Ohm’s Law
Review of last Class

Electrical Resistance

Resistance of some basic household appliances
 Typical
Lamp cord ~ << 1 ohm
 Typical
Lightbulb ~ 100 ohms
 Iron
~ 15 to 20 ohms
 Electric
toaster ~ 15 to 20 ohms
 Hairdryer
~ 15 to 20 ohms
Review of last Class

Calculating the resistance of
resistors
 1st
Band – First digit
 2nd
Band – Second digit
 3rd
Band – Multiplier ( x 10 )
 Third
digit is the power to
which 10 is raised
 4th
Band – Tolerance ( Gold5%, Silver-10%, No Band-20%)
Ohm’s Law

Is the relationship between voltage, current, and
resistance.

Ohm’s Law states
 That
the current in a circuit is directly proportional to
the voltage impressed across the circuit, and inversely
proportional to the resistance of the circuit
 Current
= Voltage / Resistance
-> I = V/R
Ohm’s Law


For a circuit of constant resistance, current and voltage
are proportional.
 What does this mean?
 You will get twice the current through a circuit for
twice the voltage across the circuit.
 I.E. – the greater the voltage, the greater the current.
What happens if the resistance is doubled? (Remember
that the relationship between current and resistance is
inversely proportional)
 If the resistance is doubled for a circuit, the current
will be half of what it would be otherwise.
 I.E. – the greater the resistance, the less the current.
Ohm’s Law

Example Problems
 Consider
a potential difference of 1 volt impressed across a
circuit that has a resistance of 1 ohm. What is the current
produced in this circuit?
I
= V/R
I
= 1Volts / 1 Ohm
I
= 1 V/Ω or 1 Ampere
 Consider
a voltage of 12 volts impressed across the same
circuit. What will the measure of the current?
I
= V/R
I
= 12V / 1Ω
I
= 12 V/Ω or 12 A
Ohm’s Law

Interactive Class Question:
 How
much current is drawn by a lamp that has a
resistance of 100 Ohms when a voltage of 50 volts is
impressed across it?
 Answer:
I
= V/R
I
= 50 V/ 100 Ω
I
=½A
Ohm’s Law and Electric Shock

What causes electrical shock in the human body?
 Current
or Voltage?
 The
damaging effects of electric shock are the result
of current passing through the body.

Knowing this what do we see from Ohm’s Law?
 Current
depends on the voltage applied and also on the
electrical resistance of the human body.
Ohm’s Law and Electrical Shock

What is the resistance of the human body?

Electrical resistance of the human body depends on condition and range

If you are soaked with salt water ~ 100 ohms

If your skin is very dry ~ 500,000 ohms

Perspective:
 If
you touch the ends (terminals) of a 12 V battery with your fingers
you would normally not feel anything.
 Your
 If
body usually provides a resistance of 100,000 ohms
you touch the terminals of a 24 V battery with your fingers you
would barely feel a tingle
Ohm’s Law and Electrical Shock
Current (Amperes)
1 mA
5 mA
Effect
Can be felt
Painful
10 mA
Involuntary muscle
contractions
Loss of muscle control
If through the heart, serious
disruption can occur,
probably fatal if the current
lasts for more than 1 second.
15 mA
70 mA
High Voltage Wires

Why can these birds perch on
these wires that carry such high
voltage?
High Voltage Wires

Answer:
 Every
part of the birds
body is at the same high
potential, therefore it does
not feel any effects.
 Same
goes for the person
in the picture to the right.
High Voltage Lines

Suppose you fall off a bridge and are able to grab onto a near by High Voltage
Power Line as you are falling. You grab onto the line with both hands. What
happens to you??
High Voltage Lines

Why do you not get shocked?
 Because
there is not a potential
difference.
 As
long as you touch nothing else,
you stay at the high potential
difference as the wires
 However,
if you reach over and
grab onto another wire of
different potential with one of
your hands you will get shocked –
ZAP!
Ground Wires

Mild shocks occur when surfaces of appliances are
at an electrical potential different from that of the
surfaces of other nearby objects.

If you touch surfaces of different potentials, you
get shocked. Why?
 Because
you are a pathway for the current to
travel

To prevent this problem, ground wires are used.
 Ground
wires connect the outside of the object
to a third prong of a three wire electric plug.
This third prong is then tied to a grounding rod
through the circuitry of the building.