Download Chapter 2 Describing Motion

Unit 13/Chapter 18
Electric Currents
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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“Motion” of Positive Charges
 Many texts define current as the direction that the “positive” charges move; however,
there are not actually any “positive” charges that contribute to current flow.
 The “positive” charges are actually holes vacated by electrons which do actually
move and form current.
 These holes may be thought of as positive charges; however, they do not move as
they are “anchored” within the material.
 Electrons move when a potential difference is applied across a conductor.
 As an electron moves leaving behind a hole, another electron occupies the hole left
behind by the first electron.
 When this electron occupies this hole, it leaves behind another hole and the process
repeats.
 The positive charges appear to move; however, it is the electrons that actually move.
 However, due to a long standing tradition in physics, we will select the opposite
direction as that which the electrons actually move to be our current direction..
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Conduction Electrons
In organic compounds, electrons are bound to specific atoms.
In metallic compounds, some of the electrons are not bound to a specific
atom.
They are free to move throughout the metal.
These electrons are called conduction electrons.
If a potential difference is placed across the wire (like when you connect
the wire to a battery), then the electrons will move.
As they move, the electrons collide with the metallic atoms.
Depending upon the number of collisions an electron has, it may move
faster or slower through the metallic structure.
Remember, moving electrons in a wire are known as current.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Batteries and Current
A complete circuit is one that connects a battery to an electrical component
back to a battery.
Remember, electrons flow from the negative end of a battery through the
light bulb (resistor) and back into the positive end of the battery.
By our standard, the current flows in the opposite direction.
The symbol “I” is used to denote current.
Current is the number of electrons passing a certain point in a circuit per
unit of time.
Q ne
I

t
t
© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Resistors
Resistors are used to control the amount of
current flowing through a circuit.
Resistors impede the flow of electrons (current).
They impede this flow because certain electrical
items have maximum limitations on the current
they can handle.
Consider the Light Emitting Diode (LED) in the
figure to the right.
The battery supplies too high a current to the
LED.
As a result, the LED is damaged by the current.
When a resistor is placed into the circuit, the
current is reduced to a level appropriate for use
with the LED, and the LED is not damaged.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Ohm’s Law
Ohm’s Law gives us a mathematical expression relating the voltage (V),
Current (I), and Equivalent Resistance (R) of a circuit.
The three forms of Ohm’s Law are listed below.
In this equation, R is the Resistance in Ohms (), I is the Current in Amps
(A), and V is the Voltage in Volts (V).
V IR
V
R
I
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
I
V
R
Resistance and Resistivity
The resistance of a material is dependant on the resistivity of the material.
This relationship is expressed by Pouillet's law.
In this equation, R is the resistance in Ohms (),  is the resistivity in Ohm
meters (m), L is the length in meters (m), and A is the cross sectional area
in square meters (m2).
The values of resistivity for different materials may be found on page 501
of your textbook.
L
R
A
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Resistance and Resistivity
Suppose a wire of length L and area A (assume a circular cross section) has
a resistance of R1.
How much resistance (R2) would a second wire, made from the same
material, have if its length was 2L?
L
R
A
L
R1  
A
R2
2L 


A
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
 L
 2     2R1
 A
Resistance and Resistivity
Suppose a wire of length l and area A (assume a circular cross section) has
a resistance of R1 (in terms of the wire’s diameter).
How much resistance would a second wire, made from the same material,
have if its diameter was halved (d2 = d1/2)?
2
 d1  1 2
2
A1   r       d1
4
2
2
1 2 1  d1 
1
A2   d 2       d12
4
4  2  16
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
R1  
L
L

4

 d12
 d12
4
R
L
A

L 
 4R1
 4  4
R2  
2 
1
  d1 
 d12
16
L
Power Equation
Power is the rate at which electrical energy is consumed.
The equation used to determine power consumption is as shown.
This equation is used to calculate the Power consumed in Watts (W) by a
resistor when the current and the voltage drop across the resistor are known.
Using Ohm’s Law, derive two more forms of the power equation.
P  IV
V IR
P  I  IR   I 2 R
V
I
R
V
V 
P  IV   V 
R
R
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Direct Current v. Alternating Current
In a direct current circuit (one
with a battery) the electrons
flow in one and only one
direction.
In an alternating current
circuit (like the electricity from
your wall outlet), the electrons
repetitively change directions.
Either way, the light remains
lit because the electrons still
move through it.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Direct Current
Batteries produce direct current (DC).
The graph below shows a plot of a direct current produced by a battery.
Direct Current
1.2
Current (A)
1
0.8
0.6
0.4
0.2
0
0
5
10
Time (s)
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Alternating Current
Your home uses Alternating Current (AC).
This current is produced by generators in electrical power plants.
The graph below shows a plot of a alternating current.
Alternating Current
1.5
Current (A)
1
0.5
0
-0.5
0
5
10
-1
-1.5
Time (s)
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
15
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Electrical Safety: Fuse
Two devices are used in household and vehicle applications in order to
ensure electrical safety: a fuse and a circuit breaker.
When the current through the fuse pictured below exceeds a certain value,
the metallic ribbon melts and opens the circuit.
A fuse must be replaced after preventing an electrical overload..
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Electrical Safety: Circuit Breaker
When the current through a circuit breaker exceeds a certain value, it opens
the circuit.
A circuit breaker can be flipped back on and does not need to be replaced.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Batteries
Batteries come in many shapes and sizes that have a variety of
voltage and currents.
Identify the voltages of the four common battery types shown below.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Batteries
This slide will explain how a battery provides electricity for use in your small
electrical appliances.
Free electrons and “holes,” which are the absences of electrons, are produced
within the battery due to electrochemical reactions.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Series Battery Configuration WS 6 #15-18
 Batteries, like other electrical components, may be connected in series or parallel
configurations.
 In a series configuration like the one below, the electrons flow from one battery
into the next battery where they remain.
 As a result, the current produced by combining batteries in series is equal to the
current produced by just one of the batteries.
 However, when batteries are connected in series, the voltages (electric potentials)
are added.
 What is the voltage produced by the four AA batteries shown below?
 These four batteries each produce 1.5 V;
therefore, the total voltage produced by
these batteries is 6.0 V.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Parallel Battery Configuration WS 6 #15-18
 Notice what happens to the current when batteries are connected in parallel.
 The current produced by parallel batteries combines resulting in a higher
current.
 AA batteries typically provide 500 mA.
 What is the total current produced by the batteries below?
 2000 mA.
 What is the total voltage of these four batteries?
 The voltage is the same as that of a single battery.
 The schematic diagram for these batteries
would be as follows.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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