Transcript Circuits and Ohm`s Law

Electrical Circuits
An electric circuit is composed of 3 elements
•
Conductor, potential difference, closed loop
•
An electrical circuit without a load that has
some resistance is essentially a short circuit
•(Think about the batteries in the simulation)
•(An ammeter has very little resistance. Think
about my poor ammeters in a circuit with no
loads or other resistance.)
Electrical Circuits
In a series electric circuit all of the electrons must flow
through the same elements
In a parallel electric circuit electrons may flow through
different circuit elements
Electrical Circuits
In a series electric circuit, the voltage drops as it flows
through each resistive circuit element
Resistors in Series: Building Analogy
R1 = Lamp
R1
Elevator
R2 = Lamp
(battery)
R3 = Lamp
R2
3 steps
To go from the top to the bottom floor, all people must take the same path. So,
by definition, the staircases are in series. With each flight people lose some of
the potential energy given to them by the elevator, expending all of it by the
time they reach the ground floor. So the sum of the V drops across the resistors
the voltage of the battery. People lose more potential energy going down longer
flights of stairs, so long stairways correspond to high resistance resistors.
The double waterfall is like a pair of resistors in series because there is only one
route for the water to take. The longer the fall, the greater the resistance.
Diagramming circuits
Series Diagram
Ohm’s Law: Ohm's Law …says that, for many
materials under a wide range of conditions, the
voltage, V, and current, I, are linearly related, which
implies resistance, R, is independent of V and I.
When does it not apply? (Most important case)
Ohm’s Law Mnemonic
Questions
When a battery is connected to a complete
circuit, charges flow in the circuit almost
simultaneously. Why?
Resistance and Ohm’s Law
• Resistance is the impedance to the flow of
charge
• Resistance is defined as the ratio of
potential difference to current
• R = V/I Unit is the Ohm, Ω
• A device is said to obey Ohm’s Law of its
resistance is independent of the Voltage
Find Resistance of a material
• Depends on four factors
– R ∞ Temp
– R ∞ RHO (ρ) resitivity
– R ∞ Length
– R ∞ A-1 (cross-sectional Area)
Low resistance
• Short
• Fat
• cold
High Resistance
• Long
• Thin
• Hot
Parallel Circuit
• More than one path for the flow of charge
• More room for the flow of charge so
Resistance goes down
• Voltage must stay the same since all
charges have the same drop