Glossary
Ammeter
An instrument used to measure the electric current flowing through a point in a circuit. It must be connected in series with the path of current and has very low internal resistance.
Example:
To measure the total current leaving a battery, you would insert an ammeter directly into the main circuit path.
Circuits
An interconnected path that allows electric current to flow, typically comprising components like resistors, batteries, and wires.
Example:
A flashlight uses a simple circuit where a battery powers a bulb through a switch.
Conventional Current
The direction in which positive charge would flow, from higher potential to lower potential. This is the standard direction used in circuit analysis, even though electrons are the actual mobile charge carriers.
Example:
When analyzing a simple circuit, we assume conventional current flows out of the positive terminal of the battery and into the negative terminal.
Current (I)
The rate of flow of electric charge through a conductor, measured in Amperes (A). It indicates how much charge passes a point per unit time.
Example:
When you turn on a light switch, current flows through the bulb's filament, causing it to glow.
Current Density (J)
A vector quantity representing the amount of current flowing per unit cross-sectional area of a conductor. It is related to the electric field and material conductivity.
Example:
In a thick wire carrying a certain current, the current density will be lower than in a thin wire carrying the same current, assuming uniform flow.
Drift Velocity (v_d)
The average velocity attained by charged particles, such as electrons, in a material due to an electric field. It is typically very slow, even though electric signals propagate quickly.
Example:
Despite electricity appearing instantaneous, the drift velocity of electrons in a copper wire powering a light bulb is only a few millimeters per second.
Electric Power (P)
The rate at which electrical energy is converted or dissipated in a circuit component, measured in Watts (W). It can be calculated as the product of voltage and current (P=IV).
Example:
A 100-watt light bulb consumes electric power at a rate of 100 joules per second when lit.
Non-Ohmic Device
A device or material that does not obey Ohm's Law, meaning its resistance changes with the applied voltage or current. Its V-I graph is non-linear.
Example:
A diode is a non-Ohmic device because its resistance changes drastically depending on the direction and magnitude of the applied voltage.
Ohm's Law
A fundamental law stating that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them (V=IR).
Example:
If a 12V car battery is connected to a 4Ω headlight, Ohm's Law predicts a current of 3 Amperes will flow through the headlight.
Ohmic Device
A device or material that obeys Ohm's Law, meaning its resistance remains constant regardless of the applied voltage or current. Its V-I graph is a straight line through the origin.
Example:
A standard carbon-composition resistor is an Ohmic device because its resistance value doesn't significantly change with the voltage applied across it.
Parallel Circuit
A circuit configuration where components are connected across the same two points, providing multiple paths for current flow. The voltage across each parallel component is the same.
Example:
Most household electrical outlets are wired in a parallel circuit, allowing multiple appliances to receive the full line voltage independently.
Resistance (R)
The opposition a material offers to the flow of electric current, measured in Ohms (Ω). It quantifies how much a component impedes charge movement.
Example:
A toaster's heating element has high resistance, which converts electrical energy into heat to toast bread.
Resistivity (ρ)
An intrinsic material property that quantifies how strongly a given material opposes the flow of electric current. It depends on the material type and temperature.
Example:
Copper has a very low resistivity, making it an excellent material for electrical wires, while rubber has a very high resistivity, making it an insulator.
Series Circuit
A circuit configuration where components are connected end-to-end along a single path, so the same current flows through each component.
Example:
Old Christmas lights were often wired in a series circuit; if one bulb burned out, the entire string would go dark because the circuit was broken.
Voltage (V)
The electric potential difference between two points in a circuit, representing the work done per unit charge to move a charge between those points. It is the 'push' that drives current.
Example:
A 9V battery provides a voltage of 9 volts, meaning it can do 9 joules of work for every coulomb of charge that moves through it.
Voltmeter
An instrument used to measure the electric potential difference (voltage) between two points in a circuit. It must be connected in parallel with the component being measured and has very high internal resistance.
Example:
To check the voltage drop across a resistor, you would connect a voltmeter in parallel across its terminals.