Simple electric circuits
• Objectives:
 draw circuits conventionally using standard symbols
and straight lines for the connecting wires
 learn how to construct circuits from circuit diagrams
 discuss the meaning and results of a circuit diagram
 using their own ideas for circuits- predict how each
circuit will behave, giving a reason, before trying it out
Current
flows due
to
a.There must be an
energy supply capable
doing work on charge to move it from a low
energy location to a high energy location and
thus establish an electric potential difference
across the two ends of the external circuit.
b.There must be a closed conducting loop
Quantitative investigation of voltage,
current, and Ohm's law
 Electric current is the flow rate
of electric charge and is
measured in amperes.
 The current transports
electrical energy along
conductors.
 Voltage (V) is a measure of
energy per unit charge
between two points in the
circuit. One may think of
voltage as the effective
"pressure difference" which
causes the current to flow.
• Resistance (R) is the
opposition to current flow and
is measured in ohms. In
practice, resistors take the
form of light bulbs, toasters,
heaters and other devices
which use electrical energy to
perform useful tasks as well as
the undesirable form of
resistance in electrical wiring
that transports the electrical
energy to you.
Notes
You need a continuous circuit to make the lamp light
• the shape of the circuit does not matter if there is only one single loop (this is
important if Worcester circuit boards are used)
• with more than one cell then the relative directions of the cells matter
• the lamps in a series circuit are all the same brightness
• more lamps in a series circuit means dimmer lamps
• the same number of cells and lamps in a series circuit means that each lamp is at
normal brightness
• one cell with two lamps connected in parallel with it shows that both lamps have
the same brightness (if 'high power, or low internal resistance cells are used)
Two lamps in series with a cell will run for longer than two similar lamps in parallel
with a cell, though students may not discover this during one lesson.
Circuit symbols & diagrams
In this circuit, there is only one source
of voltage (the battery, on the left) and
only one source of resistance to current
(the lamp, on the right).
Predict and explain
1. Circuit will be altered by adding a light bulb
in parallel. Predict & explain changes in
brightness of bulbs and current supplied by
battery.
2. The circuit will be altered by adding a light
bulb in series. Students will predict & explain
changes in brightness of light bulbs and
current supplied by the battery
Which is most dangerous, current or
voltage?
• A high voltage is not necessarily dangerous if it
cannot deliver substantial current.
• current is the killer not volts
• Static discharge to car doors in winter can involve
voltages as high as 20,000 V
• you can have thousands of volts but without flow
(current) the danger is minimal
• but if you have voltage its very easy to induce
current so be careful and let a professional take
care of it if you do not know what you are doing.
How voltage, current, and
resistance relate
• voltage -the measurement of how
much potential energy exists to move
electrons from one particular point in
that circuit to another particular point.
Without reference to two particular
points, it has no meaning
• motivate electrons from one point to
another. Before we can precisely define
what a "volt" is, we must understand
how to measure this quantity we call
"potential energy."
Coulomb- is a measure of electric charge
proportional to the number of electrons in an
imbalanced state. One coulomb of charge is
equal to 6,250,000,000,000,000,000 electrons.
International safety symbol "Caution, risk of
electric shock" (ISO 3864), also known as high
voltage symbol
•electrocution danger is mostly determined by the low electrical conductivity
•Accidental contact with high voltage supplying sufficient energy will usually result in
severe injury or death. This can occur as a person's body provides a path for current
•Low-energy exposure to high voltage may be harmless(spark touching a doorknob,
• it should not be assumed that being insulated from earth guarantees that no
current will flow to earth as grounding, or arcing to ground, can occur in unexpected
ways
•High-frequency currents can cause burns even to an ungrounded person (touching
a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil
can sustain a spark with only one endpoint).
• The means by which the resistors are
connected will have a major affect upon the
overall resistance of the circuit, the total
current in the circuit, and the current in each
resistor.
Series Circuits
• charge passes through every
light
bulb.
• As more and more light bulbs are added,
the brightness of each bulb gradually
decreases.
• This observation is an indicator that the
current within the circuit is decreasing.
• decrease in current is consistent with the conclusion
that the overall resistance increases.
In order for the devices in a series
circuit to work, each device must work.
• If one goes out, they all go out.
• Suppose that all the appliances in a household kitchen were
all connected in series.
• In order for the refrigerator to work in that kitchen, the
toaster oven, dishwasher, garbage disposal and overhead
light would all have to be on.
• If current is cut from any one of them, it is cut from all of
them. Quite obviously, the appliances in the kitchen are not
connected in series.
in parallel
• a single charge passing through the
external circuit would only pass through one
of the light bulbs.
• The light bulbs are placed within a separate
branch line, and will pass through only one
of the branches during its path back to the
low potential terminal.
a study of the overall current for
parallel connections requires the
addition of an indicator bulb.
• An indicator bulb placed outside of the
branches allows one to observe the affect of
additional resistors upon the overall current.
• The bulbs in parallel branches only provide an
indicator of current through that branch.
• So if investigating the affect of the number of
resistors upon the overall current and
resistance, one must make careful
observations of the indicator bulb, not the
bulbs that are placed in the branches.
For parallel circuits, as the number of
resistors increases, the overall current
also increases.
• This increase in current is
consistent with a decrease
in overall resistance.
• Adding more resistors in a
separate branch has the
unexpected result of
decreasing the overall
resistance!
The affect of adding resistors is quite different
if added in parallel compared to adding them in
series.
• Adding more resistors in series means that
there is more overall resistance;
• Adding more resistors in parallel means that
there is less overall resistance.
• You can add more resistors in parallel and
produce less resistance
Observe the electrical wiring below. Indicate
whether the connections are series or parallel
connections. Explain each choice.
Wires have
little
resistance
• The electrical resistance is greater for a longer wire,
less for a wire of larger cross sectional area, and would
be expected to depend upon the material out of which
the wire is made
• Resistance also depends on temperature, usually
increasing as the temperature increases.
If the connecting
wires resistance is
very little or none, we
can regard the
connected points in a
circuit as being
electrically common
• That is, points 1 and 2 in
the circuits may be
physically joined close
together or far apart, and it
doesn't matter for any
voltage or resistance
measurements relative to
those points. The same
goes for points 3 and 4. It is
as if the ends of the
resistor were attached
directly across the
terminals of the battery
Two electric circuits are diagrammed below.
For each circuit, indicate which two devices
are connected in series and which two devices
are connected in parallel.
In series? ___________________
In parallel? _________________
• As the number of resistors (light bulbs) increases,
what happens to the overall current within the
circuit?
• As the number of resistors (light bulbs) increases,
what happens to the overall resistance within the
circuit?
• If one of the resistors is turned off (i.e., a light
bulb goes out), what happens to the other
resistors (light bulbs) in the circuit? Do they
remain on (i.e., lit)?
Current is the rate at
which charge crosses
a point on a circuit.
Drift
speed
1 ampere = 1 coulomb / 1 second
Batteries are not rechargeable

