Chapter 17
Current Electricity
Conductors
 Conductors are materials in which the
electric charges move freely



Copper, aluminum and silver are good
conductors
In terms of circuits, we will generally be
using copper
When a conductor is charged in a small
region, the charge readily distributes itself
over the entire surface of the material
Electric Current
 Whenever electric charges of like signs move,
an electric current is said to exist
 The current is the rate at which the charge
flows through this surface
 Current (I) = units of charge (Q) per time

I = Q/t
 The SI unit of current is Ampere (A)

1 A = 1 C/s
Ex. 1
 The amount of charge that passes
through the filament of a certain light
bulb in 2.00s is 1.67C.
 A) Determine the current in the light
bulb.
 B) How many electrons passed through
the filament per second?
Ex. 2
 A 100.0 W light bulb draws 0.83A of
current. How many electrons pass a
given cross-sectional area of the filament
in 1 hour?
Ex. 3
 1.5 x 107 electrons pass through a given
cross section of a wire every 1.0s.

A) Find the current in the wire.

B) How much charge (in C) passes
through the wire per minute?
Electric Potential Energy of a
Charge
 Wants to move when it has
high PE
 Point b


PE = max
KE = min
 Point a


PE = min
KE = max
Voltage
 Voltage (ΔV) – the observed electrical
potential difference between two points
in a circuit
 Also the driving force behind the flow of
charge
 Analogous to a height difference with
gravitational potential energy or a
temperature difference with heat flow.
Voltage and Current
Differences
 Higher potential
difference, but less
energy flowing (therefore
less current)
 Lower potential
difference, but more
energy flowing (therefore
more current)
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In this river system, what would the
height difference in the waterfall
represent?
Charge
Voltage
Current
Wire
Electrons
Resistance
In this river system, what would the
flow of water represent?
1. Charge
2. Voltage
3. Current
4. Wire
5. Electrons
6. Resistance
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In this river system, what would the
water molecules represent?
1. Charge
2. Voltage
3. Current
4. Wire
5. Electrons
6. Resistance
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In this river system, what
would the riverbed represent?
1. Charge
2. Voltage
3. Current
4. Wire
5. Electrons
6. Resistance
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In this river system, what would the
water as a whole represent?
1. Charge
2. Voltage
3. Current
4. Wire
5. Electrons
6. Resistance
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In this river system, what would the
rocks in the river represent?
1. Charge
2. Voltage
3. Current
4. Wire
5. Electrons
6. Resistance
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Analogy to Flow of Water
 Electric Charge = Water


Coulomb = Gallons of Water
Electron = Molecule of Water
 Electric Current = Rate of Water Flow

Ampere = Gallons of Water per second
 Potential Difference or Voltage = Water
Pressure
(height of waterfall)
 Wire = Riverbed
 Resistance = Rocks in the River
Electric Current, cont
 When diagramming, conventional
current flow is the direction positive
charge (+) would flow

This is known as conventional current flow
 In
a common conductor, such as copper, the
actual current is due to the motion of the
negatively charged electrons
 In a particle accelerator, positively charged
protons are set in motion
Electrical Energy and Power,
final
 The SI unit of power is Watt (W)
 The unit of energy used by electric
companies is the kilowatt-hour (kW-hr)


This is defined in terms of the unit of power
and the amount of time it is supplied
1 kWh = 3.60 x 106 J
Meters in a Circuit -- Ammeter
 An ammeter is used to measure current

In line with the bulb, all the charge passing through the
bulb also must pass through the meter
Meters in a Circuit -- Voltmeter
 A voltmeter is used to measure voltage
(potential difference)

Connects to the two ends of the bulb
Which will turn the bulb on?
1. A.
2. B.
3. C.
4. D.
.
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Drift Velocity
 Drift Velocity is the velocity at which
electrons move opposite the electric field
(E).
 Counterintuitively, drift velocity is very
small. (eg 2.46 x 10-4 m/s in Cu wire)
 So how does the electric light turn on so
quickly??? Hmmmmm…
Charge Carrier Motion in a
Conductor
 The zig-zag black line
represents the motion of
charge carrier in a conductor

The net drift speed is
small
 The sharp changes in
direction are due to collisions
 The net motion of electrons
is opposite the direction of
the electric field
Resistance
 In a conductor, the voltage applied
across the ends of the conductor is
proportional to the current through the
conductor
 The constant of proportionality is the
resistance of the conductor
V
R
I
Resistance, cont
 Units of resistance are ohms (Ω)

1Ω=1V/A
 Resistance in a circuit arises due to
collisions between the electrons carrying
the current with the fixed atoms inside
the conductor (analogous to water
colliding with rocks in a river)
Ohm’s Law
 In general, resistance remains constant
over a wide range of applied voltages or
currents
 This statement has become known as
Ohm’s Law

ΔV = I R
Factors affecting resistance
 Length of a resistor – R increases with
length (directly prop.)
 Cross-sectional area – R increases with
smaller cross-sectional area (inv. prop.)
 Material – different metals have different
resistances
 Temperature – R increases with
temperature (dir. prop.)
Superconductors
 A class of materials and
compounds whose
resistances fall to virtually
zero below a certain
temperature, TC

TC is called the critical
temperature
Superconductors, cont
 Once a current is set up in a
superconductor, it persists without any
applied voltage

Since R = 0
Superconductor Timeline
 1911
 Superconductivity discovered by H. Kamerlingh
Onnes
 1986
 High temperature superconductivity discovered by
Bednorz and Müller
 Superconductivity near 30 K
 1987
 Superconductivity at 96 K and 105 K
 Current
 More materials and more applications
Electrical Energy and Power,
cont
 The rate at which the energy is lost in a
circuit is the power
Q
P
V  IV
t
 From Ohm’s Law, alternate forms of
power are
( V )
P I R 
R
2
2
Electrical Activity in the Heart
 Every action involving the
body’s muscles is initiated by
electrical activity
 Voltage pulses cause the
heart to beat
 These voltage pulses are
large enough to be detected
by equipment attached to the
skin
Electrocardiogram (EKG)
 A normal EKG
 P occurs just before the
atria begin to contract
 The QRS pulse occurs in
the ventricles just before
they contract
 The T pulse occurs when
the cells in the ventricles
begin to recover
Abnormal EKG, 1
 The QRS portion is
wider than normal
 This indicates the
possibility of an
enlarged heart
Abnormal EKG, 2
 There is no constant relationship between P and QRS
pulse
 This suggests a blockage in the electrical conduction
path between the SA and the AV nodes
 This leads to inefficient heart pumping
Abnormal EKG, 3
 No P pulse and an irregular spacing between the QRS
pulses
 Symptomatic of irregular atrial contraction, called
fibrillation
 The atrial and ventricular contraction are irregular
Implanted Cardioverter
Defibrillator (ICD)
 Devices that can
monitor, record and
logically process
heart signals
 Then supply different
corrective signals to
hearts that are not
beating correctly