Current, Voltage, and Resistance
Define current, potential difference, resistance, and Ohm law. Interpret circuit symbols and simple circuit measurements.
A lesson of Physics, in Electricity and Magnetism, about 10 minutes.
By the end of this lesson, you will be able to define current, voltage, and resistance; apply Ohm's Law; and calculate power in electrical circuits.
Electric Current
Current ($I$): the rate of charge flow.
$$I = \frac{\Delta q}{\Delta t}$$
Units: Ampere (A) = C/s.
Conventional current: defined as the direction positive charges would flow (opposite to electron flow).
Voltage (Potential Difference)
Voltage ($V$): the energy per unit charge between two points.
$$V = \frac{W}{q}$$
Units: Volt (V) = J/C.
A battery acts like a "charge pump" — it does work to move charges from low to high potential, maintaining a potential difference.
Resistance
Resistance ($R$): opposition to current flow.
Units: Ohm (Ω) = V/A.
Ohm's Law:
$$V = IR$$
Not all devices obey Ohm's law (only ohmic devices like resistors).
Resistivity: $R = \rho L/A$ where $\rho$ = resistivity of material, $L$ = length, $A$ = cross-sectional area.
Power in Electric Circuits
$$P = IV = \frac{V^2}{R} = I^2R$$
Units: Watts (W).
Energy dissipated: $E = Pt = IVt$
Example 1 — Ohm's Law
A 60 Ω resistor has 12 V across it. Find current and power.
$I = V/R = 12/60 = 0.20$ A
$P = IV = 0.20 \times 12 = 2.4$ W
Example 2 — Household Electricity
A 100 W light bulb at 120 V. Find resistance and current.
$I = P/V = 100/120 = 0.83$ A
$R = V/I = 120/0.83 = 144$ Ω
Summary
Current: $I = \Delta q/\Delta t$; units A.
Voltage: energy per charge; units V.
Ohm's Law: $V = IR$; linear relationship for ohmic devices.
Power: $P = IV = V^2/R = I^2R$; units W.