Why is voltage not a force

Various types of batteries are available, with emfs determined by the combination of chemicals involved. We can view this as a molecular reaction what much of chemistry is about that separates charge. The lead-acid battery used in cars and other vehicles is one of the most common types.

A single cell one of six of this battery is seen in Figure 3. The cathode positive terminal of the cell is connected to a lead oxide plate, while the anode negative terminal is connected to a lead plate.

Both plates are immersed in sulfuric acid, the electrolyte for the system. Figure 3. Chemical reactions in a lead-acid cell separate charge, sending negative charge to the anode, which is connected to the lead plates. The lead oxide plates are connected to the positive or cathode terminal of the cell. Sulfuric acid conducts the charge as well as participating in the chemical reaction.

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. Figure 4 shows the result of a single chemical reaction.

Two electrons are placed on the anode, making it negative, provided that the cathode supplied two electrons. This leaves the cathode positively charged, because it has lost two electrons. In short, a separation of charge has been driven by a chemical reaction.

Note that the reaction will not take place unless there is a complete circuit to allow two electrons to be supplied to the cathode. Under many circumstances, these electrons come from the anode, flow through a resistance, and return to the cathode. Note also that since the chemical reactions involve substances with resistance, it is not possible to create the emf without an internal resistance.

Figure 4. The chemical reaction in a lead-acid battery places two electrons on the anode and removes two from the cathode. It requires a closed circuit to proceed, since the two electrons must be supplied to the cathode. In the case of a lead-acid battery, an energy of 2 eV is given to each electron sent to the anode. Voltage is defined as the electrical potential energy divided by charge.

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. It is the energy produced in each molecular reaction that produces the voltage. A different reaction produces a different energy and, hence, a different voltage.

The voltage output of a device is measured across its terminals and, thus, is called its terminal voltage V. Terminal voltage is given by. I is positive if current flows away from the positive terminal, as shown in Figure 2. You can see that the larger the current, the smaller the terminal voltage.

And it is likewise true that the larger the internal resistance, the smaller the terminal voltage. Suppose a load resistance R load is connected to a voltage source, as in Figure 5. Figure 5.

Schematic of a voltage source and its load Rload. Since the internal resistance r is in series with the load, it can significantly affect the terminal voltage and current delivered to the load. 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.

As batteries are depleted, r increases. If r becomes a significant fraction of the load resistance, then the current is significantly reduced, as the following example illustrates. A certain battery has a The analysis above gave an expression for current when internal resistance is taken into account. Once current is found, the power dissipated by a resistor can also be found. Entering the given values for the emf, load resistance, and internal resistance into the expression above yields.

The terminal voltage here is only slightly lower than the emf, implying that This terminal voltage exhibits a more significant reduction compared with emf, implying 0. The power dissipated by the 0. Entering the known values gives. Here the internal resistance has increased, perhaps due to the depletion of the battery, to the point where it is as great as the load resistance.

As before, we first find the current by entering the known values into the expression, yielding. We see that the increased internal resistance has significantly decreased terminal voltage, current, and power delivered to a load. Battery testers, such as those in Figure 6, use small load resistors to intentionally draw current to determine whether the terminal voltage drops below an acceptable level.

They really test the internal resistance of the battery. If internal resistance is high, the battery is weak, as evidenced by its low terminal voltage. Figure 6. These two battery testers measure terminal voltage under a load to determine the condition of a battery. The large device is being used by a U. Navy electronics technician to test large batteries aboard the aircraft carrier USS Nimitz and has a small resistance that can dissipate large amounts of power.

Johnston The small device is used on small batteries and has a digital display to indicate the acceptability of their terminal voltage. Some batteries can be recharged by passing a current through them in the direction opposite to the current they supply to a resistance. This is done routinely in cars and batteries for small electrical appliances and electronic devices, and is represented pictorially in Figure 7.

The voltage output of the battery charger must be greater than the emf of the battery to reverse current through it. Figure 7. A car battery charger reverses the normal direction of current through a battery, reversing its chemical reaction and replenishing its chemical potential.

There are two voltage sources when a battery charger is used. Voltage sources connected in series are relatively simple. When voltage sources are in series, their internal resistances add and their emfs add algebraically. See Figure 8. Series connections of voltage sources are common—for example, in flashlights, toys, and other appliances. Usually, the cells are in series in order to produce a larger total emf. But if the cells oppose one another, such as when one is put into an appliance backward, the total emf is less, since it is the algebraic sum of the individual emfs.

The scientific symbol for voltage is an "E", dating to early days of electricity when it was called the "Electromotive force". Scientists and engineers use the "E" symbol for voltage, while electricians and wiring books use "V" as the voltage symbol. This can create some confusion, since either may be encountered. In this title, we'll use the practical symbol "V" for voltage. English Spanish. Potential difference and electric field are intertwined. If there is a potential difference in a circuit, then there is an electric field; if there is an electric field in a circuit, then there are forces acting on charged particles in the circuit.

Therefore, if there is a voltage in a circuit, then there might be a current the circuit needs to be complete before there is a current. Since voltage is the difference in electric potential energy per charge, we should measure it in terms of energy per charge — and we do. Explore the relationships between ideas about voltage in the Concept Development Maps — Electricity and Magnetism. Other textbooks do not even use the word at all. This is partly because the concept is a very difficult and completely abstract one.

However it is such a commonly used term that it must be part of teaching of these ideas at this level — concerns with its precision and value are much better left until specialist physics courses. As with the focus idea Electric circuits , quantitative teaching approaches can impede the development of conceptual understanding. At Level 6 these are still best avoided, or, if they must be introduced, used only after the development of understanding.

They certainly should not be the central aspect of assessment of student learning. A teaching focus on mathematical relationships leads to a very strong student learning focus on how concepts are linked from a logical, mathematical viewpoint; it is then very hard for students to explain why concepts are linked this way, what causes those linkages, and, most importantly at this level, why the science is used in the world in the ways it is.

An appropriate model is a cliff. An object at the edge of the top of a cliff has gravitational potential energy because of its position in the gravitational field. Now ask students to predict whether the globe will shine more brightly, less brightly or the same when the AA sized battery is replaced by a D size battery i. Have students in groups connect up a light globe with a battery so the globe is lit. As a whole class, discuss the responses and draw attention to how the models suggested by the groups are both consistent with and different from the electric circuit.