowing electrons to transfer.

Charging by Induction

Charging an object without direct contact by bringing a charged object nearby, which rearranges internal charges.

Grounding

The process of safely transferring excess charge to Earth by providing a path for charge to flow away.

Polarization

The temporary separation of charges within a neutral object caused by a nearby charged object.

Conductor

A material that allows electric charges to move freely due to free-moving electrons.

Insulator

A material that resists the flow of electric charge because electrons are tightly bound.

Free-Moving Electrons

Electrons in conductors that can move easily and carry electric charge.

Coulomb’s Law

The electrostatic force between two charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

Point Charge

A charged object treated as if all its charge is concentrated at a single point.

Coulomb (C)

The SI unit of electric charge.

Electrostatic Constant (k)

A constant in Coulomb’s law equal to approximately 8.99 × 10⁹ N·m²/C².

Inverse Square Law

A relationship where force decreases with the square of the distance between charges.

Like Charges

Charges with the same sign that repel each other.

Opposite Charges

Charges with different signs that attract each other.

Charge–Force Relationship

Increasing the amount of charge increases the electrostatic force.

Distance–Force Relationship

Increasing the distance between charges decreases the electrostatic force.

Newton’s Third Law (Electric Forces)

Electric forces between two charges are equal in magnitude and opposite in direction.

Electric Field

A region around a charged object where other charges experience an electric force.

Electric Field Strength (E)

The force experienced by a positive test charge divided by the magnitude of that charge.

Electric Field Unit

Electric field strength is measured in newtons per coulomb (N/C) or volts per meter (V/m).

Electric Field Lines

Imaginary lines that represent the direction and strength of an electric field.

Direction of Field Lines

Electric field lines point away from positive charges and toward negative charges.

Field Line Density

The closer the field lines, the stronger the electric field.

No Crossing Rule

Electric field lines never cross because the force at a point has only one direction.

Electrostatic Equilibrium

A state where charges in a conductor are not moving and the electric field inside the conductor is zero.

Charge Distribution on Conductors

In electrostatic equilibrium, excess charge resides on the surface of a conductor.

Faraday Cage Effect

The phenomenon that keeps people safe inside a car during lightning because charge stays on the outside.

Capacitor

A device that stores electric charge between two conductive plates.

Function of a Capacitor

To store electric charge and electrical energy in a circuit.

Electric Potential Energy

The energy a charged object has due to its position in an electric field.

Electric Potential (V)

Electric potential energy per unit charge.

Volt (V)

The unit of electric potential, equal to one joule per coulomb (J/C).

Electric Potential Difference (Voltage)

The difference in electric potential between two points that drives charge flow.

Conservative Force (Electric)

A force where work depends only on starting and ending positions, not the path taken.

Electric Field Between Plates

The electric field created between two oppositely charged parallel plates.

Electric Field Formula (Plates)

Electric field strength equals voltage divided by distance between plates.

Voltage-Field Relationship

Increasing voltage increases electric field strength.

Distance-Field Relationship

Increasing distance between plates decreases electric field strength.

Scalar Quantity

A quantity with magnitude only, such as voltage.

Vector Quantity

A quantity with both magnitude and direction, such as electric field strength.

Static Electricity

The buildup of electric charge on an object, often caused by friction.

Lightning

A large electrical discharge caused by charge buildup between clouds and the ground.

Touchscreens

Devices that detect changes in electric fields caused by your finger.

Photocopiers and Laser Printers

Machines that use electric forces to attract toner to paper.

Cell Membrane Electric Fields

Electric fields across cell membranes that allow nerve signaling.

Air Purifiers

Use electric fields to attract and remove dust and pollen from air.

Electrophoresis

The use of electric fields to separate DNA or proteins by charge and size.

Electric Fish

Animals that generate electric fields to navigate and communicate.

Solar Panels

Devices that use electric fields to direct electron flow and generate electricity.

Defibrillators

Medical devices that use high electric potential differences to restore heart rhythm.

Battery

A device that converts chemical energy into electrical energy by creating a potential difference.

Lightning and Electric Potential

Lightning occurs when electric potential difference becomes large enough to overcome air resistance.

Neural Action Potential

A rapid change in electric potential across a neuron’s membrane that transmits signals.