2. Resistance and Ohm’sResistance and Ohm’s
LawLaw
October 10, 2013October 10, 2013

3. ObjectivesObjectives
1.1. Describe the parts of Ohm’s LawDescribe the parts of Ohm’s Law
- Volts, amps, and resistanceVolts, amps, and resistance
1.1. Given two known values, calculateGiven two known values, calculate
the unknown using Ohm’s Lawthe unknown using Ohm’s Law

4. Ohm’s LawOhm’s Law
 Relationship between current,Relationship between current,
resistance, and voltageresistance, and voltage

5. Electrical ResistanceElectrical Resistance
 Resistance ofResistance of
electrons to flowelectrons to flow
in a circuitin a circuit
 What affectsWhat affects
resistance?resistance?
• Length, and widthLength, and width
and type of wireand type of wire

6.  Shown by a zig-zagShown by a zig-zag
symbol in a circuitsymbol in a circuit
 Controls currentControls current
 Measured in ohmsMeasured in ohms

7. Ohm’s LawOhm’s Law
 Current is proportional to the voltageCurrent is proportional to the voltage
and inverse to the resistanceand inverse to the resistance
 Current(I) = Voltage/ResistanceCurrent(I) = Voltage/Resistance
 Units:Units:
• Amperes = volts/ohmsAmperes = volts/ohms

8.  1 volt across a circuit with a resistance of1 volt across a circuit with a resistance of
1 ohm produces a current of 1 ampere1 ohm produces a current of 1 ampere
 What is the current if there are 12 voltsWhat is the current if there are 12 volts
across the same circuit?across the same circuit?
 What about If the resistance increased toWhat about If the resistance increased to
two ohms with 1 volt?two ohms with 1 volt?

9.  How much current flows through aHow much current flows through a
lamp with a resistance of 60 ohmslamp with a resistance of 60 ohms
when the voltage across the lamp iswhen the voltage across the lamp is
12V?12V?
 What is the resistance of an electricWhat is the resistance of an electric
frying pan that draws a current offrying pan that draws a current of
12A when connected to a 120-V12A when connected to a 120-V
circuit?circuit?

10. Electric Shock and Ohm’sElectric Shock and Ohm’s
 Result from the current through theResult from the current through the
bodybody
 Depends on voltage and body’sDepends on voltage and body’s
electrical resistance (100 ohms toelectrical resistance (100 ohms to
500,000 ohms)500,000 ohms)

11. Effect of Electric Current on theEffect of Electric Current on the
BodyBody
Current(A)Current(A) EffectEffect
0.0010.001 Can be felt`Can be felt`
0.0050.005 Is painfulIs painful
0.0100.010 Causes involuntary muscleCauses involuntary muscle
contractions (spasms)contractions (spasms)
0.0150.015 Causes loss of muscle controlCauses loss of muscle control
0.0700.070 Goes through the heart, seriousGoes through the heart, serious
damage, probably fatal if lastsdamage, probably fatal if lasts
longer than 1slonger than 1s

12.  At 100,000 ohms how much currentAt 100,000 ohms how much current
will flow through your body if youwill flow through your body if you
touch the terminals of a 12-Vtouch the terminals of a 12-V
battery?battery?
 If your skin is very moist, so thatIf your skin is very moist, so that
your resistance is only 1000 ohms,your resistance is only 1000 ohms,
and you touch the terminals of a 12-and you touch the terminals of a 12-
V battery, how much current do youV battery, how much current do you
receive?receive?

13.  To receive a shock there must be aTo receive a shock there must be a
difference in voltage ( a circuit mustdifference in voltage ( a circuit must
be formed)be formed)
 Electron flow will follow the path ofElectron flow will follow the path of
least resistanceleast resistance

14. Plugs and socketsPlugs and sockets
 Wired with three connectionsWired with three connections
 Two flat prongs are for current-Two flat prongs are for current-
carryingcarrying
 The larger round prong connects to aThe larger round prong connects to a
wire in the ground (grounded)wire in the ground (grounded)