Electric Cells Section 5.3 (p. 217 – 226)

(chemical) Cells i.e. batteries Direct current (DC) devices Electron flow = leave negative side, re-enter positive terminal Conventional current = positive terminal  negative terminal Higher potential = positive terminal Primary Cell Internal cell Used until power supply is exhausted Secondary Cell Rechargeable When exhausted, charger reverses chemical reaction to recreate original chemical composition

Chemistry connection: What are the chemicals used in a primary cell dry-cell or battery? Which is associated with the positive terminal and which is associated with the negative terminal? What is different about a rechargeable battery? Email me your answers, with citation of your source(s), by Wednesday morning (9:55 AM), for up to 3 assignment e.c. points

Cell Capacity If two cells have the same chemical composition, they will each be able to generate the same emf (electromotive force) for a circuit. Capacity: the measure of the ability of a cell to release its charge. High rate of discharge = short-lasting cell Low rate of discharge = long-lasting cell Determined by the constant current that can be supplied during discharge Units = Amp-hours (Ah)

Lab set-up Select someone in the class to set up a scenario such as is found on page 221 of your textbook. Use a LabQuest, a 1.5 V battery, a light-bulb, and a Vernier Voltage Sensor. We will be leaving this set up and collecting data overnight. (set the data collection accordingly—for about 30 hours, with data collected every minute or so) If you are not the one setting it up, you need to check on it. Journal entry for everyone: Sketch a circuit diagram of the set-up, and predict what the graph of terminal voltage as a function of time will look like for the next 26 hours. Leave space for a sketch of the actual results.

eMF Electromotive Force: Terminal Voltage: the open circuit potential difference across the terminals of a power source The terminal voltage when no current is supplied The energy per unit charge made available (supplied) by the source Terminal Voltage: The potential difference measured across the terminals of a cell or battery

Internal Resistance 𝜀=𝑉+𝐼𝑟 How would internal resistance affect the emf of a cell? How would internal resistance affect the terminal voltage of a cell? Internal Resistance Present in all electrical cells, to some extent Consumes some of the potential from the eMF, so the terminal voltage will decrease when current is running through a circuit. eMF = terminal voltage + potential drop across internal resistance or 𝜀=𝐼(𝑅+𝑟) 𝜀=𝑉+𝐼𝑟

Packet q #6 In the circuit below an electrical device (load) is connected in series with a cell of emf 2.5 V and internal resistance r. The current in the circuit is 0.10 A. The power dissipated in the load is 0.23 W Calculate: The total power of the cell Resistance of the load Internal resistance of the cell

Q6, continued A second identical cell is connected into the circuit as shown below The current in this circuit is 0.15 A. Deduce that the load is a non-ohmic device.