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Chapter 5 - Electricity
Observatory: pages 140 to 171
5.1
Review Package
What is electricity?
 Def: All phenomena caused by positive and negative particle
Static – electrical charges are at rest
Dynamic – electrical charges are in motion
5.1.1 Electrical Charge
 In the nucleus of an atom there are protons and neutrons, while electrons
surround the nucleus.
o Protons are positive: they cannot move.
o Electrons are negative: they can move.
o Neutrons do not have a charge
 A positively charged body has more protons than electrons.
 A negatively charged body has more electrons than protons.
 Electrical charges are measured in coulomb (C). A single electron or protons
has a charge of only 1.602 X 10 -19C because they are so small. For that
reasons, one coulomb corresponds to the charge of either 6.25 X 1018
electrons or protons.
 LIKE charges repel each other (ex: two positive charges).
 OPPOSITE charges attract each other (ex: one positive with one negative).
 The Law of conservation of charges states: “electrical charges can neither
be created nor destroyed”.
5.1.2 Conductors VS Insulators
 Conductors are substances that permit the free flow of electrical charge.
o They include metals, electrolytic solutions
 Insulators are substances that impede the flow of electrical charge.
o They include non metals, wood, plastic, glass, rubber and pure air.
 Semi-conductors exhibit variable levels of conductivity depending on
factors such as temperature. They are widely used in electronics.
o They include metalloids and carbon.
5.2
Static Electricity
 Remember when you use to rub balloons and stick them on a wall?
 “static” means at rest.
 Normally matter is neutral, meaning that it has the same number of
protons and electrons. However electrons can move!
 Objects that gain electrons become negative.
 Objects that lose electrons become positive
 Again, like charges repel while opposite charges attract each other.
 Electroscope: An apparatus used to detect the presence of static electricity.
5.2.1 Charging an object
 There are three ways to charge an object:
friction, conduction and induction.
1. Friction: you can rub two different
substances on one another. One will
give electrons and become positively
charged. The other substance will
receive electrons and become
negatively charged.
o Some substances give electrons
more easily than others; you must
refer to the electrostatic series.
o The substance higher on the list
will become positive the one
further down, negative.
2. Conduction: an object can be electrically charged by putting it in
contact with another object that already has a charge. The two
objects will then charge the charge between them. The electroscope
remains open!
3. Induction: You can create a temporary electrical charge by simply
bringing a charged object in close proximity to another neutral
object. The electrons, on the neutral object, will move, creating
partial charges. As soon as you remove the charged object, the other
object will lose its charge.
5.3
Dynamic Electricity
 Def: describes all the phenomena related to electrical charges in motion.
5.3.1 Electrical Current
 Def: the orderly flow of negative charges (electrons) in matter.
 In reality, electrons flow in a circuit from the negative terminal of a power
supply, to the positive terminal (electrons are attracted to the positive
end).
 BUT: a long time ago, scientist had limited knowledge about electricity and
simply assumed that positive particles travelled from negative to positive.
This is referred to as: conventional current! At home circuits are still
labelled this way!
Power supply
DIAGRAM OF A CIRCUIT
Intensity (amperage)
 The number of charges that flow past a given point per second.
 The symbol is: I
Measured in: amperes (A).
 To calculate the current, use :
 Hence, 1 A = 1 C/s
 The instrument used to measure the current intensity is the
ammeter and must be connected in series.
Potential difference (voltage)
 The amount of energy transferred between two points in an
electrical circuit.
 The symbol is: V
Measured in: volts (V).
 To calculate the current, use :
 Hence, 1 V = 1 J/C
 The instrument used to measure the current intensity is the
voltmeter and must be connected in parallel.
Resistance
 The ability of a material to hinder the flow of electric current.
 The symbol is: R
Measured in: ohms (Ω).
1Ω = 1 V/A
 In circuits, resistors offer resistance and transform energy.
 A resistor can be a light bulb, a heater, a TV: any electrical device.
 The instrument used to measure the current intensity is the
ohmmeter.
 Many factors influence the resistance of a material:
1. The nature of the material.
2. Length: longer the wire = more resistance.
3. Diameter: smaller the diameter of a wire = more resistance.
4. Temperature: warmer material usually offers more resistance.
5.3.1 Ohm’s Law – * REMEMBER *
 It states that, for a given resistance, the potential difference in an electrical
circuit is directly proportional to the current intensity.
5.3.2




Electrical Power
The amount of work an electrical device can perform per second.
The symbol is: P
Measured in: watts (W).
1W = J/s
The amount of watts required is specific to each electrical device
To calculate the current, use :
E
P
t
 In a circuit if you know the voltage (V) and the amperage (I), you can
calculate the number of watts (W).
 Use the following formula:
5.3.3 The relationship between power and electrical energy
 We pay for electricity. Therefore it is important to know the amount used
up. Do do this we multiply the power by the time.
 KILLOWATTHOUR (kWh) = The number of kilowatts used in per hour
 Formula: E = Pe∆t
(Where: E in kWh, Pe in kW and ∆t in hours)
 In Quebec, we pay roughly 8 cent per killowatt hour
 Buisnesses compete to develop product that
consume the lest amount of electricity as possible!
5.3.3. Electrical Circuits
 Two types of circuits: SERIES and PARALLEL
1) SERIES : It contains a single loop where the elements are connected end
to end.
2) PARALLEL: The circuit contains at least one branch.
5.4 What is magnetism?
 Magnets: objects that can attract other objects containing iron, cobalt or
nickel.
5.4.1 Magnets
 Some substances may act as magnets, not always!
 Under a microscope, matter is made of a set of regions called: domains.
 When domains (tiny little magnets) are lined up, matter is magnetized.
 Magnets can be temporary or permanent.
 The NORTH POLE of a magnet is the end that naturally seeks the Earth’s
magnetic pole
near the
geographic
North Pole. The
other end of the
magnet is its
SOUTH POLE.
 The earth geographic north pole is the south magnetic pole.
 Opposite magnetic poles attract each other, while like poles repel.
5.4.2 Magnetic Fields
 Def: the area of space in which the magnetic
force of a magnet can act on another magnet.
 The directions of the lines go from the north
pole to the south pole.
 Even, if you break a magnet in half, you will
still have a north and south pole.
 The amount of force will be the strongest near
the poles.
 The lines indicate the relative strength of the force. The closer the lines, the
stronger the force!
5.5 Electromagnetism
 Def: All phenomena resulting from the interaction between electricity and
magnetism.
5.5.1
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Magnetization by electricity
You can generate a temporary magnetic field with dynamic electricity.
As soon as the current stops, the magnetic field disappears.
THE RIGHT-HAND RULE
+
-
 The thumb points in the conventional current direction.
(positive terminal to negative terminal)
 The fingers grasping the wire show the direction of the magnetic field lines.