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Electric Current
Chapter 13
What is Electricity?
Some Key Terms:
Conductor: Allows Electricity to
flow easily
Insulator: Does NOT allow
electricity to flow easily
Potential Difference: A
difference in Electrical
Potential Energy. Also
called Voltage or an
Electromotive force (EMF)
What is a circuit
An electric circuit is a closed
conductive path with a power
source, for electricity to flow.
Potential Difference:
We have seen Potential
before…where….What
does it mean?
Where can
you find
Potential for
electricity…?
What is potential difference?
Potential difference is a measure of the difference in
electrical potential energy between two points in a circuit.
Potential difference is also called voltage.
Voltage is measured in volts
(V) using a voltmeter.
The voltage of a battery or
cell is a measure of the force
or “push” it gives the current.
A 1 V cell gives 1 joule (J) of
energy to each coulomb of
charge.
So, a 12 V battery gives 12 J of
energy to each coulomb of charge.
V
A voltmeter must be
connected in parallel
because it measures
the potential difference
across the component.
What can we measure in a
circuit? - Voltage
• Voltage
– Is a measure of __________
________ _________.
– When there is a voltage difference;
there is _______ _____ ____ _____.
– The Volt has a unit of ____.
– Example: a 9 Volt battery has an
electrical potential energy difference of
_____.
What is current?
Current is a measure of the rate of flow of electric charge in
a circuit. Electric charge is measured in coulombs.
How is current measured?
Current is measured in amperes
(A) using an ammeter.
This unit is named after Andre
Ampere, one of the early
scientists to study electricity.
A current of 1 A is 1 coulomb of
charge flowing every second.
So, a current of 5 A is 5 coulombs
of charge flowing every second.
A
An ammeter must be
connected in series
because it measures
the current flowing
through it.
What can we measure in a
circuit? - Current
• Current What it actually means!
– When there is a potential difference
(V) electrons will flow in the circuit.
– The electrons FLOWING is the actual
_________.
– We say the current flows from the +
end to the – end of the battery.
– The symbol for current is I and the unit
is A (ampere or amp)
“Viva La Resistance!”
- Eric Cartman
What is Resistance…in a circuit?
Resistance is a measure of how hard it is for
electrons to move in an electrical circuit.
–Symbol is R and it’s unit is Ω (Ohm).
-If we add things to a circuit like an light bulb,
it is harder for electricity for flow through it,
increasing the resistance.
Water model of a circuit
Battery and Bulb
• Can you make a bulb light using
only a bulb, a D-cell battery and one
piece of wire?
• Draw a picture of all the ways you
have tried to light the bulb.
• Identify which ones worked and
which did not.
1 step back, 2 steps
forward
• Current Electricity- The FLOW of
electric charge. Charge flows from
higher potential to lower charge
potential (more on this later)
• Q: How do we describe current
flow?
How did you light the bulb?
Congratulations! You
made a circuit!!!, but what
is a circuit?
You built this…
BUT…we would draw
it like this…
OR
Symbollogy…How to draw
circuit diagrams
Without the simple symbols,
We would have to draw complex
Pictures like this…
Current Flow
• Circuit-The path or paths which
current flows. MUST BE
COMPLETE!
• Conventional Current-How electrons
would flow if they had a positive
chargefrom Positive to Negative
• Current cannot flow through an
OPEN circuit
Closed circuit Vs Open Circuit
Closed Circuit
Open Circuit
Electric Current Summary
• Electrons in motion, move from +  • Current: The number of electrons that
pass a specific point in a circuit in one
second
• Current is brought to us by the letter I
I = Q/t
Q-Charge (Coulombs (C)) t-Time (Seconds (s))
• Circuit: electric current flows through a
closed, continuous path.
Units and Examples
•SI Units for Current is the Ampere
(Amp)(A)
•From the equation I = Q/t we see 1 A
= C/s
•Ex: A charge of 3 coulombs moves
through a circuit in .5 seconds. What is
the current in the Circuit?
q
I
t
3C
I
.5 S
C
I  6  6A
s
Conditions for current
• For current to flow one needs 2
things: (think back to the bulbs)
• A closed path of conducting
material. This path is called a _____
• A difference of potential (electric
charge) between two points in the
circuit.
Potential
Difference/Voltage
•Remember, another name for
Potential Difference is Voltage
emf
•Voltage is the difference in
electrical potential between two
places where e¯ are flowing
•Voltage is what “pushes” the
electricity through the circuit.
•But HOW????
Stuff you know! 
Explain the following
diagram:
How does this relate to a circuit?
Jumping back to Electricity
• Think of the Current as water. Water
flows from Higher PE to lower PE
• Eventually the water settles and
“loses” its energy
• The pump acts like a battery,
because it brings the water from
Low E to High E
• Battery keeps charge difference so
that current continues to flow.
How much Current?
•The amount of current flowing through
a circuit depends on 2 things:
1. How big the voltage is (how hard the
electrons are being pushed
2. How big the resistance is.
What is resistance?
How do you measure
current and voltage
Ammeter (smart wire)
Voltmeter
Resistance
• Resistance- degree of opposition
(sort of like friction) that charges
encounter as they move through a
conductor, measured in Ohms (Ω)
• HUH?
• Can you use the water analogy to
explain this in English?
• How can we have a small
resistance?
Ohms Law
• Voltage = Current X Resistance
V=IR
• The Real lyrics to Twinkle Twinkle
Little Star!
• Twinkle Twinkle Little Star,
Voltage= I x R
A second verse, coming soon 
Practice Problems!