More on simple battery circuits
Review
• A simple voltaic cell
diagram shown at right
• What direction do the
electrons flow? Do they
change at all?
• If the electrons only
flow in one direction,
this is a called a Direct
Current
Series circuits
• Batteries can be connected in series to increase
voltage
• For any circuit, the total voltage is the sum voltage
of the batteries connected in the series
More about Direct Current
• The current flowing from a battery through a
circuit only flows in ONE direction
• The current is described as Direct Current (or
DC) power
• Voltage of a battery diminishes as the
chemical reactions driving the battery run out
of reagents (run out of chemical potential)
What is happening in an electrical
circuit?
•
The reason electrons are
– there is a
First – what flowing
does “circuit”
positive charge at the
mean? cathode
they are attracted
to
– What is a circuit workout?
Here, kinetic energy of
electrons is put to work,
in this case powering a
light
– What is a racing circuit
– In an electrical circuit,
“charge” flows in a circular
pathway
• The energy of electrons
may be transformed
multiple ways throughout
their journey
Anode – where
electrons are
generated
This switch is
closed, meaning it
is connected and
charge is flowing
through it
Things that affect flow of charge
• Voltage – symbol is V (sometimes written as E)
• Amperage – symbol is I
• Ohms (resistance) – symbol is R (sometimes shown as
Ω)
Ohm’s law
• Ohm’s law relates how Voltage, current and resistance
relate to each other
• For any electrical circuit, ΔV = I • R
• Put into words, the current flow depends on the
Voltage divided by the resistance
• Given constant resistance, to get more flow what must
be changed?
• The voltage must be increased!
• Given constant voltage, to get more flow what must be
changed?
• The resistance must be lowered!
What about Watts?
• Simply put, Wattage (power) is Voltage times current,
or
• W=IxV
• So electrical devices can be described in terms of either
the current they like to take (because Voltage in
American electrical outlets is always the same), or –
• They may be described in terms of their wattage
• In American homes, electrical outlets are standard at
120 V
• European systems are standard at 220 V
Parallel circuits and current
Capacity of a battery is how long it can
generate a current
• Current (measured in
Amperes or “Amps”),
uses symbol I
• A measure of the
amount of electrical
charge passing through
a point in a circuit in a
given time
• Sometimes this is
described in terms of
“Amp-hours”
Gift wrap
available!
Increasing capacity
• The capacity of a battery
(amount of current it
generate in a unit time) is
fixed
• But when batteries are
joined in parallel, the
capacities add up
• A parallel connection
links the battery’s anodes
to each other, and the
battery’s cathodes to
each other
• Voltage is unchanged,
capacity increases
Series versus Parallel
• In a series connection,
voltages add, but
capacity is not changed
• In parallel connections,
voltages remain the
same, but capacity
increases
Series connection – only Voltage
increases
Parallel connection – only capacity
increases
Series versus parallel
Sometimes a circuit can be both
What is Voltage/Capacity of this
system?
Parallel
connection
Parallel
connection
Series connection
Series connection
Resistance…
…is futile
Resistors
• A resistor is a device to increase the
resistance at a particular point in a circuit by
reducing the current that can flow through a
point
• Think of a valve in pipe, in our “water
analogy”
• Unit is the Ohm
• Resistors are used to generate heat, lower
voltage, produce light, etc.
• Essentially, a light bulb can be thought of as
a resistor
Resistors are color-coded
Resistors in series
• When resistors are in series – the total resistance is
the sum of the individual resistor’s value in Ohms
• So it works just like batteries connected in series
Resistors in parallel
Practice problems
• Three resistors are connected in a series. The
value of each resistor is 10 Ω. What is the
resistance of the circuit?
• 30 Ω
• The same three resistors are now assembled
in parallel. What is the resistance of the
circuit?
• 3.33 Ω
Conclusion - resistance
• All electrical circuits will have some resistance
• Even though Copper is a good conductor, good is
not “perfect”
• So all circuits will lose some of their electrical
energy as heat (electronics get warm when you
use them because of resistance in the circuits)
• With heat – resistance increases!
• A superconductor is a material that can carry
current with little – or even no – resistance! But
the material often has to be cooled to very low
temperatures to become superconductive
Super-cooled substances show strange
effects