Download Current – Power Current Power Preamble Conductors Insulators

Current - Power
(R. Bolton - 2012)
Current - Power
(R. Bolton - 2012)
Preamble

Current – Power

Physics, 8th Edition Custom Edition
Cutnell & Johnson
Chapter 20
20.1,
1 20
20.4
4
Pages 599-601, 605-607
(R. Bolton - 2012)
Current - Power



1
(R. Bolton - 2012)
Conductors


Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
2
Physics 155.3: Introduction to Electricity and Magnetism
2
Current - Power
(R. Bolton - 2012)
Insulators
Conductors are those materials where
electrons can move freely under the
influence of an electric field.
Conductors are important in Physics
155.3 because they allow us to create
electric circuits
circuits. Recall the copper wires
introduced earlier?
(R. Bolton - 2012)
We must have a potential difference
between two points
points.
We need a means by which the charge can
move
move.
(R. Bolton - 2012)
1
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
Prof. Achenbach discussed stationary
charge in his lectures.
We are interested in moving charge.
To accomplish this
Current - Power




3
Materials where all electrons are bound to either a
single atom or to a few atoms (always the same few
atoms) are called insulators
insulators.
Electrons in these materials do not move under the
influence of even a strong electric field.
I
Insulators,
l t
h
however, can be
b made
d to
t breakdown
b kd
(i.e.,
(i
fail) with a very high electric field. The classic
example is a spark plug in an engine (air is the
i
insulator).
l t )
More on this later (capacitors and dielectrics).
(R. Bolton - 2012)
3
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
4
4
Current - Power
(R. Bolton - 2012)
Current - Power
Semiconductors



Current
These materials have a few free electrons
that are able to move under the influence of
an electric field.
Semiconductors are important (but not in
Physics 155.3) because they are the building
blocks of all modern computer systems.
Th
These
building
b ildi blocks
bl k are referred
f
d to
t as
integrated circuits.
(R. Bolton - 2012)
Current - Power

5
Physics 155.3: Introduction to Electricity and Magnetism
(R. Bolton - 2012)
Average Current



| ΔQ |
Δt
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
6
Physics 155.3: Introduction to Electricity and Magnetism
6
Current - Power
(R. Bolton - 2012)


Current has
h units off coulombs/second.
l b /
d
A coulomb/second is called an ampere
(A). That is, 1 coulomb/second = 1 A.
(R. Bolton - 2012)
Current - Power
Current Direction
The symbol for average current is I
I=
If charge is moving in a conductor, for
example a copper wire, then, by
definition, current is the rate at which
charge passes a plane.
(R. Bolton - 2012)
5
Current - Power
(R. Bolton - 2012)

7
Current has direction and, by definition, the
direction is opposite to the flow of electrons.
The reason for this is that when current was
defined it was thought that the positive
charge was what flowed.
In the figure shown below, if Q is negative
charge
h
then
th the
th currentt fl
flow will
ill be
b from
f
right to left.
(R. Bolton - 2012)
7
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
8
8
Current - Power
(R. Bolton - 2012)
Current - Power
(R. Bolton - 2012)
Power

Power is the rate at which energy is
transferred or converted from one form to
another:




W
Δt
where:
P=




W is
i the
th workk done
d
in
i time
ti
interval
i t
l Δt.
Δt
In Physics 155.3 we are interested in power
that is converted to heat
heat.
(R. Bolton - 2012)
Current - Power
9
Physics 155.3: Introduction to Electricity and Magnetism
Note: No
Note
Now W has ttwo
o meanings
meanings: it is the symbol
s mbol for
fo work,
ok
and it is the units of power (i.e., 1 W = 1 J/s).
It is common for a symbol or an abbreviation to have more
than one meaning.
meaning The context in which it is used
determines exactly what is being talked about.
For example, C can mean coulombs and C can mean
capacitance. This can be quite confusing since coulombs and
capacitance are sometimes referred to in the same
sentence.
(R. Bolton - 2012)
9
Current - Power
(R. Bolton - 2012)
Current - Power
10
Physics 155.3: Introduction to Electricity and Magnetism
10
Current - Power
(R. Bolton - 2012)
Example 1
Solution:
Example
p 1
Light Energy to Electrical Energy

The units of power are J/s
1J/s = 1 watt. The abbreviation of watt is W.
(100W)
Over a period of 50 seconds, -5000 C of
charge is moved from the positive terminal of
a photovoltaic
h t
lt i cellll to
t th
the negative
ti terminal.
t
i l

What is the average rate of energy conversion
from light energy to electrical energy over the 50
second interval?

Note: Voltage across the plates is constant at 1 Volt
Volt.
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
11
(R. Bolton - 2012)
11
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
12
12
Current - Power
(R. Bolton - 2012)
Current - Power
Example 2
A cold day in Saskatoon

Example 2
Solution:
What is the average rate at which the
batteryy converts chemical energy
gy to
electrical energy?
(R. Bolton - 2012)
Current - Power
13
Physics 155.3: Introduction to Electricity and Magnetism
(R. Bolton - 2012)
13
Current - Power
(R. Bolton - 2012)
Power Equation (P=VI)

W
Δt
VAB
(R. Bolton - 2012)

(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
P=
Current - Power
14
Current - Power
Q = IΔt
Considering
d
+ve charge
h
(W
( BA is –ve))
So
∴W = V AB Q = V AB IΔt
14
Physics 155.3: Introduction to Electricity and Magnetism

− WBA
=
Q
Current - Power
Energy
Recall
P=

(2.4kW)
On a cold day in Saskatoon, a truck’s
battery separates charge at a rate of
I=200A for 15 seconds while the
electric starter motor is operating.


(R. Bolton - 2012)
Power is the rate at which energy is
converted.
Here are some interesting (?) energy
values.
values
W
= V AB I
Δt
15
(R. Bolton - 2012)
15
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
16
16
Current - Power
(R. Bolton - 2012)
Current - Power
Table of Energy Values
SaskPower
Energy

1J
Lifting a medium apple 1 meter
.35x106J
Apple (snack)
1x106J
Mars Bar (snack)
10x106J
2500 food calories
1.5x109J
Lightning bolt
1.6x109J
Tank of gas (45 liters)
36x109J
Storm (Halle Berry) tornado generation (X
(X-Men)
Men)
2x1017J
Sun’s Energy (Earth, 1 second)
1.33x1020J
2004 Tsunami/earthquake
4.74x1020J
World energy consumption (2008)
(R. Bolton - 2012)
Current - Power

17
Physics 155.3: Introduction to Electricity and Magnetism
We pay for energy from the provincial
power utility (SaskPower) in units of
kW·hours.
The current rate (as of August 1
1, 2010)
is approximately 10.61¢/kW·hour.
(R. Bolton - 2012)
17
Current - Power
(R. Bolton - 2012)
Example 3
Coffee Percolator

(R. Bolton - 2012)
Current - Power
18
Physics 155.3: Introduction to Electricity and Magnetism
18
Current - Power
(R. Bolton - 2012)
Example 3
Solution:
(0.42¢)
How much will it cost, on February 7,
2012, to percolate a coffee pot if it
takes 3 minutes (the ideal time) to
percolate the cold water and the coffee
pot draws 800W from the 120V outlet?

Conclusions??
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
19
(R. Bolton - 2012)
19
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
20
20
Current - Power
(R. Bolton - 2012)
Potential Renewable Energy
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Current - Power
21
21