Ch 20 Electricity
20.1 Electric Charges

Electric charge is a property that causes
subatomic particles such as protons and
electrons to attract or repel each other.
• Recall that Protons have a positive charge and
electrons have a negative charge.
The
atom is neutral when it has equal
numbers of positive and negative charges.
• If an atom gains electrons, it becomes a
negatively charged ion.
• If an atom loses electrons, it becomes a
positively charged ion.
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Electric Force
 Like
charges repel, and opposite
charges attract.
The force of attraction or repulsion
between electrically charged objects is
electric force.
 Inside an atom, electric forces are much
stronger than gravitational forces.

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Electric Fields
 An
electric field is a region of space
that exerts electric forces on charged
particles
 The strength of an electric field
depends on the amount of charge that
produces the field and on the distance
from the charge.
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Static Electricity & Charging
Static electricity is the study of the behavior of
electric charges.
 According to the law of conservation of charge,
the total charge in an isolated system is constant.
 Charge can be transferred by friction, by contact,
and by induction.

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Charging by Friction
Rubbing a balloon on your
hair causes charging by
friction.

• Electrons move from your
hair to the balloon because
atoms in rubber have a
greater attraction for
electrons than atoms in hair.
• The balloon picks up a net
negative charge.
• Your hair loses electrons
and becomes positively
charged.
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Charging by Contact
A Van de Graaff generator builds a charge on a
metal sphere. Touching the sphere transfers charge
by contact.
The sphere is still charged, but its net charge is
reduced.

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Charging by Induction
•Why do you get a shock
from a doorknob?
•The spark you feel is a
static discharge.
Walking on a carpet builds a negative charge on
your body. The negative charge in your hand repels
electrons in a metal doorknob.
The doorknob is still neutral, but charge has moved
within it. This is induction, a transfer of charge
without contact between materials.
 Static discharge occurs when a pathway
through which charges can move forms
suddenly.
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Lightning is Static Discharge Too
Lightning is a more dramatic discharge.
• Charge builds up in a storm cloud from friction
between moving air.
• Negative charge in the lower part of the cloud
induces a positive charge in the ground below the
cloud.
• As the charge in the cloud increases,
the force of attraction between charges
in the cloud and charges in the ground
increases.
• Eventually the air becomes charged,
forming a pathway for electrons to
travel from the cloud to the ground.
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20.2
 An
electric current can only flow when
there is a closed path in which
electrons can move.
 There are 2 types of current;
 Direct (DC)
 Alternating (AC)
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Current
The
continuous flow of electric charge is an
electric current.
• Charge flows only in one direction in
direct current (DC). A flashlight and
most other battery-operated devices use
direct current.
• Alternating current (AC) is a flow of
electric charge that regularly reverses its
direction. Electric current in your home
and school is mostly alternating current.
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Conductors and Insulators

Electrical conductor – material through which a
charge can easily flow. Example metal wire

Electrical insulator – material through which a
charge cannot easily flow. Example the plastic
coating around metal wire

Metals such as copper and silver are good
conductors. Wood plastic rubber and air are good
insulators
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Resistance
Resistance
is opposition to the
flow of charges in a material.
The SI unit of resistance is the
ohm.
 A material’s thickness, length, and
temperature affect its resistance.
 Using a thick straw to drink a
milkshake is easier than using a
thin straw. Similarly, electrons flow
more easily through a thick wire
than they flow through a thin wire
of the same material.
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Resistance
is greater in a longer wire
because the charges travel farther. As
temperature increases, a metal’s
resistance increases because electrons
collide more often.
A superconductor is a material that
has almost zero resistance when it is
cooled to low temperatures.
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Voltage
In order for charge to flow in a conducting
wire, the wire must be connected in a
complete loop that includes a source of
electrical energy.
 Potential Difference – the difference in
electric potential energy between two
different points measured in volts (V), often
called voltage

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How are current, resistance and
voltage related?
Increasing the voltage increases the
current. Keeping the same voltage and
increasing the resistance decreases the
current.
 This relationship can be expressed in
Ohm’s Law
 According to Ohm’s law, the voltage (V) in
a circuit equals the product of the current
(I) and the resistance (R).

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Practice Ohm’s Law
 What
is the voltage if the
resistance is 5 ohms and the
current is 3 amps?
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V=?
Given:
current (I) = 3 amps
Resistance (R) = 5 ohms
V=IR
V=(3 amps)(5 ohms)
V=15
volts
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20.3 Electric Circuits
Circuit – a closed
path through which
electrons can flow
 Circuit diagrams –
use symbols to
represent parts of a
circuit, including
source of electrical
energy and devices
that run by the
electrical energy

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Two Types of Circuits: Series & Parallel

Series circuits – charge has one path of flow. If
one element stops working, none of the elements
can operate. The amount of current is the same
through every part. When any part of a series is
disconnected, no current flows through the circuit.
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
Parallel circuit- Two or more paths through
which charges can flow. Voltage is the same in
each branch. When one branch is open current
continues to flow through the other branches. If
one element stops functioning in a parallel
circuit, the rest of the elements still can operate.
Homes are wired in parallel circuits
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Electrical Power
– the rate at which work is done
or energy used. Electrical power is a
measure of the rate at which electricity
does work or provides energy
 Calculating power
 Power
 Power = Voltage x Current
P=VI
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Practice Calculating Power
 An
electric oven is connected to a
240-volt line, and it uses 34 amps of
current. What is the power used by
the oven?
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Identify the variables
P=?
 I= 34 amperes
 V= 240 volts
 P=VI
 P= (240 volts)(34 amperes)

P=
8200 watts
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