It is integral of electromotive intensity both electric and non-electric around the circuit. It is called force for lack of better term and for historical reasons. Minus 1 for effort. How is it related to potential difference? Add a comment. Active Oldest Votes. Improve this answer.
Featured on Meta. Now live: A fully responsive profile. The emf source acts as a charge pump, moving negative charges from the positive terminal to the negative terminal to maintain the potential difference.
This increases the potential energy of the charges and, therefore, the electric potential of the charges. In order for the negative charges to be moved to the negative terminal, work must be done on the negative charges. This requires energy, which comes from chemical reactions in the battery. The potential is kept high on the positive terminal and low on the negative terminal to maintain the potential difference between the two terminals. An ideal battery is an emf source that maintains a constant terminal voltage, independent of the current between the two terminals.
An ideal battery has no internal resistance, and the terminal voltage is equal to the emf of the battery. In the next section, we will show that a real battery does have internal resistance and the terminal voltage is always less than the emf of the battery. The combination of chemicals and the makeup of the terminals in a battery determine its emf. The lead acid battery used in cars and other vehicles is one of the most common combinations of chemicals. The cathode positive terminal of the cell is connected to a lead oxide plate, whereas the anode negative terminal is connected to a lead plate.
Both plates are immersed in sulfuric acid, the electrolyte for the system. Knowing a little about how the chemicals in a lead-acid battery interact helps in understanding the potential created by the battery. Two electrons are placed on the anode , making it negative, provided that the cathode supplies 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 does 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.
The amount of resistance to the flow of current within the voltage source is called the internal resistance. The internal resistance r of a battery can behave in complex ways. Most popular tags between explain describe define difference mean differentiate water compounds account different meant types characteristics organic reaction example formation process carbon human examples compound acid formula salts justify plants computer acids equilibrium reactions formed does electric uses effects sound simple give breathing functional structure energy circuit potential prepared light group note system present reflection pollution convex waves kidney soluble produced state alkanes alkenes charge discuss salt importance petroleum principle advantages prepare working reproduction used internet truth pendulum ethene carbohydrates lewis hydrocarbons sources understand vegetative found point components coordination reasons information alcohol proteins diagram concave presence important normal necessary body dioxide through.
Send feedback. Powered by Question2Answer. Email me at this address if a comment is added after mine: Email me if a comment is added after mine.
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. A battery is a multiple connection of voltaic cells, as shown in Figure 9.
The disadvantage of series connections of cells is that their internal resistances add. One of the authors once owned a MGA that had two 6-V batteries in series, rather than a single V battery. This arrangement produced a large internal resistance that caused him many problems in starting the engine. Figure 8. A series connection of two voltage sources. Figure 9. Batteries are multiple connections of individual cells, as shown in this modern rendition of an old print.
0コメント