ohms law physics dr. robert mackay. voltage (volts) electrical pressure + - + - v
TRANSCRIPT
Ohm’s LawOhm’s LawOhm’s LawOhm’s Law
Physics
Dr. Robert MacKay
Voltage (Volts)Electrical Pressure
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-
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V
Current (Amps)Current (Amps)Charge FlowCharge Flow
Current (Amps)Current (Amps)Charge FlowCharge Flow
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++ + + + +
+ +
I
V
Resistance, R (Ohms, Resistance, R (Ohms, Ω))ImpedenceImpedence
Resistance, R (Ohms, Resistance, R (Ohms, Ω))ImpedenceImpedence
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I
VR Depends on Temperature Material of wire Area of wire Length of wire
Resistance, R (Ohms, Resistance, R (Ohms, Ω))ImpedenceImpedence
Resistance, R (Ohms, Resistance, R (Ohms, Ω))ImpedenceImpedence
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R Depends on: * Temperature * Material of wire * Cros sectional Area * Length
Area
Length
Resistance and ResistivityResistance and ResistivityResistance and ResistivityResistance and Resistivity
R L
A
= resistivityR=resistance
Material (ž- m) (°C-1)Silver 1.59x10-8 6.1x10-3
Copper 1.70x10-8 6.8x10-3
Carbon 3.6x10-5 -5.0x10-4
Silicon 2.5x102 -7.0x10-2
Glass 1012
Rubber 1015 1
R R0T
R R0(1T)
0T
0(1T)
As a wire heats it up its resistance increases.
Some thermometers work on this principle.
Ohm’s LawOhm’s LawV= I RV= I R
Ohm’s LawOhm’s LawV= I RV= I R
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I
V
ExerciseExerciseExerciseExercise Find the electric voltage required to have
10 Amps of current flow through a 2 Ω resistor.
Given: I= 10 A and R=2 Ω Wanted: V Solution: V=I R V= (10 A) (2 Ω) = 20 Volts
ExerciseExerciseExerciseExercise Find the electric current flowing in a 10 Ω
resistor (light bulb) when connected to a 20 Volt battery.
Given: V= 20 Vand R=10 Ω Wanted: I Solution: V=I R or I=V / R I= (20 V) / (10 Ω) = 2 Amps
ExerciseExerciseExerciseExercise When a 60 Volt battery is connected to a
circuit 4.0 amps of current flow from the battery. What is the circuit’s resistance.
Given: V= 60 V and I=4.0 Amps Wanted: R Solution: V=I R or R=V / I I= (60 V) / (4.0 Amps) = 15 Ω
Electric Power (Watts)Electric Power (Watts)
Power = Current x Voltage 1 Watt = Amp x Volt
CurcuitsCurcuitsR=10 Ω
A
V= 20 V
I =?P= ?
R
I
V
P
10 Ω
20 V
CurcuitsCurcuitsR=10 Ω
A
V= 20 V
I =?P= ?
R
I
V
P
10 Ω
20 V
2.0 A
V= I RorI = V / R
CurcuitsCurcuitsR=10 Ω
A
V= 20 V
I =?P= ?
R
I
V
P
10 Ω
20 V
2.0 Amps
40.0 Watts
P= I V = 2A(20V) = 40W
V= I RorI = V / R
CurcuitsCurcuitsR=?
A
V= 120 V
I =?P= 60 W
R
I
V
P
120 V
60 W
CurcuitsCurcuitsR=?
A
V= 120 V
I =?P= 60 W
R
I
V
P
120 V
60 W
V= I R
P= I V or I=P/V=60W/120V=0.5A
CurcuitsCurcuitsR=?
A
V= 120 V
I =?P= 60 W
R
I
V
P
120 V
60 W
0.50A
CurcuitsCurcuitsR=?
A
V= 120 V
I =?P= 60 W
R
I
V
P
120 V
60 W
0.50A
R=V/I=120V/0.5A=240
CurcuitsCurcuitsR=?
A
V= 120 V
I =?P= 60 W
R
I
V
P
120 V
60 W
0.50A
240
CurcuitsCurcuitsR=10 Ω
A
V= 20 V
I =2 A
P= 40 Watts
R
I
V
P
10 Ω
20 V
2 A
40 Watts
Two light bulbs are connected in series to a battery the resistance of each is 20 . What is the total effective resistance of this series combination?
Two light bulbs are connected in series to a battery the resistance of each is 20 . What is the total effective resistance of this series combination?
Req=R1+R2= 20 20 40
Two light bulbs are connected in series to a 40 Volt battery. The resistance of each is 20 . What is the total Current leaving the battery?
Two light bulbs are connected in series to a 40 Volt battery. The resistance of each is 20 . What is the total Current leaving the battery?
V=I R or I=V/R (R=Req=40
I=40V/40 =1.0 Amp
CurcuitsCurcuitsR=10 Ω
A
V= 20 V
I =2 A
P= 40 Watts
R
I
V
P
10 Ω
20 V
2 A
40 Watts
A bit of triviaA bit of trivia When 2 equal resistors are connected in parallel the effective
resistance is half the value of each. (two 10 resistor Req=5) When 3 equal resistors are connected in parallel the effective
resistance is one-third the value of each. (three10 resistor Req=3.33)
When 4 equal resistors are connected in parallel the effective resistance is one-forth the value of each. (four10 resistor Req=2.5)
When N equal resistors are connected in parallel the effective resistance is 1/N the value of each. (N 10 resistor Req=10/N)
What is the effective total resistance when 10 equal 10 resistors are connected in parallel?
What is the effective total resistance when 10 equal 10 resistors are connected in parallel?
Req=10/10=1.0
Two 20 Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?
Two 20 Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?
V=V1=V2=20 V
Two 20 Resistors are connected in parallel to a 20 V battery. What is the current flowing through each resistor?
Two 20 Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?
I=V/R I1=V1/R1 and I2=V2/R2
I1=20V/ 20 =1.0 A
I2=20V/ 20 =1.0 A
Two 20 Resistors are connected in parallel to a 20 V battery. What is the total effective resistance of this circuit?
Two 20 Resistors are connected in parallel to a 20 V battery. What is the current flowing through each resistor?
R=V/I=20V/2.0A=10
