CHAPTER 5
Scientists’ Ideas
ELECTRIC CIRCUIT INTERACTIONS
By the 18th century experiments with electricity were the rage and scientists such as
Stephen Gray and Charles Dufay began to notice the effect of connecting electrical
materials together using a wire or a thread.
A vital experimental finding in the history of the electric circuit interaction was
discovered largely by accident by Luigi Galvani who produced an electrical convulsion
in a frog’s leg. This accidental discovery led Alessandro Volta to construct the first
“voltaic cells” or batteries in the early 19th century. Experiments with this device led to
the discovery that a noticeable quantity of heat was generated in the wire connecting
the battery. This discovery was astonishing from the standpoint of energy because no
external energy was being supplied to the battery. Therefore, scientists had to generate
an understanding of energy transformations in order to explain the source of this heat.
Some scientists’ current ideas about the electric circuit interaction are listed below. Read
through these ideas with your team and, below each idea, make a note of the evidence
or examples you have seen in your investigations that support each idea.
Idea EC1 - An electric circuit interaction occurs when a source of electrical energy is
connected in a closed path of conductors to an energy receiver:
If the path is opened, or if a non-conductor (insulator) is placed in the direct path, then
the electric circuit interaction will cease occurring.
Evidence/example:
(a) Examples of energy sources for electric circuits: battery, generator, and solar cell.
(b) Examples of energy receivers for electric circuits: light bulb, motor, and buzzer.
In the electric circuit descriptions that follow we will assume the battery is the energy
source in the circuit. However, the ideas still hold for any other type of energy source.
Idea EC2 - Each device in an electric circuit is two-ended; and each end must be
directly connected in the circuit
(If only one end of a device is connected in the circuit, then the device or circuit will not
work.)
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Evidence/example:
Idea EC3: Electric circuit interactions can be described in terms of electrical energy,
and the law of energy conservation applies to all devices in the electric circuit
During an electric circuit interaction, electrical energy is transferred from the energy
source to the energy receiver. No energy is “used up” or “destroyed.” Instead, each
device in the circuit transforms one type of energy into one or more other types. It is
convenient to draw input/output energy diagrams to describe the interactions between
the device and other objects. In all cases, the law of conservation of energy applies to
the energy description.
Evidence/example:
Idea EC4: All electric circuit devices warm up when operating and transfer heat
energy to the surroundings. (In fact, in any type of interaction where something is
moving, or some part of a device is moving, there is always friction present and
consequently the device will warm up and transfer heat energy to the surroundings.)
Evidence/example:
Idea EC5: The efficiency of an electrical device is a measure of how efficient the
device is in transforming its input energy into useful energy output. (For a battery,
the decrease in chemical potential energy is used instead of input energy.)
Evidence/example:
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Scientists’ Ideas: Electric Circuit Interactions
Idea EC6: An important variable in electric circuits is the rate at which electrical
energy is transferred from the energy source to one or more energy devices. By
definition, the rate of energy transfer is the amount of energy transferred from the
source to the device in one second. The unit is a joule/sec or watt.
Evidence/example:
Idea EC7: The brightness of a bulb depends on the rate of electrical energy
transferred to it: the higher the rate, the brighter the bulb.
Evidence/example:
Idea EC8: Electric circuit interactions can be described in terms of electric current,
which is the amount of electric charge moving past any point in the circuit in one
second.
Electric current is measured in units of amperes (A) or milli-amperes (mA) using a
device called an ammeter. The electric charges flow in one direction around the circuit:
out of one end of the battery, through the devices, and back into the other end of the
battery.
Evidence/examples:
Idea EC9: The value of the electric current in a circuit depends both on the value of
the battery voltage and the value of the resistance in the circuit. The relationship
between these quantities is given by Ohm’s Law:
,
where battery voltage is measured in volts and resistance is measured in ohms.
Evidence/examples:
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Idea EC10: The resistance of the filament is a measure of how much of an obstacle
the bulb filament is to the flow of electric charges through it. The amount of
resistance of a bulb filament depends on the length, thickness, temperature and type
of material of the filament:
Longer filaments offer more resistance than shorter filaments. Thicker filaments offer
less resistance than thinner filaments. As the temperature of the filament increases, its
resistance also increases. Some metals, like nichrome, have a much greater resistance
than other metals, like copper, for the same length, thickness and temperature.
Evidence/examples
Idea EC11: Electric current does not change in value around a single loop in a circuit:
As the electric charges move through the circuit, none of them disappear (or are lost)
and no new charges are created. The charges just keep circulating around the loop like
cars at a racetrack. The electric current has the same value at each position within a
single loop.
Evidence/examples:
Idea EC12: Electrical devices can be connected to a battery either in series or in
parallel:
(a) In a series circuit, all devices are connected in a single loop with the battery. The
greater the number of devices, the lower the value of the electric current.
Evidence/examples:
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(b) In a parallel circuit, each device is connected in its own separate branch or
pathway to the same battery. The greater the number of devices, the greater the
value of the electric current.
Evidence/examples:
Idea EC13 - A short circuit is a special situation that happens when a wire with low
resistance (e.g. copper) directly connects the positive and negative terminals on a
battery (without a bulb in the same pathway).
During a short circuit the battery can get very warm because the rate of increase in
thermal energy is very high. Also, the rate of decrease in chemical potential energy is
very large, and this means the battery can “die” much sooner than it would in a circuit
that is not shorted. In the shorting wire, because it has a very low resistance, the value
of the electric current can be very high. In a household circuit, to prevent a large
increase in current if there is a short, a fuse or circuit breaker will open up causing the
circuit to stop functioning.
Evidence/examples:
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