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Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Lecture 11
Chapter 28
Resistors
Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII
Physics II
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Today we are going to discuss:
Chapter 28:
Section 28.1-6 Resistors 28.7 (Example 28.29)
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Circuit Elements
Slide 31-22
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Resistorsin series/parallel
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
+ +
Resistors in ParallelConsider three resistors connected in parallel.
I
Rea
l cir
cuit
Equ
ival
ent c
ircu
it
ΔV
Resistors in parallel have the same potential difference, ΔV
I + +
;
We have replaced 3 resistors with an “equivalent” resistor.
+ +
Conservation of current
Req is inserted without changing the operation of the circuit, so I and ΔV are same as in the real circuit
Equivalent resistance of resistors in parallel.
=
I1
I2
I3
Ohm’s law;
ΔV
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
+ +
Resistors in SeriesConsider three resistors connected in series.
Rea
l cir
cuit
Equ
ival
ent c
ircu
it
ΔV
+ +
Ohm’s law ∆
Req is inserted without changing the operation of the circuit, so I and ΔV are same as in the real circuit
Equivalent resistance of resistors in series.
ΔV
ΔV1 ΔV2 ΔV3
∆∆
+ +
Are headlight wired: A) in parallel?
B) in series?
ConcepTest Headlights
ConcepTest Resistors I
The battery current I is
A) 3 A
B) 2 A
C) 1 A
D) 2/3 A
E) ½ A
=2/3 A
+
ConcepTest Resistors II
The battery current I is
A) 3 A
B) 2 A
C) 1 A
D) 2/3 A
E) ½ A
=3 A
+
=4
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Exa
mpl
e:
Ana
lyzi
ng a
com
plex
cir
cuit
a)Find the equivalent resistance.b)Find the current through and the potential difference across each of the resistors in the circuit.
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Real batteries
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Give me a break! I do it as fast as I
can!
Real Batteries. Internal resistanceTo drive a current in a circuit we need a “charge pump”, a device that by doing work on the charge carriers maintains a potential difference.Let’s look at a gravitational analog of a battery:
A person does work to maintain a steady flow of balls through “the circuit”.However, this guy cannot move balls instantaneously. It takes time. So there is a natural hindrance to a completely free flow. To describe this hindrance we can introduce the internal resistance, r.It is inside a battery and it cannot be separated from the battery.
Pot. difference of a battery without an internal resistance is called an electromotive force.(EMF, ε)
∆Terminal voltage
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Why is electric energy useful?
It can be easily transformed into other forms of energy.
Electric energy
Mechanical energy Thermal
energy
LightE/M waves
How to find the power transformed by these electrical devices
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Electric PowerConsider any electrical device:
ΔVSome charge Q
Δt (for Q to go through the device)
As some charge Q moves through the potential difference ΔV, the potential energy of the charge changes by: ∆ ∆Let Δt be the time required the charge to move through the potential difference ΔV. Then, the power P (the rate energy is transformed, Physics I) is:
∙ ∆∆ ∆ ∙ ∆ ∙ ∆
power transformed by an electrical device
The power transformed by a resistor can be written like this: ∙ ∆ ∆
∙ ∆ /
∆∆
A) 2 AB) 3 AC) 4 AD) 5 AE) 25 A
Most loudspeakers are designed to have a resistance of 4 Ω. If it is connected to an amplifier with a rating of 100 W, what is the current to the loudspeaker?
ConcepTest Electric Power
∙ ∆ ∆ ∙
Department of Physics and Applied PhysicsPHYS.1440 Lecture 11 Danylov
Thank youSee you next time
