Transcript Electrical Resistance and Ohm`s Law

 Georg
Simon Ohm
 1789-1854
 German
 Physicist
 HIGH
SCHOOL TEACHER!!!!
 Using
equipment he built himself
 Determined there was a direct
proportionality between voltage applied
across a conductor and the resultant
current (OHM’s LAW)
 Good
conductors easily allow electrons to
move through their atoms
 In a good conductor little energy is lost as
the electrons pass along the conductor’s
atoms
 In other materials such as a light bulb the
electrons do not move as easily.
 This ability of a substance to impede the
flow of electrons is called electrical
resistance
In other materials such as a light bulb the
electrons do not move as easily.
 This ability of a substance to impede the flow
of electrons is called electrical resistance
 Different loads have different resistances to
achieve the desired conversion of energy
 e.g. : light bulb filament = high resistance
(produces light)

toaster = lower resistance (produces heat)
 The symbol for electrical resistance is R
 The standard unit is (SI) is the Ohm (Ω)

The symbol for electrical resistance is R
 The standard unit (SI) is the Ohm (Ω)
 When electrons flow through different
materials, the electrical resistance causes a
loss of voltage
 There is a difference in the amount of voltage
(electric potential) that an electron has after
it has flowed through the material
 This is known as potential difference or voltage
drop
 e.g. After electrons flow through a light bulb,
they have less energy than before they went
through it (some is used)


e.g. After electrons flow through a light bulb,
they have less energy than before they went
through it (some is used)
 Ohm’s
Law
 Ohm’s Law describes the relationship
between potential difference and
current in a conductor


Potential difference = electric current x Electrical
Resistance
(voltage drop)
V = Voltage
(Potential Difference)
I = electric current
(measured in Amps)
R = electrical resistance
(measured in Ω)
V
= IxR (units for V are Volts)
 V/I = R or R = V/I (Units for R are Ω)
 V/R = I or I = V/R (Units for I are Amps)
 Sample problem:
 What is the voltage drop across the tungsten
filament in a 100W light bulb? The resistance
of the filament is 144 Ω and the current is
0.833A.
V = ?
I = 0.833A R = 144 Ω
V = I x R
V = 0.833A x 144 Ω = 120V
 Therefore the voltage drop at the light bulb
is 120V