Download Ohm`s Law

Ohm’s Law
Ohm’s Law provides a basic understanding of how
voltage, current, and resistance interact in a closed
circuit.
Assembled By
Ken Mitchell
Livermore TOPScience
Water Tank
Sprinkler
On-Off
Valve
Water Analogy to Electrical Circuit
Guess where the return circuit is.
Replaces
water flow
in pipe
Electron Flow
Replaces the
sprinkler
Replaces
the water
tank
L
O
A
D
Battery
Switch
Replaces on-off
water valve
See notes
below
Simple Electrical
Circuit Equivalent
to the Water
System
Voltage – Source - DC – Batteries (dry cells, Lead Acid, LiH, etc.)
Expressed in “Volts.”
Physical example – The depth of water in a water tank.
Current – Electron flow in the circuit, from + to – side of the power
source (battery). Expressed in Amperes.
Physical example – Water flowing through a hose.
Resistance -- An electrical component that operates on voltage
applied across it. Because it does work and dissipates the energy
as heat it’s simply called a “Load” and is expressed in “ohms.”
Physical example – The lawn sprinkler.
The Three Control Variables
Ohm's Law is a formulation of the of voltage, current, and resistance,
expressed as:
Where:
V - is the Voltage measured in volts
I - is the Current measured in amperes
R - is the resistance measured in Ohms
Therefore: Volts = Amps times Resistance
Ohm’s Law Simplified
The Variables:
Divide
across
here
V
V = Voltage
Multiply
across here
I = Current
R = Resistance
I R
Solution:
Cover the unknown
variable and the
remainder gives the
solution.
How to find the Unknown Variable
Using Ohm’s Triangle
V
I R
Example: What happens to
current (I) as the resistance
(R) goes to zero?
Hold a finger over current (I)
and observe what happens
when the resistance (R)
goes to zero.
********
Current (I) becomes infinite
(actually it will be limited by
the internal resistance of the
voltage source).
An Example Using Ohm’s Triangle
Example Calculation
Given: Voltage = 10 volts and the Resistance = 10 ohms
How does the current, I, change as the resistance goes
to zero?
Place finger over I in the triangle, Voltage is over
Resistance so we divide voltage by the resistance.
Make small table:
Voltage
/
Resistance = Current (I)
10 volts
/
10 ohms
= 1 amp
10 volts
/
5 ohms
=
10 volts
/
2 ohms
= 5 amps
10 volts
/
1 ohm
= 10 amps
10 volts
/
½ ohms
= 20 amps
10 volts
/
zero ohms = Unlimited amps*
2 amps
*Current is actually limited by the battery’s internal resistance.
Ohms' Law Example
Current in Amps
25
20
Voltage
15
Resistance
10
Current
5
0
1 2
3 4 5
6 7 8 9 10 11
Resistance in Ohms