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Grade 11 Physics Unit 5: Electricity and Magnetism Summary
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- Physics
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- 11th Grade
A detailed summary and test preparation aid of unit 5: Electricity and Magnetism of Grade 11 Physics.
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Physics Unit 5: Electricity and Magnetism● Electrical Power: The rate at which electrical energy is produced or consumed in a given
time
● Electrical energy is measured in units of kilowatt hours (kWh) because the joule is
sometimes too small to be a convenient measurement
○ One kWh = 3.6 million joules (3.6 x 10^6 J)
● The kilowatt hour is a useful unit of measurement for homes, but when describing the
electrical energy generated by a power plant we use megawatt hours (MWh)
● To meet the demand for electrical energy, we can use two approaches: conservation and
generating energy more efficiently
● Electric Potential: The measurement of the electric potential energy associated with
charges (the amount of energy needed to move a quantity of electrons closer to one
another)
● Free electrons that are able to move are essential for the transfer of electrical energy
● In a circuit, electrons are closely packed together in the conducting wires and moving the
electrons closer together increases their electric potential
● Electric potential measures both the electrical energy and the quantity of electrons
● The unit for measuring electric potential is joules per coulomb (J/C) (the joule is a
measurement of the electric potential energy and the coulomb is a measurement of the
number of electrons/amount of charge)
● This unit is more commonly known as the volt (V)
● 1 J/C = 1 V
● Differences in electric potential at different points in a circuit exist because energy is
transformed in electric circuits
● Electric Potential Difference: The difference in electric potential energy associated with a
coulomb of charge at two different points in a circuit (also referred to as voltage)
○ A positive electric potential difference is called a voltage gain
○ A negative electric potential difference is called a voltage drop
● Voltmeter: An electrical device that measures electric potential difference
○ Voltmeters only work accurately if they are connected in parallel in the circuit
● In a circuit, sources of electrical energy cause an increase in the electric potential
(voltage gain), whereas loads cause a decrease in the electric potential (voltage drop)
● The movement of electrons (an electric current) is required for an electrical device to
operate
● Electric current is responsible for the transfer of electrical energy along a conducting wire
● Direct current (DC): 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 and instead move around randomly and are considered “free electrons”
○ When a source of electrical energy is supplied to a circuit, free electrons will
move in one direction through the conductor
○ This movement of the free electrons is the direct current
, ○ The more free electrons that are moving in one direction, the greater the direct
current
● 1 coulomb = 6.2 × 10^18 electrons
● The nerve cells in your body communicate with each other by creating very small electric
currents, but if a larger current is transmitted through your body, it can overload your
nervous system and cause:
○ Muscle contraction, inability to let go of wire
○ Burns (on both surface and deep tissue)
● Ammeter: A device that measures electric current
○ Ammeters must be connected in series in a circuit so that all the electrons flowing
through the wire also have to flow through the ammeter, giving an accurate
reading of the current
○ If the ammeter were connected in parallel, there would be more than one path for
the current to flow along (one path would be into the circuit while the other path
would be through the ammeter), so you would not be sure how much of the
current went through the path
● Real = Electron Flow (⊖ ⇨ ⊕)
● Not Real = Conventional Current (⊕ ⇨ ⊖)
● Kirchhoff’s Voltage Law: In any complete path in an electric circuit, the total electric
potential increase at the source(s) is equal to the total electric potential decrease
throughout the rest of the circuit (the sum of the voltage gains and drops in a complete
path in a circuit is zero and the electric potential in a complete path in a circuit remains
constant)
● Kirchhoff’s Current Law: In a closed circuit, the amount of current entering a junction is
equal to the amount of current exiting a junction
● Electrical Resistance (R): A property of matter that describes how difficult it is for electric
current to travel through a material
○ Insulators like rubber and plastic have high resistance
○ Conductors like metals have low resistance
● Resistor: An electrical device that has a specific resistance value
● Ohm’s Law: The voltage in a conductor is proportional to the current if the temperature
remains constant
○ When voltage increases/decreases, so does current
○ A graph of voltage vs. current is a straight line with the slope of the graph being
constant (this constant is the electrical resistance, R)
● All of the electrical components in a circuit have electrical resistance (the connecting
wires and control devices, such as switches, typically have small resistances)
● Resistance causes some of the electrical energy to be converted into thermal energy
● Some loads depend on this conversion to function, such as incandescent light bulbs
● Superconductors are special materials that have no electrical resistance
○ There is a lot of interest in these because of the potential to eliminate the thermal
energy wasted in wires
, ○ The goal is to be able to make circuits from superconducting material at room
temperature (imagine a computer that does not need cooling, or power plant
transmission wires that is 100 % efficient)
○ Even though superconductivity requires very low temperatures, it is still being
actively used in technologies around the world such as in the magnetic
resonance imaging (MRI) device
● Ohmmeter: A device used to measure electrical resistance
○ Connected in parallel
○ Must never be used on a circuit if the circuit is live (turn off power before using)
○ Useful for testing whether a load works (loads have low resistance so if a load is
tested with an ohmmeter and the resistance reads off the scale, the load likely
has a bad connection and will not work)
● Adding more resistors to a series circuit decreases the current in the circuit
○ The decreased current is a result of the increased resistance
● Equivalent Resistance: The total resistance of a group of resistors in a circuit connected
in series or parallel
● Resistors in series can be reduced to a single resistor with a resistance equivalent to the
total resistance of the resistors denoted as Rseries
● The more resistors you connect in parallel, the greater the current because with each
additional parallel connection, you increase the number of pathways for the electric
current
○ If there are more pathways, you would expect more electric current
○ Therefore, the total resistance must decrease
● Force Fields: A region of space surrounding an object that can cause another object to
experience a force
○ Force fields are all around us
● Magnetic Field: The three-dimensional region of space surrounding a magnet that will
exert a force on magnetic objects
○ Exists around the magnet whether the magnetic field is causing a force or not
○ Earth has a magnetic field that exists both inside and surrounding it
○ A gravitational field exists around Earth because of Earth’s mass, and it causes
objects with mass to be attracted toward Earth’s centre
○ Electric fields surround charged particles such as protons and electrons and
cause the charged particles to experience forces of attraction or repulsion
● When a magnetite sliver is suspended on a string, it orients itself with Earth’s magnetic
field
● Since these slivers had two magnetic ends, they were labelled poles
● This discovery led to the invention of the compass, a very useful navigational tool
● The compass needle is a permanent magnet
● Unlike poles attract each other, and like poles repel each other
● The force of repulsion or attraction increases as the magnets get closer to one another
● Since the north pole of the compass needle points north, it means that Earth’s north pole
must actually be a south magnetic pole, similarly, Earth’s south pole must actually be a
north magnetic pole
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