Download Resistance and Ohms

 When
electric charges flow
they experience opposition
or resistance which reduces
the amount of energy they
have
 Greater resistance ->
greater amount of energy
each charge has to give up
Filament – high
resistance
Therefore, lot’s of
energy taken from
each electron and
turned into light
Wire – low
resistance
Therefore, little
energy lost –
turned mainly into
heat
 The
amount of energy
(voltage) required to push
electrons (current) through a
conductor
 German
Physicist Georg Ohm
determined that for a given
conductor, the ratio of voltage
to current (V/I) is constant
 We call this constant
resistance (Measured in Ohms
- )
Resistance
Constant
V
=
I
 Relates
voltage, current and resistance
 Note: E stands for electric potential
(commonly known as voltage)
V
R
I
 Electrical
Insultator
 Electrical
conductor
◦ Prevents the transfer of electric
charges
◦ Ex: Air, glass, rubber, paper
◦ Low resistance which allows
electric charges to flow easily
◦ Ex: copper
A single cell is set-up in a circuit with
a switch and a resistor. For the
resistor, a voltmeter is set-up and it
measures 1.3V and an ammeter is
set-up and it measures 3.5A.
a) Draw the circuit with the correct setup of a voltmeter and ammeter
b) Calculate the resistance of the
resistor

 If
a resistor has a resistance
of 1000 and the current is
2.0A. What will be the
voltage drop across the
resistor?
 Pg
330 # 1a
 Pg 332 # 1, 3-5
 Pg 331 # 5ab,7,8
 Series
◦Only one path for current to
flow
 Parallel
◦More than one path for
current to take
 We
have four tools we can use
to analyze circuits:
◦ Kirchoff’s Current Law (KCL)
◦ Kirchoff’s Voltage Law (KVL)
◦ Ohm’s Law
◦ Equivalent Resistance (Series and
Parallel)
 At
any junction (Point) in the
circuit, the current going in
equals the current going out
 At a point,
Iin = Iout
 Note: In any series circuit, every
point has the same current
 In
any complete path in a
circuit, the sum of voltage
rises (sources) is equal to the
sum of voltage drops (loads)
 For
a path,
Vrises = Vdrops
 Pg
337 # 1-3
 Pg 343 # 1, 2, 4
 If
you have many resistors (or
loads) in a circuit, finding the
equivalent resistance allows you
to replace multiple resistors with
a single resistor
 This allows easier analysis of
circuits
Rs  R1  R2  R3  ...  Rn
1
1
1
1
1
 
  ... 
RP R1 R2 R3
Rn
The easiest way to put this formula into a
calculator is to use your inverse button (x-1)
 Pg
339 # 4-6
 Pg 340 # 7