Physics: An Introduction Chapter 21 -- Reference Tools and Resources

Chapter 21: Electric Currents and Direct-Current Circuits
Reference Tools and Resources

Reference Tools & Resources

I. Key Terms and Phrases

electric current: results from the flow of electric charge.

electric circuits: closed paths containing circuit elements through which current can flow.

direct current (DC) circuits: circuits in which the current always flows in one direction.

electric battery: a device, that maintains a potential difference, used as an energy source for electric circuits.

electromotive force (emf): the potential difference across an ideal battery.

resistance: the opposition to the flow of charge through a wire due to the properties of the wire.

ohm: the SI unit of resistance.

resistivity: the property of a substance that partially determines the resistance of objects made of that substance.

connected in series: circuit elements connected one after another such that the same current flows through them.

connected in parallel: circuit elements connected across the same potential difference.

Krichhoff's rules: two rules that apply the conservation of charge (junction rule) and the conservation of energy (loop rule) to electric circuits.

junction: a point in a circuit where three or more wires meet so that the current in the circuit may take different paths into or out of this point.

loop: any closed path in a circuit.

time constant: the characteristic amount of time, t = RC, for an initially uncharged capacitor to charge up to 63.2% of its maximum value in an RC circuit.

ammeter: a device designed to measure the current through a circuit element.

voltmeter: a device designed to measure the potential difference across a circuit element.

II. Important Equations

Name/Topic Equation Explanation
Ohm's law V = IR The relationship between potential difference, current, and resistance.

resistivity How the resistance of a wire depends on the properties of the wire.

electrical power P = IV = I²R = V²/R The electrical power transformed by a device in a circuit.

resistors in series How resistors in series combine.

resistors in parallel How resistors in parallel combine.

capacitors in parallel How capacitors in parallel combine.

capacitors in series How capacitors in series combine.

Name/Topic	Equation	Explanation
Ohm's law	V = IR	The relationship between potential difference, current, and resistance.
resistivity		How the resistance of a wire depends on the properties of the wire.
electrical power	P = IV = I²R = V²/R	The electrical power transformed by a device in a circuit.
resistors in series		How resistors in series combine.
resistors in parallel		How resistors in parallel combine.
capacitors in parallel		How capacitors in parallel combine.
capacitors in series		How capacitors in series combine.

III. Know Your Units

Quantity Dimension SI Unit
electric current (I) [A] A

resistance (R) [M][L²][A^-2][T^-3] W

resistivity () [M][L³][A^-2][T^-3]

Quantity	Dimension	SI Unit
electric current (I)	[A]	A
resistance (R)	[M][L²][A^-2][T^-3]	W
resistivity ()	[M][L³][A^-2][T^-3]