Electric Circuits
Ohm's law, series-parallel circuits, internal resistance
Electric Circuits
Ohm's law: V = IR. In series: current is the same, voltages add up. In parallel: voltage is the same, currents add up. Internal resistance (r) reduces terminal voltage: V_terminal = EMF - Ir.
Example
Worked Example
Battery: EMF = 12 V, r = 0.5 ohm, connected to 5.5 ohm resistor. Total R = 6 ohm. I = EMF/R = 12/6 = 2 A. Terminal voltage: V = 12 - 2(0.5) = 11 V. Lost volts: Ir = 1 V.
Note
Exam Tip
Always account for internal resistance. Kirchhoff's current law: sum of currents entering a junction equals sum leaving. Kirchhoff's voltage law: sum of EMFs equals sum of voltage drops around a loop.
Key Vocabulary
Ohm's lawV = IR — voltage equals current times resistance
EMFElectromotive force — the total energy per unit charge supplied by a battery
Internal resistanceThe resistance within a battery that causes energy loss
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Ohm's law
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