21.2 Electromotive Force: Terminal Voltage

21.2 Electromotive Force: Terminal Voltage

• A clear circuit diagram can be drawn.
• A list of knowns for the problem is included in this step.

• Identifying the unknowns will help determine exactly what needs to be determined in the problem.
• A written list can be useful.

• Determine if the resistors are in series, parallel, or a combination of both.
• This assessment can be made by examining the circuit diagram.
• If the same current must pass through them, they are in series.

• To solve unknowns, use the appropriate list of major features.
• There are two lists for series and parallel.
• If your problem has a combination of series and parallel, you can reduce it in steps by considering individual groups of series or parallel connections.

• The reciprocal must be taken care of when finding.

• Check to see if the answers are reasonable.
• Units and numerical results have to be reasonable.
• For example, the total parallel resistance should be smaller.
• In parallel, power should be greater for the same devices.

• As the battery runs down, your car lights dim as you forget to turn them off.
• As the battery depletes, the battery output voltage decreases.

• Even if the wires to the lights have low resistance, they will be dim even if the battery is fresh.
• This means that the battery's output voltage is reduced.

• There are many different types of voltage sources.
• There are many different types of batteries.
• There are many different types of mechanical/electrical generators.
• Both solar cells and thermoelectric devices create voltage from temperature differences.

• The electric field is caused by the potential difference on the small scale.

• Emf is a type of potential difference that is not a force.
• When no current is flowing, the emf is the potential difference of a source.
• The emf are units of electricity.

• The source of potential difference is related tomotive force.
• As the battery depletes or is loaded down, the voltage across the terminals of the battery is less than the emf.
• If the device's output voltage can be measured without drawing current, then it will equal emf.

• A 12-V truck battery contains more charge and energy and can deliver a larger current than a 12-V motorcycle battery.
• The lead-acid batteries have the same emf, but the truck battery has a smaller internal resistance.
• The source's resistance to the flow of current is inherent.

• Figure 21.9 is a representation of the two fundamental parts of a voltage source.
• The emf is represented by a script E in the figure.
• The bigger the internal resistance, the more power the source can supply.

• A carbon-zinc dry cell has an emf related to its source of potential difference and an internal resistance related to its construction.
• The output terminals are where the terminal voltage is measured.
• If there is no current flowing, terminal voltage is equal to emf.

• The resistance can behave in many different ways.
• As a battery depletes, the increase increases.
• The magnitude and direction of the current through a voltage source, its temperature, and even its history can all affect internal resistance.
• The internal resistance of nickel-cadmium cells depends on how many times they have been used.

• The emfs of different types of batteries are determined by the combination of chemicals.
• We can see this as a reaction that separates charge.

• The lead-acid battery is used in cars and other vehicles.
• The positive terminal of the cell is connected to a lead oxide plate, while the negative terminal is connected to a lead plate.
• The plates are immersed in acid.

• The negative charge is sent to the anode, which is connected to the lead plates.
• The lead oxide plates are connected to the cell.

• Sulfuric acid is involved in the chemical reaction.

• The results of the chemical reaction are left to the reader to study in a chemistry text, but they help explain the potential created by the battery.
• If there are two electrons on the anode, it will be negative.
• The cathode has lost two electrons.
• A separation of charge has been driven by a chemical reaction.

• If there is a complete circuit that allows two electrons to be supplied to the cathode, the reaction will not take place.
• The electrons come from the anode and go through a resistance to return to the cathode.
• Since the chemical reactions involve substances with resistance, it is not possible to create the emf without an internal resistance.

• An artist's conception of two electrons being forced onto the anode of a cell and two electrons being removed from the cathode of the cell.
• A lead-acid battery has two electrons on the anode and two on the cathode.

• Since the two electrons must be supplied to the cathode, it requires a closed circuit to proceed.

• The energy states of the atoms and electrons in a molecule can be predicted with the help of quantum mechanical descriptions.

• An energy of 2 eV is given to each electron sent to the anode in a lead-acid battery.
• The electrical potential energy is divided by charge.
• The energy given to a single electron is called an electron volts.
• The energy produced in each reaction is what produces the voltage.
• A different reaction produces a different energy.

• The current is flowing at the time of the measurement.

• The bigger the current, the smaller the terminal voltage.
• The bigger the internal resistance, the smaller the terminal voltage.

