Physics HL · Chapter 11: Current and Circuits
11.2 Voltage, Power, and EMF
Track electrical energy transfer rates and distinguish terminal voltage from source emf.
Estimated time: 32 minutes
Voltage Across Components and Loop Accounting
In a simple loop, the source raises potential and resistors lower potential. Across ideal wires, potential drop is approximately zero. This means measured potential differences appear mainly across elements with significant resistance or active source behavior, which is why resistor and component terminals are where voltmeters matter most.
When two components are in series, the source emf is partitioned among them by current and resistance. The same current flows through each series element, so larger resistance gets larger voltage drop. This partition logic is the precursor to potential-divider analysis later in the chapter.
Power as Rate of Electrical Energy Transfer
These equivalent forms are valid when symbols refer to the same component state.
Power tells you how fast electrical energy is converted by a device. In a resistor this appears mostly as thermal transfer. In a lamp, part appears as light and part as thermal transfer. In a motor, part appears as mechanical output and part as thermal losses. Units are joules per second, i.e. watts.
Rated values matter: a lamp labeled 60 W at 220 V reaches that power only at 220 V. If applied voltage changes, operating current and power change. Assuming fixed resistance gives useful first estimates, but real filament temperature changes can shift resistance too.
EMF as Nonelectrical Work per Unit Charge
Emf is set by source mechanism (chemical, photovoltaic, mechanical), not by resistor arrangement alone.
A battery uses chemical processes to separate charge and maintain potential difference between terminals. That process does non-electrical work inside the source. Emf quantifies this per coulomb energy lift available before external circuit drops and internal losses are considered.
In idealized models, emf equals terminal voltage. In real sources, internal resistance causes part of the emf to be consumed inside the source when current flows. The terminal voltage then falls below emf by the internal drop Ir, which becomes central in Section 11.4.
Simulation: Source Power and EMF Efficiency
Use source mode to vary emf, internal resistance, and load, then track terminal voltage, load power, and internal heating losses.
Current
1.111 A
Terminal voltage
11.111 V
Load power
12.346 W
Efficiency
92.6 %
Vout (no load)
7.143 V
Vout (loaded)
6.033 V
Divider branch I
8.78e-3 A
Load current
2.74e-3 A
Terminal model follows V = emf - I r. Divider model compares open-circuit Vout to loaded Vout so you can see why low-output-resistance dividers are needed when the next stage draws current.
Test Yourself
A light bulb is rated 60 W at 220 V. Assuming constant resistance, enter its power when connected to 110 V.
Hint: For fixed R, use P proportional to V^2.