Learning with PurposeSlide 1

Learning with PurposeSlide 1

Chapter 3: Ohm’s 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

𝐼 =𝑉

𝑅𝑉 = 𝑅𝐼

Examples

𝐼 =𝑉

𝑅=

Ohm’s law

Georg Simon Ohm, German scientist, 1787-1854

5 V

100 W

𝑉 = 𝑅𝐼 =

V

50 WI 100mA

Learning with PurposeSlide 4

Ohm’s law

1.57 A

To solve for resistance, Ohm’s low is 𝑅 =𝑉

IWhat is the resistance of the bulb?

Learning with PurposeSlide 5

Experiment

Conductance & Resistance

Slope = Conductance

𝐺 =250 − 0 (𝑚𝐴)

10 − 0 (𝑉)= 25𝑚𝑆

Resistance (reciprocal)

𝑅 =1

𝐺=

1

25 (𝑚𝑆)= 4 Ω 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

𝑷 =𝑾

𝒕

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 𝑷 = 𝑽 𝑰

With Ohm’s law 𝑉 = 𝑅𝐼 then 𝑷 = 𝑹𝑰𝟐

This is the power dissipated in a resistance, “Joule effect”

With 𝐼 =𝑉

𝑅you can obtain 𝑷 =

𝑽𝟐

𝑹

Electrical Power

Watt’s 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