# Ohm's law

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```					***Ohm’s law: logo
Ohm’s law

V=I*R
Ohm’s law relates currents to voltage in a circuit
A simple circuit
Why are birds not killed sitting on high-voltage
power lines?
A bird stands on an electric transmission line carrying 2100 A.
The line has 2.8x10-5 Ω resistance per meter and the bird's feet
are 4 cm apart. What voltage difference does the bird feel?
Thousands of hawks, eagles, and other large birds die every
year from electrocution by power lines.
What happens to people
who touch power lines?
Talk about birds on a power line!
Would it be safe to swing from a power line? What
should you do if a high-voltage power line falls close
to you?
***electrocution logo
***Short circuit…
Barefoot in the kitchen: After washing his dishes,
Brian decides to have fun by touching a 120-V line.
Would this be dangerous?

dry skin: R=104-106 Ω
wet skin: R=103 Ω

I=120/1000=120 mA
Consider this short circuit. The person touching the case is wet and
presents only 3 kΩ to the ground. What is the voltage on the case if 5
mA flows through the person? If the resistance of the ground wire is
0.2 Ω, will the 20-A circuit breaker be triggered?

V = IpR = 15 V
I = 15/0.20 = 75 A
Itot = 0.005+75=75 A
Why are two-prong plugs potentially
dangerous?
The third prong provides proper
grounding
How much current will a person who presents a
resistance of 100,000 Ω draw from a 10,000-V,
improperly grounded device?
The third prong
Fatal attraction? In the summer of 1999, two women were killed
by lightning in Hyde Park, London. The coroner's report
implicated underwired bras. What might have been their
rationale?

According to the coroner: It's far better to be
naked in a storm than to wear your bra.
Why are burns due to electrocution often
characterized by fern-like patterns?
True or false: A 1-amp current is often
less damaging than a 70-mA current.
Is it correct to say that an electrician is safe from
shock if he uses the back of his hand when they
touch live wires?
Effects of electrical shock as a function of current*

Current (mA)                                              Effect

1                                                         Threshold of sensation

5                                                         Maximum harmless current

10-20                                                     Sustained muscular contraction (can’t let go);
contraction of chest muscles may stop breathing

50                                                        Onset of pain

100-300+                                                  Ventricular fibrillation, often fatal

300-600                                                   Burns

6000                                                      Sustained ventricular contraction and respiratory
paralysis; both cease when shock ends; can be useful
for defibrillation
*For an average male shocked through trunk of body for 1 s by 60 Hz AC. Values for females are typically 60-
80% those of males.
Is it correct to say that an electrician is safe from
shock if he uses the back of his hand when they
touch live wires?
Physiological effects of
electrocution

Effects of Current through Human Body

Current (mA)                        Effect

<10            tingling or imperceptible

10-20          painful and cannot let go (muscle
spasms)
20-30         Muscle paralysis and breathing
difficulties
>70           breathing very difficult (could be fatal if
prolonged)
100           death due to ventricular fibrillation

>200           Heart and breathing stops; severe burns
Suppose that the motor of a motorized hospital bed shorts out to
the bed frame, and the bed frame's connection to a ground has
broken (or was not there in the first place). If a nurse touches the
bed and the patient at the same time, she becomes a conductor and
a complete circuit can be made through the patient to ground
through the ECG apparatus, as shown. If V = 220 V, calculate the
current through the patient.

I = 220/(3∙104) = 7.3 mA
An electric current can cause muscular contractions with
varying effects. Here, the victim is thrown backward by
involuntary muscle contractions that extend the legs and torso.
All the muscles in the hand are being stimulated by the current.
Why can’t he let go?
Heart attack or electrocution?
***Current…
SPEAR, a storage ring 72 m in diameter at the Stanford
Linear Accelerator, has a 20-A circulating beam of
electrons that are moving at nearly the speed of light.
How many electrons are in the beam?
End
The disk drive in a portable CD player is connected to a battery
that supplies a current of 0.22 A. How may electrons pass
through the drive in 4.5 seconds?
The higher the voltage, the greater the
current
The unit of electrical current is called
Ampere
A diode does not obey Ohm’s law
Light bulbs turn on at different times. Why
the delay?
How does a lightbulb work?
The electric eel can generate a current as large as 1
ampere at a voltage of up to about 500 volts. What is
the resistance of the water?
How does the hair drier work?
End
The falling water
can do work in
turning the water
wheel only as
long as the pump
maintains the
potential
difference
between the upper
and lower
reservoirs.
High voltage cables such as these distribute
electricity throughout the country.
A standard flashlight battery can deliver about 2 W · h of
energy before it runs down. (a) If a battery costs 80 ¢, what
is the cost of operating a 60 W lamp for 10 h using
batteries? (b) What is the cost if the power is provided by an
electric utility company, at 6 ¢/kW∙h?

