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•Cells may be connected together to form a battery:
Review: Electrochemical Cells
• A basic electrochemical cell is constructed from two dissimilar metals (we used Copper and Zinc) and a chemical solution called the electrolyte (we used soft drink)
• The electrolyte is typically a strong acid or base, but sometimes a concentrated solution of chemical salts can also be used
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Battery Cell
Review: Electrochemical Cells• Used to establish electric
current in a circuit• Transformation of
chemical energy stored in the cell to kinetic energy of the charge carriers (electrons)
• Two oppositely charged electrodes (terminals)
• An electrical potential difference (voltage) exists between the terminals
• Electric potential is potential energy per unit charge: Electirc Potential Energy = Charge x Voltage
EPE qV
Review: Cells in Series and Parallel
• A battery pack consists of four 1.2-volt cells in series (end-to-end)..
• The nominal voltage of the battery is the sum of the individual cell voltages = V × 4 = 4.8 V
• With parallel cells, the voltage stays the same = 1.2 V
• The amount of available charge increases → larger current
In series voltages add
In parallel current and capacity add
Review: Current• Definition: Current is the
amount of charge moving past a point per unit time
• Charge flows in a confined channel (wire) like a river
• Current is caused by potential differences (voltages)
• Charge is measured in Coulombs (C)
• The unit of current is the Ampere (A): 1 A = 1 C/s
“One Coulomb per second”
• 1 A = lots of moving charge (1 C = 6.25 x 1018 electrons)!
• Typical house: 200 A• Direct current (DC):
flows in one direction • Alternating current
(AC): flows back and forth
Ampere
Review: Cells, Batteries & Current
• When connected to a circuit, the potential difference between the cell or battery terminals creates an electric force on the charges in the conductor causing them to move and establishing an electric current
• The conventional current is from regions of higher potential to regions of lower potential, positive to negative
Conventional current
Review: Conventional Current and Electron Flow• Truth: the particles which
carry charge through wires in a circuit are mobile electrons
• Ben Franklin: positive charges move (oops!)
• The direction of current is by convention the direction in which a positive charge would move (so current flows from high potential to low potential)
Electron flow
Conventional Current
Good and Bad Wiring
• Good: • Bad:
• Ugly:
=
Broken!
Short!
No device!
• Power has a precise definition in physics:
Power is the rate at which work is done or how much work is done per unit of time
The Mechanical Definition of Power
WP
t
Units: J/s Watt (W)
James Watt invented the steam engine
In an electric circuit power is the product of current and voltage:
Power I V
Resistors
When an extension cord is used with a space heater, the cord must have a resistance that is sufficiently small to prevent overheating of the cord
Resistors are used in appliances to convert electrical energy into thermal energy (heat) or light
Stove Heating Element
Toaster
Space Heater Light Bulb
Series Resistors
• For resistors R1 & R2 connected in series (sequentially), the current i passing through each resistor must be the same
• The voltages across
R1 & R2 must add up
to V
V+
Parallel Resistors• Resistors R1 & R2
connected in parallel have the same potential difference (voltage) V across them
• Charge must be conserved, so the currents I1 & I2 flowing through the two resistors must add up to the total current I leaving the battery (continuity for electric current!)
1 2I I I
+V
