Glossary
Current Density (J)
Current density is a vector quantity representing the amount of charge flowing per unit area through a specific cross-section of a material. It indicates the microscopic flow of current.
Example:
In a high-power transmission line, the current density within the conductor must be carefully managed to prevent overheating and material damage.
Electrical Power (P)
Electrical power is the rate at which electrical energy is converted into other forms of energy (such as heat, light, or mechanical work) in a circuit component.
Example:
A microwave oven has a high electrical power rating, indicating it converts electrical energy into microwave radiation and heat at a rapid rate to cook food quickly.
Electromotive Force (EMF or ε)
Electromotive force is the total energy per unit charge supplied by a source (like a battery) when no current is flowing, representing its ideal voltage before any internal losses.
Example:
A fresh AA battery might have an electromotive force of 1.5V, which is its maximum potential to drive current before it's connected to a circuit.
Internal Resistance (r)
Internal resistance is the inherent resistance within a real voltage source (like a battery) that causes a drop in the terminal voltage when current flows through it.
Example:
When a car battery struggles to start an engine, its high internal resistance might be causing a significant voltage drop, leaving insufficient power for the starter motor.
Non-Ohmic Device
A non-Ohmic device is a component that does not obey Ohm's Law, meaning its resistance changes with voltage or current, resulting in a non-linear V-I graph.
Example:
A light-emitting diode (LED) is a non-Ohmic device; its resistance drops sharply once a certain 'turn-on' voltage is reached, allowing current to flow.
Ohm's Law
Ohm's Law is a fundamental principle stating that the voltage across a conductor is directly proportional to the current flowing through it, given a constant resistance (V = IR).
Example:
An engineer uses Ohm's Law to calculate the necessary resistance for an LED to operate safely without burning out when connected to a specific voltage source.
Ohmic Device
An Ohmic device is a component that obeys Ohm's Law, meaning its resistance remains constant regardless of the applied voltage or current, resulting in a linear V-I graph.
Example:
A standard carbon-film resistor is considered an Ohmic device because its resistance value does not significantly change with varying voltage or current.
Parallel Resistors
Resistors connected across the same two points, providing multiple paths for current, where the voltage across each is the same, and their total equivalent resistance is less than any individual resistance.
Example:
Household electrical outlets are wired with appliances in parallel resistors, ensuring that each device receives the full voltage independently and can operate even if another is turned off.
Resistance (R)
Resistance is the overall opposition to current flow in a specific object or component, dependent on its material (resistivity), length, and cross-sectional area.
Example:
A long, thin nichrome wire used in a toaster has high resistance, causing it to heat up and glow when current passes through it.
Resistivity (ρ)
Resistivity is an intrinsic material property that quantifies its opposition to the flow of electric current, independent of its shape or size. It determines how easily a material conducts electricity.
Example:
Silver has a very low resistivity, making it an excellent conductor, which is why it's sometimes used in high-performance electrical contacts.
Series Resistors
Resistors connected end-to-end in a single path, where the same current flows through each, and their total equivalent resistance is the sum of their individual resistances.
Example:
If you connect multiple decorative lights in series resistors, and one bulb burns out, the entire string will go dark because the circuit is broken.
Terminal Voltage (Vₜ)
Terminal voltage is the actual voltage available across the terminals of a real battery or power source when current is being drawn from it, which is less than the EMF due to internal resistance.
Example:
When you measure the voltage across a battery while it's actively powering a flashlight, you are reading its terminal voltage, which will be slightly lower than its stated EMF.