SPH3U­Sec.11.1­11.4.notebook
Section 11.1 ­ 11.4
January 02, 2014
Power Ratings
The power rating is how many joules of energy are required per second to run the machine.
Electricity
• A form of energy resulting from the existence of charged particles (such as electrons or protons), either statically as an accumulation of charge or dynamically as a current
• The supply of electric current to a house or other building for heating, lighting, or powering appliances
­ the electricity was back on
Jan 2­3:05 PM
This value gives you a snapshot of the power required to operate an appliance, but it doesn't tell you how much energy was consumed during its use. Electrical energy is measured in units of kilowatt hours (kWh) because the joule (J)
is sometimes too small to be a convenient measurement. One kilowatt hour is equal
to 3.6 million joules. A typical home in Ontario uses 1000 kWh of electrical energy
each month.
Jan 2­4:37 PM
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Electrical Power ­ the rate of transformation of electrical energy
How a battery works...
(expressed in Joules per second, or Watts)
Battery demonstration
Hydro rates; Thunder Bay
1. water glasses
2. fruit/potato battery
What is the cost to operate the bulb if electricity is sold for $0.11/kWh?
Jan 2­4:48 PM
Jan 2­2:48 PM
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SPH3U­Sec.11.1­11.4.notebook
Section 11.3 ­ Electric Potential Difference
January 02, 2014
(building up of pressure­water analogy)
Electric potential is the measurement of the electric potential energy associated with
charges (in this case, electrons). You can think of electric potential as the amount
of energy needed to move a quantity of electrons closer to one another. In a circuit,
electrons are closely packed together in the conducting wires. Moving the electrons
closer together increases their electric potential. The more work that is done to move
the electrons closer together, the larger is the electric potential.
(higher voltage = higher pressure)
Electric potential measures both the electrical energy and the quantity of electrons.
The unit for measuring electric potential is the joule per coulomb (J/C). The joule (J) is
a measurement of the electric potential energy, and the coulomb (C) is a measurement
of the number of electrons or amount of charge. This unit is more commonly known
as the volt (V), in honour of scientist Alessandro Volta. 1 J/C is equal to 1 V.
Electric potential difference is the difference in electric potential energy associated
with a coulomb of charge at two different points in a circuit. Electric potential difference
is also referred to as voltage (V). A positive electric potential difference is sometimes
called a voltage gain. A negative electric potential difference is sometimes called a voltage
drop. Electric potential difference can be represented by the following equation:
where V = electric potential difference (joules per coulomb or volts), ΔE = change
in electric potential energy (joules), and Q = the amount of charge (electrons) (coulombs).
1 Coulomb is equal to the charge of approximately 6.241×1018 electrons.
Jan 2­4:57 PM
Jan 2­5:16 PM
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Measuring Electric Potential Difference (Voltage)
Section 11.5 ­ Electric Current
A voltmeter is used to measure electric potential difference in circuits. Voltmeters only work accurately if they are connected in parallel in the circuit. To connect in parallel, you must create a separate path for the electrons to follow in the circuit.
The FLOW (movement) of electrons is required for electric devices to operate; the electric current is what is responsible for the transfer of electrical energy along a conducting wire.
12 V
4 V
8 V
12 V
12 V
12 V
_
Conventional Current (old model)
"+" to "­"
+
Electron Flow Convention
"­" to "+"
Jan 2­5:20 PM
Jan 2­5:42 PM
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DC or Direct Current
Direct current (DC) is the flow of electrons in only one direction through a circuit. The
electrons flow from the negative terminal of the source of electrical energy and travel
through the conducting wires toward the positive terminal.
The outer electrons in the atoms of a metal conductor are not tightly held to their
nuclei; rather, they move around randomly and are considered “free electrons.” When
a source of electrical energy (for example, a battery) is supplied to a circuit, free electrons
will move in one direction through the conductor (Figure 1). This movement
of the free electrons is the direct current. The more free electrons that are moving in
one direction, the greater is the direct current.
In 1820, French physicist Andre­Marie Ampere performed experiments on direct currents in wires. He was interested in measuring the intensity of the current, which is why today we use the symbol I to represent electric current.
where I = current (A), Q = amount of charge (C), and Δt= time interval (s). One coulomb of charge (or electrons) passing one point every second in a circuit is equivalent to one ampere. Therefore, 1 A = 1 C/s. In illustrations of current, we often depict only a few electrons. However, a coulomb of electrons is 6.2 x 10 electrons. So a current of 1 A means over 6 billion billion electrons moving past a point each second.
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The standard electrical breaker for your home is a 15 Amp rating, that means it will turn itself off when more than 15 Amperes of current are requested (a safety feature).
Jan 2­6:07 PM
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