There are many types of mechanical/electrical generators, driven by many different energy sources, ranging from nuclear to wind.

As before, we first find the current by entering the known values into the expression, yielding \[I = \dfrac{emf}{R_{load} + r} = \dfrac{12.0 \, V}{1.00 \, \Omega} = 12.0 \, A.\], Now the terminal voltage is \[V = emf - Ir = 12.0 \, V - (12.0 \, A)(0,500 \, \Omega)\]\[= 6.00 \, V,\] and the power dissipated by the load is \[P_{load} = I^2R_{load} = (12.0 \, A)^2(0.500 \, \Omega) = 72.0 \, W.\]. What are examples of 1st, 2nd, and 3d class levers? Suppose you were told that you needed only 18 W (but no required voltage). Assume each card has an output of 0.5 V and a current (under bright light) of 2 A. Two electrons are placed on the anode, making it negative, provided that the cathode supplied two electrons. TAKE-HOME EXPERIMENT: VIRTUAL SOLAR CELLS. All such devices create a potential difference and can supply current if connected to a resistance. Electromotive force is defined as the electric potential produced by either electrochemical cell or by changing the magnetic field. See Figure \(\PageIndex{10}\), for example, which shows a circuit exactly analogous to the battery charger discussed above. (See Figure \(\PageIndex{8}\).) Quantum mechanical descriptions of molecules, which take into account the types of atoms and numbers of electrons in them, are able to predict the energy states they can have and the energies of reactions between them. They really test the internal resistance of the battery. Do the same with the old batteries. What are some examples of Newton’s third law?

Some batteries can be recharged by passing a current through them in the direction opposite to the current they supply to a resistance. This terminal voltage exhibits a more significant reduction compared with emf, implying \(0.500 \, \Omega\) is a heavy load for this battery. If internal resistance is high, the battery is weak, as evidenced by its low terminal voltage. We see from this expression that the smaller the internal resistance \(r\), the greater the current the voltage source supplies to its load \(R_{load}\). One of the authors once owned a 1957 MGA that had two 6-V batteries in series, rather than a single 12-V battery.

The voltage across the terminals of a battery, for example, is less than the emf when the battery supplies current, and it declines further as the battery is depleted or loaded down. The details of the chemical reaction are left to the reader to pursue in a chemistry text, but their results at the molecular level help explain the potential created by the battery. Electromotive force is directly related to the source of potential difference, such as the particular combination of chemicals in a battery. In this simple case, the total emf is the same as the individual emfs. Units of emf are volts. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. An electron volt is the energy given to a single electron by a voltage of 1 V. So the voltage here is 2 V, since 2 eV is given to each electron. Both plates are immersed in sulfuric acid, the electrolyte for the system. Their gradual dimming implies that battery output voltage decreases as the battery is depleted.

The internal resistance \(r\) of a voltage source affects the output voltage when a current flows. This implies that the battery’s output voltage is reduced by the overload. The potential difference across the resistor is measured as 2.5 V. The voltage output of a device is called its terminal voltage \(V\) and is given by \(V = emf - Ir\), where \(I\) is the electric current and is positive when flowing away from the positive terminal of the voltage source. >> Now let us look at some of the basic components and relationships of magnetic fields. The reason for the decrease in output voltage for depleted or overloaded batteries is that all voltage sources have two fundamental parts—a source of electrical energy and an internal resistance. The parallel connection thus can produce a larger current. (d) If the internal resistance grows to \(0.500 \, \Omega\) find the current, terminal voltage, and power dissipated by a \(0.500 \, \Omega\) load. One can assemble a “virtual” solar cell array by using playing cards, or business or index cards, to represent a solar cell. However, if the device’s output voltage can be measured without drawing current, then output voltage will equal emf (even for a very depleted battery).

Electric eels use their own electric fields produced by the electroplaques to stun their prey or enemies. A solar-cell array or module usually consists of between 36 and 72 cells, with a power output of 50 W to 140 W. The output of the solar cells is direct current. For most uses in a home, AC is required, so a device called an inverter must be used to convert the DC to AC. The numerical value of the emf depends on the source of potential difference. Both are lead-acid batteries with identical emf, but, because of its size, the truck battery has a smaller internal resistance \(r\). Under bright noon sunlight, a current of about \(100 \, mA/cm^2\) of cell surface area is produced by typical single-crystal cells. You can think of many different types of voltage sources. Voltage is defined as the electrical potential energy divided by charge: \(V = \frac{P_E}{q}\). 21.2: Electromotive Force - Terminal Voltage, [ "article:topic", "authorname:openstax", "electromotive force (emf)", "internal resistance", "potential difference", "terminal voltage", "electromotive force", "emf", "license:ccby", "showtoc:no" ], terminal voltage \(V\). Once the current is found, the terminal voltage can be calculated using the equation \(V = emf - Ir\). Terminal voltage is given by \[V = emf - Ir,\] where \(r\) is the internal resistance and \(I\), Creative Commons Attribution License (by 4.0). Electromotive force is directly related to the source of potential difference, such as the particular combination of chemicals in a battery. Another example dealing with multiple voltage sources is that of combinations of solar cells—wired in both series and parallel combinations to yield a desired voltage and current. Using your cards, how would you arrange them to produce an output of 6 A at 3 V (18 W)?

The voltage output of the battery charger must be greater than the emf of the battery to reverse current through it. If the series connection of two voltage sources is made into a complete circuit with the emfs in opposition, then a current of magnitude \(I = \frac{(emf_1 - emf_2)}{r_1 + r_2}\) flows.

The power dissipated by the \(0.500 \, \Omega\) load can be found using the formula \(P = I^2 R\). Figure \(\PageIndex{2}\) is a schematic representation of the two fundamental parts of any voltage source. The voltage output of a device is measured across its terminals and, thus, is called its terminal voltage \(V\). The analysis above gave an expression for current when internal resistance is taken into account. Despite its name, the electromotive force is not actually a force.