• The total resistance in the circuit is because the resistances are in series.

• The terminal voltage and current delivered to the load can be affected by the internal resistance.

• The expression shows that the smaller the internal resistance, the greater the current the source supplies to its load.
• As batteries deplete, the increase increases.
• The current is reduced if the load resistance becomes a significant fraction.

• The internal resistance of the battery is 12.0-V emf.

• When internal resistance is taken into account, the analysis gave an expression for current.
• The terminal voltage can be calculated using the equation once the current is found.
• The power dissipated by a Resistor can be found once current is found.

• The terminal voltage here is slightly lower than the emf, which is a sign of a light load for this particular battery.

• The reduction in the terminal voltage is more significant than the reduction in emf.

• The formula can be used to find the dissipated power.

• The power can be obtained using the expressions or where is the terminal voltage.

• The internal resistance has increased to the point where it is comparable to the load resistance.

• Increased internal resistance has resulted in a decrease in terminal voltage, current, and power delivered to a load.

• The internal resistance of the battery is tested.
• The battery is weak if its internal resistance is high.

• Two battery testers measure terminal voltage under a load to determine the condition of a battery.
• The large device is being used by a Navy electronics technician to test large batteries on the aircraft carrier.

• Some batteries can be recharged by passing a current through them in the opposite direction to the resistance.
• The battery's emf must be greater than the battery's voltage to reverse current through it.
• The terminal voltage of the battery will be greater than the emf because of this.

• A car battery charge reverses the normal direction of current through the battery and replenishes its chemical potential.

• When a battery charger is used, there are two different sources of power.
• It is relatively easy to connect voltage sources in a series.
• The internal resistances and emfs of the voltage sources add and subtract.
• In toys and other appliances, series connections of voltage sources are common.
• In order to produce a larger total emf, the cells are usually in series.

• Since it is the sum of the individual emfs, the total emf is less if the cells oppose one another.

• The internal resistances of cells add to the disadvantage of series connections.
• One of the authors once owned a 1957 MGA that had two 6-V batteries in series instead of a single 12-V battery.
• The arrangement made it difficult for him to start the engine.

• The emfs are labeled with a script E and give a total emf of and a total internal resistance.

• There are multiple connections of individual cells in batteries, as shown in this modern rendition of an old print.
• The AA or C cells are not batteries.

• The emfs are connected in opposition to the two voltage sources.
• Current flows in the direction of the greater emf and is limited to the sum of the internal resistances.
• There is a connection between a battery and a battery charger.
• The battery must have a larger emf to reverse current through it.

• This schematic shows a flashlight with two cells and a single bulb.

• New and old batteries can be found if you find a flashlight that uses several batteries.
• Predict the brightness of the flashlight when different combinations of batteries are used based on the discussions in this module.
• Put new batteries in the flashlight and leave it on for a while.
• The same thing should be done with the old batteries.

• Two voltage sources with the same emfs are connected to a load resistance.
• The total emf is the same as the individual emfs.
• Since the internal resistances are in parallel, the total internal resistance is reduced.
• A larger current can be produced by the parallel connection.

• The load is less than the individual batteries.
• Some diesel powered cars use two 12-V batteries in parallel; they produce a total emf of 12 V but can deliver the larger current needed to start a diesel engine.

• Two voltage sources with the same emf but with different total internal resistance are connected in parallel.
• More current is often delivered by parallel combinations.
• Here moves through the load.

• Several animals produce and detect electrical signals.
• Electric fields generated by prey are detected by fish, sharks, and platypuses.
• Electric eels produce their own emf through biological cells, which are arranged in both series and parallel as a set of batteries.

• The electric eel has a voltage of 0.15 V across each of its flat, disk-like cells.
• In the case of the electric eel, the cells are found along the entire body.
• The South American Eel has 140 rows with each row stretching along the body and holding 5,000 electroplaques.
• This can produce an emf of 600 V and a current of 1 A--deadly.

• The mechanism for detecting external electric fields is similar to that for producing nerve signals in the cell through depolarization and repolarization.
• Weak electric fields in the water produce a current in a gel-filled canal that runs from the skin to the cells.
• The Australian platypus, one of the few mammals that lay eggs, can detect fields of 30 while sharks can sense a field in their snouts as small as 100.
• Electric eels use their own electric fields to stun their prey or enemies.

• Sand tiger sharks use their snouts to locate their prey.