Electric charges in motion
Electrical circuits
Just like the flow of H2O molecules in a hose is a
water current, the flow of electrons in a wire is an
electric current. The electrical current is the rate
that charge moves through some part of a circuit
(I=q/t). One Ampere is a Coulomb per second.
When the switch is "open", current can not
flow on a continuous conducting path from
one pole of the battery to the other. By
closing the switch, we allow current to flow
unobstructed around the complete circuit.
A short circuit is a path with very little
resistance to the flow of electrical current.
How much electrical charge flows through a light
bulb in 8 seconds if the current is 100 mA?
Voltage sources
The simple water
circuit is analogous to
an electrical circuit.
Electrical charges flow when they are "pushed" or
"driven" by energy sources such as batteries, solar
cells, or generators. The energy sources do work on
charges by increasing their electrical potential energy.
Two batteries are in series if they form a
single pathway so that the current flows
through both batteries. The voltage across
the series connection is the sum of the individual voltages.
Two batteries are in parallel if they form
side-by-side pathways for the current flow.
Electrical Resistance
The amount of electrical current that flows through the bulb
depends on the resistance that the bulb offers to the flow
of electrical charges. The resistance of the filament
depends on the type of material used to make the filament
and its length and diameter. The electrical resistance is
measured in units of Ohms (Ω).
A flashlight has two 1.5-V batteries placed
end to end. What voltage rating should the
flashlight bulb have?
9.1
What happens when a light bulb "burns out"?
PHYS 1010Q
© D.S. Hamilton
Ohm's law
9.2
PHYS 1010Q
Series and Parallel circuits
The relationship between voltage, current, and resistance is
summarized by a statement called Ohm's law. Ohm discovered
that the current in a circuit is directly proportional to the
voltage applied to the circuit and is inversely proportional to
its resistance, ie
current =
© D.S. Hamilton
voltage
;
resistance
I=
V
;
R
Amps =
For the series connection,
if one bulb burns out or is
removed, the other bulb
goes out.
Volts
Ohms
Consider the circuit shown below. If the battery is a 1.5V
“D” cell, what would be the electrical current if the light bulb
has a 15 Ohm resistance.
For the parallel connection,
if one bulb burns out or is
removed, the other bulb
goes continues to give off
light.
Household AC circuits
Suppose I replace the light bulb with a different one so
that the current was measured to be 50 mA. What is the
resistance of this new bulb?
9.3
PHYS 1010Q
© D.S. Hamilton
Ohm's law works for both direct current
(dc) and alternating current (ac) circuits.
The ac voltage in ordinary household
circuits is about 120 V.
If an iron draws 6 A on its high setting, what is the
electrical resistance at this setting?
9.4
PHYS 1010Q
© D.S. Hamilton
Itotal = I1 + I2 + I3 + ....
Appliances are connected in
parallel to a household circuit.
The whole circuit is protected
from drawing excessive
current by a fuse or “circuit
breaker”.
Electrical Power
Power is the rate that energy is being transferred or
transformed. A Watt of power is one Joule of energy per
second.
Power =
energy
time
=
q⋅V
t
= IV = P;
Watt =
Joule
sec
The sticker on the back of my Toshiba portable DVD player
indicates that the unit requires 12V and that it draws 3A of
electrical current. How much power is being used?
If the current exceeds the rated
amperage, the metallic ribbon in a fuse
will melt and break the circuit. A
circuit breaker uses the difference in
thermal expansion of a bimetallic strip
to break the flow of current.
If I watch a 3 hr movie on this DVD player, how much
energy did I consume?
If the circuit above is protected by a 15 amp fuse and the
table lamp draws 1 A, the toaster 8 A and the TV set 4 A, will
the fuse blow when I plug in my 3 A coffee grinder?
What is the resistance of a 40-W light bulb?
A ground wire attached to the case of an
appliance keeps the voltage of the case at
ground potential (zero volts). This protects
against electrical shock. The ground wire, a
GFI (ground fault interrupter) outlet and the
fuse or breaker are standard safety devices
used with household electric circuits.
9.5
PHYS 1010Q
© D.S. Hamilton
Power and Energy
PHYS 1010Q
© D.S. Hamilton
Electrical shock
You pay "Connecticut Light & Power" for
the energy you have used during the
month, not for power. The energy used is
the rate of energy consumption multiplied
by the time duration of energy use.
The kilowatt hour (kW·hr) is commonly used by electrical
distribution providers for purposes of billing. The energy
consumption of a residential customer in a one month billing
cycle typically ranges from a tens to several hundred kilowatt
hours.
A 1500-W heater for a sauna requires 30 min to heat the
sauna to a temperature of 190°F. How much electrical
energy does this represent in kW·hr?
What does this cost if electricity sells for 12 ¢/kW·hr?
9.7
9.6
PHYS 1010Q
© D.S. Hamilton
The damaging effects of electrical shock are the result of
current passing through the body over some extended time.
The physiological effects of varying amounts of electrical
current are tabulated below.
CURRENT
0.005A = 5 mA
0.010A = 10 mA
0.015A = 15 mA
0.070A = 70 mA
EFFECT
painful tingling
involuntary muscle contractions
loss of muscle control, breathing disturbed
fatal if through heart and > 1 sec
From Ohm’s law, the current through your
body is I=V/R. The resistance of your body
to the flow of current involves both a contact
resistance of the skin and a bulk resistance
of the surrounding tissue. The current will increase with
increasing voltage or decreasing resistance. So for example,
skin that is wet with saline solution with a contact resistance
of 1,000 Ω will result in a larger current than dry skin whose
contact resistance is about 1 MΩ.
Compare the electrical current through an object if the
voltage is 120 V and the resistance is 1,000 Ω to when the
resistance is 1 MΩ?
9.8
PHYS 1010Q
© D.S. Hamilton