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Electric Cells
Section 5.3 (p. 217 – 226)
(chemical) Cells
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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:
• 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
• 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
𝜀 = 𝑉 + 𝐼𝑟
or
𝜀 = 𝐼(𝑅 + 𝑟)
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.
e.m.f. = 2.5V
• The power dissipated in the load is 0.23 W
• Calculate:
r
• The total power of the cell
I = 0.10A
• Resistance of the load
load
• Internal resistance of the cell
Q6, continued
• A second identical cell is connected into the circuit as shown
below
I = 0.15A
load
• The current in this circuit is 0.15 A. Deduce that the load is a
non-ohmic device.