Electromotive force (EMF, symbol ε) is the energy per unit charge that a source like a battery or generator converts from another form (chemical, mechanical) into electrical energy, measured in volts. Despite the name, it's not a force; it's the ideal voltage of a source before internal resistance takes its cut.
Electromotive force (EMF) is the voltage a source would deliver if it were perfect. A battery converts chemical energy into electrical energy; a generator converts mechanical energy. EMF measures how many joules of energy the source gives to each coulomb of charge passing through it, so its unit is the volt (J/C). The symbol is ε (epsilon), and the worst-named quantity in physics is not a force at all. It's an energy-per-charge quantity, exactly like potential difference.
Here's the catch that makes EMF exam-relevant rather than just vocabulary. Real sources have internal resistance (r), so some of that energy gets dissipated inside the source itself when current flows. The voltage you actually measure across the battery's terminals is the terminal voltage, V = ε − Ir. EMF is the full paycheck; terminal voltage is what's left after the battery pays its own internal 'tax.' When no current flows (I = 0), terminal voltage equals EMF, which is why a voltmeter across a disconnected battery reads the EMF.
EMF lives in Topic 4.2 (Resistivity and Resistance) in AP Physics 2, where you build realistic circuit models that include internal resistance instead of treating batteries as ideal. Once you write ε = I(R + r), a whole family of problems opens up. You can explain why a battery's terminal voltage sags under load, why a 'dead' battery can still read 1.5 V on a voltmeter, and how to find r from a graph of terminal voltage versus current. EMF is also the energy-conservation backbone of Kirchhoff's loop rule, since going around a loop, the energy per charge gained from EMF sources equals the energy per charge dissipated in resistors. Later, in electromagnetism, EMF returns as induced EMF from changing magnetic flux, so the concept literally bridges the circuits unit and the magnetism unit.
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Internal Resistance (Unit 4)
EMF and internal resistance are a package deal. The equation V = ε − Ir says terminal voltage drops below EMF whenever current flows, and the slope of a terminal-voltage-vs-current graph gives you r while the y-intercept gives you ε. That graph is a classic AP setup.
Battery (Unit 4)
A battery is the textbook EMF source. It does work on charges using chemical energy, lifting them from low to high potential inside the cell. Model a real battery as an ideal EMF in series with a small internal resistor.
Generator (Unit 5)
A generator produces EMF mechanically instead of chemically. A coil rotating in a magnetic field has changing flux through it, which induces an EMF (Faraday's law). Same energy-per-charge idea, totally different energy source, which is exactly the kind of cross-unit connection AP loves.
Volt (Unit 4)
EMF, terminal voltage, and potential difference all share the volt as their unit because all three measure energy per charge (J/C). The difference is what's doing the measuring. EMF describes energy given by a source, while potential difference describes energy gained or lost between two points.
EMF shows up in circuit analysis whenever the problem says a battery has internal resistance, or gives you both an EMF value and a terminal voltage and expects you to notice they're different. Common multiple-choice stems ask what happens to terminal voltage as more bulbs are added in parallel (current rises, so V = ε − Ir falls), or what a voltmeter reads across a battery in an open circuit (the EMF itself). On free-response, you'll typically apply Kirchhoff's loop rule with ε = I(R + r), interpret a graph of terminal voltage versus current to extract ε and r, or justify in words why a battery delivers less than its rated voltage under load. The skill being tested is energy accounting per charge around a loop, not memorizing the definition.
EMF is the ideal energy per charge the source supplies; terminal voltage is what actually appears across the battery's terminals once internal resistance eats some of it. They're related by V = ε − Ir, so they're only equal when I = 0 (open circuit). If a problem treats the battery as ideal (r = 0), the distinction vanishes, but the moment internal resistance appears, mixing them up is the number-one way to lose circuit points.
EMF (ε) is the energy per unit charge a source converts into electrical energy, measured in volts, and despite its name it is not a force.
Terminal voltage equals EMF minus the drop across internal resistance, V = ε − Ir, so a battery delivers its full EMF only when no current flows.
For a complete circuit, ε = I(R + r), which is just Kirchhoff's loop rule applied to a real battery driving an external resistance R.
On a graph of terminal voltage versus current, the y-intercept is the EMF and the magnitude of the slope is the internal resistance.
Batteries produce EMF from chemical energy and generators produce EMF from mechanical energy via changing magnetic flux, so the same concept links Unit 4 circuits to electromagnetism.
EMF is the energy per unit charge that a source such as a battery or generator converts from chemical or mechanical energy into electrical energy. It's measured in volts and represents the ideal voltage of the source before internal resistance reduces it.
No. The name is a historical leftover. EMF is measured in volts (joules per coulomb), making it an energy-per-charge quantity like potential difference, not a force measured in newtons.
EMF is the source's ideal voltage; terminal voltage is what you actually measure across its terminals while current flows. They're connected by V = ε − Ir, so a battery with ε = 9 V and r = 0.5 Ω delivering 2 A only shows 8 V at its terminals.
Because current flowing through the battery's internal resistance dissipates some energy inside the battery itself. The bigger the current drawn, the larger the Ir drop, so terminal voltage sags further below the EMF.
Only when no current flows, meaning an open circuit. That's why an ideal voltmeter (which draws essentially zero current) placed across a disconnected battery reads the EMF directly.