Transcript intro to circuits and ohms law

Electrical
The amountPotential
of work that can be done by
some charge moving in a circuit.
Work
electrical potential 
charge
W
V= q
Joules
units:
= Volt = V
Coulomb
Current
The movement of electrons through a
conductor. The rate at which charge flows.
charge
current 
time
I=
q
t
Coulombs
units:
= Ampere = Amp = A
Second
DC (Direct Current) – All charges move in one direction in the
circuit.
AC (Alternating Current) – charges move one way and then the
other, changing direction from moment to moment.
CIRCUIT:
A path where electrons flow
and their energy is used.
RESISTANCE:
Opposition to the flow of
electrons in a circuit.
THE DAM ANALOGY
Dam = Battery, Outlet,
Power Supply
Water Depth =
V = Voltage or Potential
Pipe =
Wire
Water Wheel =
Energy
User/Converter
(Light Bulb, Motor…)
Valve = Resistance or Current Control
GROUND or Lowest Potential
How can the resistance
change?
• What are the variables that effect the
resistance of the flow of the water in the
Dam Analogy?
• Resistance is much like friction. The more
“friction” against a current, the more
resistance.
First variable
• What would happen if we widen the
path for the water?
• The resistance would be less.
• Therefore: R is in proportion to 1/A
2nd variable
• What would happen if we shorten the path
of the valve?
• There would be less to flow through,
therefore the resistance would be less.
• Therefore, R is proportional to L
3rd variable
• What would happen if we thicken the walls
of the pathway for the water?
• There would be more resistance.
• Therefore, R is proportional to density of
the material.
4th variable
• What would happen if we heat up the
pathway of the water?
• The resistance would increase
• As resistance goes up, temperature goes
up.
Summary
•
1)
2)
3)
4)
Resistance of an object depends on
Cross-sectional area of the resistor
Length of the resistor
Density of the resistor
Temperature of the resistor
What does the dam analogy tell us about
the relationship between I and R?
As R increases, I ……
DECREASES!
or
How could the current be increased in a
circuit whose resistance is held constant?
Increase the “push” or VOLTAGE
Georg Simon Ohm
Ohm’s Law:
V
Voltage
I  UNITS FOR
Current 
RESISTANCE
Resistance
R
Volts
Can
Be
Written As:

Ohm


More Typically Written As:
Amp
V
V

IR
R
This tells you the number of volts necessary
to push 1
amp of current through the device.
I
OR
Power
• Deriving power of a circuit
• P = Work/time
• How is work related to potential of a
circuit?
PRACTICE PROBLEMS
1. If it took 5 minutes for 10 C to flow in a
circuit, what was the current while the
circuit was working?
I = 0.033A
2. How long would it take for a 10 A stove
top to pass 500 C of charge?
t = 50 s
PRACTICE PROBLEMS
3. What is the voltage on a circuit that has a
resistance of 95Ω and a current flowing
through it of 0.5 A?
V = 47.5 V
4. What is the resistance of a circuit that is
hooked up to a 12 V battery and has
0.02 A flowing through it?
R = 600Ω