Transcript Current and Power - Spring Branch ISD

Current and Power
Batteries
•
•
•
•
Negative terminal is ground (0 V)
Not bottles of electrons!
Supply energy so charge can move
Cause electrons in wire to move by changing
the amount of Peelectric per q or ∆V
• Ex) 12V battery
– Positive is 12V and negative is 0V so the potential
difference is 12V
•Work is done to move the charge inside the battery from the
negative terminal (at 0V) to the positive terminal (at +12V).
•As the charge leaves the positive terminal it leaves with 12J
of PEelectric .
•As the charge moves through the circuit it gives up it’s 12J of
energy (lighting bulbs, running motors, etc.)
•The charge reaches the negative terminal with PEelectric = 0J
Electric Current
Current (I) is measured in Amperes (A)
Q = charge that passes through a given area
Current = the rate at which charges flows
Note: The direction current flows is the direction POSITIVE charges would move.
• Electrons are SLOW!
• Drift Velocity is approximately
0.000246 m/s
• 68minutes to travel 1 m
• Need an E field to make them move
faster from one place to another
Sources of Current
• Current requires a potential difference
• Positive charges (and current) moves from
high V to low V
• Batteries
• Chemical Energy to Electrical Energy
• Generators
• Mechanical Energy to Electrical Energy
Types of Current
• Direct Current (DC)
– Move in direction
– Batteries are always DC
• Alternating Current (AC)
– Motion of charges continuously change direction
– Terminals are constantly changing sign
– Occurs very fast (60times a sec)
– Supplied to homes and buildings
Resistance
• Opposition to the flow of charge
• Depends on the material
• Resistance (R) has units of Ω (ohm)
Factor
Lesser R
Greater R
Length
shorter
longer
Cross Sectional Area
Larger
Smaller
silver, copper
aluminum
lower
higher
Material
Temperature
Resistors continued
• A resistor is an electrical element that
– provides a specified resistance
– is used in circuit boards
• Our skin’s resistance
• when wet, resistance decreases
• when wet with salt water, resistance
decreases more
• perspiration decreases resistance
Ohm’s Law
To figure out the
strength of a resistor
you would need a color
chart that tells what
each color means.
Electric Power
Power = rate at
which work is done
(energy is
converted)
Electric work = rate
at which electrical
energy is converted
(rate of energy
transfer)
Electric Company
Electric Companies charge for Energy, not power