Learning with Purpose Slide 1 Learning with Purpose Slide 1 Chapter 3: Ohmβs Law, Energy, and Power Instructor: Jean-FranΓ§ois MILLITHALER http://faculty.uml.edu/JeanFrancois_Millithaler/FunElec/Spring2017
Learning with PurposeSlide 1
Learning with PurposeSlide 1
Chapter 3: Ohmβs Law, Energy,
and Power
Instructor: Jean-François MILLITHALER
http://faculty.uml.edu/JeanFrancois_Millithaler/FunElec/Spring2017
Learning with PurposeSlide 2
Review of V, I, and R
Voltage: amount of energy per charge available to move electrons from one point to another in a circuit and is measured in volts
Current: rate of charge flow and is measured in amperes
Resistance: opposition to current and measured in ohms
Learning with PurposeSlide 3
Most important fundamental law in electronics
Relates voltage, current, and resistance
πΌ =π
π π = π πΌ
Examples
πΌ =π
π =
Ohmβs law
Georg Simon Ohm, German scientist, 1787-1854
5 V
100 W
π = π πΌ =
V
50 WI 100mA
Learning with PurposeSlide 4
Ohmβs law
1.57 A
To solve for resistance, Ohmβs low is π =π
IWhat is the resistance of the bulb?
Learning with PurposeSlide 5
Experiment
Conductance & Resistance
Slope = Conductance
πΊ =250 β 0 (ππ΄)
10 β 0 (π)= 25ππ
Resistance (reciprocal)
π =1
πΊ=
1
25 (ππ)= 4 Ξ© Voltage (V)
0 2 4 6 8 10 12
Curr
ent (m
A)
0
50
100
150
200
250
300?
Learning with PurposeSlide 6
Work = force x distance newton-meter (N-m) or joule (J)
Energy
James Prescott Joule, Bristish, 1818-1889
Force
Distance
d =1 m
1 J = raising an apple over 1 m
Energy is the ability to do work(same unit)
Symbol is W
F = 1 N
W = F x d
Learning with PurposeSlide 7
Power is the rate of doing work, the energy per amount of time
π· =πΎ
π
The unit is joule per second (J/s), which defines a watt (W)
For electrical appliances we use the kilowatt-hour (kWh)
1 kWh = 3.6 x 106 J
Power
James Watt, Scottish, 1736-1819
Learning with PurposeSlide 8
What amount of energy is converted to heat in sliding a box along a floor for 5 meters if the force to move it is 400 N?
What power is developed if the box is moved in 10 s?
What is the energy used in operating a 1200 W heater for 20 minutes?
Questions
Learning with PurposeSlide 9
Power = Voltage x Current π· = π½ π°
With Ohmβs law π = π πΌ then π· = πΉπ°π
This is the power dissipated in a resistance, βJoule effectβ
With πΌ =π
π you can obtain π· =
π½π
πΉ
Electrical Power
Wattβs law
Learning with PurposeSlide 10
What power is dissipated in a 27 W resistor if the current is
0.135 A?
What power is dissipated by a heaterthat draws 12 A of current from a 120 V supply?
What power is dissipated in a 100 W resistor with 5 V
across it? ?
Questions
Learning with PurposeSlide 11
Loss of energy by electrons as they flow through a resistance
Voltage drop
Voltage = energy divided by charge (V = W/Q)
Same number of electrons but less energy
Voltage drop
Most energy
Less energy
Least energy
Learning with PurposeSlide 12
For a resistive device (ex: Lamp, resistor): NO
For other device (ex: diode, transistor, rotor, motor): YES
Current path important?
Learning with PurposeSlide 13
Expected battery life of batteries is given as the ampere-hours specification. Various factors affect this, so it is anapproximation. (Factors include rate of current withdrawal,age of battery, temperature, etc.)
How many hours can you expect to have a battery deliver 0.5 A if it is rated at 10 Ah?
Ampere-hour rating of batteries