An electrochemical cell supplies the energy needed to move a charge from
a low potential location to a high potential location

The charge that flows through a circuit originates in the wires of the
circuit. The charge carriers in wires are simply the electrons possessed by
the atoms that make up the wires.

Charge moves abnormally slowly - on average, about 1 meter in an hour - through a circuit

The rate at which charge flows is everywhere the same within an electric circuit. The rate at
which charge flows into a light bulb is the same as the rate at which charge flows out of a
light bulb.

An electrical appliance such as a light bulb transforms the electrical energy of moving charge
into other forms of energy such as light energy and thermal energy. Thus, the amount of
electrical energy possessed by a charge as it exits an appliance is less than it possessed when
it entered the appliance
Electric circuits are all about
energy, not charge.
•
When a battery no longer works, it is out of energy.
•
A battery (or single cell) operates by packing a collection of reactive
chemicals inside. These chemicals undergo an oxidation-reduction
reaction that produces energy. This energy-producing reaction is
capable of pumping the charge through the battery from low energy
terminal to high energy terminal and establishing the electric potential
difference across the external circuit. And when a battery no longer
works, it is because the chemicals have been consumed to the point
that the ability of the battery to move the charge between terminals
has been severely diminished.
•
When a battery no longer works, it is because the conversion of
reactants to products have occurred to the extent that the energyproducing reaction is no longer able to do its job of pumping charge.
• By placing the cell into a so-called recharger, the
energy of a household electrical circuit can be used
to drive the reaction in the reverse direction and
transform the chemical products back into chemical
reactants. This reverse process requires energy; it is
the recharger which supplies the energy. With
reactants replenished, the cell can now be used again
to power the electric circuit.
Ohm's Law
• If you know 2 pars, and wish to determine
another, just eliminate it from the picture and
see what's left:
REVIEW:
• With resistance steady, current follows voltage
(an increase in voltage means an increase in
current, and vice versa).
• With voltage steady, changes in current and
resistance are opposite (an increase in current
means a decrease in resistance, and vice versa).
• With current steady, voltage follows resistance
(an increase in resistance means an increase in
voltage).
REVIEW:
• Devices called resistors are built to provide precise amounts
of resistance in electric circuits. Resistors are rated both in
terms of their resistance (ohms) and their ability to
dissipate heat energy (watts).
• Resistor resistance ratings cannot be determined from the
physical size of the resistor(s) in question, although
approximate power ratings can. The larger the resistor is,
the more power it can safely dissipate without suffering
damage.
• Any device that performs some useful task with electric
power is generally known as a load. Sometimes resistor
symbols are used in schematic diagrams to designate a
non-specific load, rather than an actual resistor
What is the amount of current (I) in
this circuit?
• What is the
amount of
current (I) in
this circuit?
What is the amount
of voltage provided
by the battery?