Conceptual Physics: Electricity:

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Conceptual Physics Electricity
What is an electric field?
- The space surrounding a charged object is
affected by the presence of the charge an
electric field is established in that space. A
charged object creates an electric field
How do different electric fields interact?
These two examples are when a test charge is the
same sign as that of the electric field.
These two examples are when a test charge is the
opposite sign as that of the electric field.
How do charged objects create electric potential?
Gravitational Potential V.s Electric
Potential
1) Mass - a property of the object experiencing
the gravitational field. 2) Height - the location
within the gravitational field.
1) Electric charge - a property of the object
experiencing the electrical field. 2) Distance
from source - the location within the electric
field.
PE Potential Energy Q Charge
Electric Potential
While electric potential energy has a dependency
upon the charge of the object experiencing the
electric field, electric potential is purely
location dependent. Electric potential is the
potential energy per charge.
What is the electric potential difference?
Consider the task of moving a positive test
charge within a uniform electric field from
location A to location B as shown in the diagram
below. In moving the charge against the electric
field from location A to location B, work will
have to be done on the charge by an external
force. The work done on the charge changes its
potential energy to a higher value and the
amount of work that is done is equal to the
change in the potential energy. As a result of
this change in potential energy, there is also a
difference in electric potential between
locations A and B.
This difference in electric potential is
represented by the symbol V and is formally
referred to as the electric potential difference.
Relation to circuits Relationship to batteries
What is a circuit ?
Consider the following situation. There are two
metal plates parallel to each other and each
being charged with an opposite type of charge -
one being positive and the other being negative.
This arrangement of charged plates would create
an electric field in the region between the
plates that is directed away from the positive
plate and towards the negative plate. A positive
test charge placed between the plates would move
away from the positive plate and towards the
negative plate.
Now suppose that the two oppositely charged
plates are connected by a metal wire. What would
happen? The wire serves as a sort of charge pipe
through which charge can flow.
An example of a simple circuit
The fact that the light bulb lights and remains
lit is evidence that charge is flowing through
the light bulb filament and that an electric
circuit has been established.
How can we make a circuit? What's required?
What is a light bulb?
The difference between a successful and not
successful circuit.. closing the loop!
Electrons doing Work!
An electrical circuit is simply an energy
transformation tool. Energy is provided to the
circuit by an electrochemical cell, battery,
generator or other electrical energy source. And
energy is delivered by the circuit to the load at
the location of the load. The rate at which this
energy transformation occurs is of great
importance to those who design electrical
circuits for useful functions
Power - the rate at which mechanical work is done
Power is the rate at which electrical energy is
supplied to a circuit or consumed by a load.
UNIT OF POWER! WATTS!!! 1 watt 1 joule /
second
EX When it is observed that a light bulb is
rated at 60 watts, then there are 60 joules of
energy delivered to the light bulb every second.
A 120-watt light bulbs draws 120 joules of energy
every second. The ratio of the energy delivered
or expended by the device to time is equal to the
wattage of the device.
How do you calculate power?
P Watts V Volts I Amps
EX Determine the ... a. ... current in a 60-watt
bulb plugged into a 120-volt outlet. b. ...
current in a 120-watt bulb plugged into a
120-volt outlet. c. ... power of a saw that draws
12 amps of current when plugged into a 120-volt
outlet. d. ... power of a toaster that draws 6
amps of current when plugged into a 120-volt
outlet. e. ... current in a 1000-watt microwave
when plugged into a 120-volt outlet.
a. The current in a 60-Watt bulb plugged
into a 120-Volt outlet is 0.5 A. b. The
current in a 120-Watt bulb plugged into a
120-Volt outlet is? c. The power of a saw
that draws 12 amps of current when plugged into a
120-Volt outlet is? d. The power of a toaster
that draws 6 amps of current when plugged into a
120-Volt outlet is? e. The current in a
1000-Watt microwave when plugged into a 120-Volt
outlet is?
What is a Kilowatt-hour?
Electrical utility companies who provide energy
for homes provide a monthly bill charging those
homes for the electrical energy that they used. A
typical bill can be very complicated with a
number of line items indicating charges for
various aspects of the utility service. But
somewhere on the bill will be a charge for the
number of kilowatt-hours of electricity that were
consumed. Exactly what is a kilowatt-hour? Is it
a unit of power? time? energy? or some other
quantity? And when we pay for the electricity
that we use, what exactly is it that we are
paying for?
A Kilowatt hour is a unit of Power time. If
Power Energy / time, then Power time
Energy. So a unit of power time is a unit of
energy. The kilowatt hour is a unit of
energy. Example Remember 1000 watts in a
Kilowatt
10/kilowatt-hour (i.e., .10/kWhr) for their
electrical energy.
Power Rating (Watt) Time (hrs) Energy Used (kilowatt-hour) Costs (cents) Costs ()
60 Watt Bulb 1 0.060 kWhr 0.6 0.006
60 Watt Bulb 4 0.24 kWhr 2.4 cents 0.024
120 Watt Bulb 2 0.24 kWhr 2.4 cents 0.024
100 Watt Bulb 100 10 kW-hr 100 cents 1.00
60 Watt Bulb 1000 10
100 60 kW-hr