60 10 / 2  300 batteries  \$240
60 10 / 1000   \$0.06  3.6 ¢
Charge tends to flow
downhill much like a
fluid
By convention, current is a flow of
positive charge.
The falling
water can do
work in turning
the water wheel
only as long as
the pump
maintains the
potential
difference
between the
upper and lower
reservoirs.
An EMF, often a battery, is the charge
pump
A simple electric circuit has a voltage source (such as a generator or
battery) that maintains the electrical potential, some device (such as a
lamp or motor ) where work is done by the potential, and continuous
pathways for the current to follow.
A simple electric circuit carrying a current of
1.00 coulomb per second through a cross section
of a conductor has a current of 1.00 amp.
A conventional current describes positive charges moving from the
positive terminal (+) to the negative terminal (-). An electron current
describes negative charges (-) moving from the negative terminal (-)
to the positive terminal (+)
***What is the nature of the electric current
carried by these conducting lines? It is an
electric field that moves at near the speed of
light. The field causes a net motion of electrons
that constitutes a flow of charge, a current.
Determine the number of electrons that pass
between the terminals of a 12-V car battery when a
60-W headlight burns for one hour.

P = I•V or I = 60/12 = 5 A
Q = I•t = 5 •3600 = 1.8x104 C
***Ohm’s law: bra animation
How does a digital voltmeter work?
A 12-V storage battery consists of
six 2-V cells connected in series
Current vs resistance: Water flow
analogy
Example of Ohm’s Law
Ohm’s law applet
Applet: bulb amperage
Any voltage source (in this case, a
carbon-zinc dry cell) has an
internal resistance
Electrical power…
If you connect a 40-W light bulb in series with a 60-
W light bulb, which would be brighter?
Why is the 60-W
lightbulb the
brightest?
If my bill for Nov of 2002 was about \$72, what was
the average cost of one kWh?
Electric pickles
Charging electric cars
Why do batteries warm up when a flashlight is in use?
Typical neuron
Membrane charge

Positive and negative charge layers form on the outside and
inside surfaces of a membrane during its resting state.
In the technique of impedance plethysmography, the
electrical resistance of the calf is measured to diagnose
deep venous thrombosis (blood clotting in the veins).
What is the resistance the light bulb if the
current is 0.4 A?
***Power…
An electrical heating coil is immersed in 4.2 kg of
water at 22°C. The coil, which has a resistance of
210 W, warms the water to 35°C in 3.5 min. What is
the potential difference at which the coil operates?
A heating coil has a resistance of 3.7 and operates
on 117 V. What does is the cost of operating the
heating coil 30 min per day for 30 days?
A 50 W electrical heating element is placed in a well-insulated
container into which has just been placed 500 g of water at
10°C and 280 g of ice at 0°C. How long will it take for the
contents to evaporate completely? (The latent heat of fusion
for water is 333 J/g, the heat of vaporization is 2260 J/g,
and the specific heat is 4.18 J/g.)
A standard toaster has a power cord with a resistance of 0.02 W
connected in series with a 10-W nichrome heating element. The
potential difference between the terminals of the toaster is 120
V. How much power is dissipated in the power cord and in the
heating element?
Extension Cords and a Potential Fire Hazard

When an extension cord is used with a space heater, the cord must have a
resistance that is sufficiently small to prevent overheating of the cord.
Electrical Power Sent to a Loudspeaker
***Ohm’s law: carbon button…Darren’s presentation
***ohm…How do turn signals work?
Battery: Chemical energy of a battery can be
converted to mechanical energy by an engine.
Plasma ball

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