PhysicsChapter 11: Electricity

Current, Voltage, and Resistance Review

Current, potential difference and resistance describe different features of a circuit. Current measures charge flow, potential difference measures energy transferred per coulomb, and resistance measures how strongly a component opposes current.

Current

Key equation

I=Qt

I: current (A)
Q: charge (C)
t: time (s)

One ampere means one coulomb of charge passes a point each second. An ammeter measures current and is connected in series.

Potential difference

Key equation

V=WQ

V: potential difference (V)
W: energy transferred (J)
Q: charge (C)

One volt means one joule of energy is transferred per coulomb. A voltmeter compares two points and is connected in parallel.

Resistance

Key equation

R=VI

R: resistance (Ω)
V: potential difference (V)
I: current (A)

Key equation

R=ρLA

Resistance depends on material, length, cross-sectional area and temperature. For an ohmic conductor at constant temperature, V is directly proportional to I.

Circuit comparison

Series: the same current passes through every component; potential differences add.
Parallel: every branch has the same potential difference; branch currents add at a junction.
Ammeter: series connection and very small resistance.
Voltmeter: parallel connection and very large resistance.

Quick solved example

A resistor carries 0.50 A when 6.0 V is applied:

Key equation

R=VI=6.00.50=12\ Ω

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