Transcript 12.1 Introducing Current electricity

11.3 Ohm’s Law
Relating Current,
Voltage, and Resistance
p. 458-464
German
physicist Georg
Ohm (1787-1854) found
relationship between
potential difference &
current.
 He kept potential
difference & current
CONSTANT but varied the
length of the conductor
(changes resistance).
Ohm’s Experiment



Observations: as the length of the
conductor increased, the current
decreased.
Recall: increasing length of wire
increases resistance (if resistance
increases current decreases provided that
voltage stays the same)
Findings:
 For certain materials, when you plot a
graph of the voltage versus the
current, you get a straight – line
relationship.
 The slope of the straight line represents
the resistance of the material.
 The steeper the slope of the straight
line, the greater the resistance
Relationships…
The
relationship written:
R = resistance in ohms
V = voltage OR potential difference in
volts (v)
I = current in amperes (A)
This Relationship is called Ohm’s Law
which states: “as the potential
difference across a load increases so
does current”
Ohm’s relationship to calculate resistance
of a load in a circuit…
Ohm’s equation rearranged to solve
for current:
Ohm’s relationship rearranged to
calculate voltage across a load…
Current through Loads in Series

If you have a circuit with one load the total Resistance of
the circuit will be different than if you have two or more
loads connected in series

Electrons have only one path to follow and with two or
more loads they have more bumps to deal with along the
way, and from this the current flowing through a circuit
with two or more loads will be less than the current flowing
through a circuit with one load.

A) Electrons leaving the battery will have two possible
paths to follow and since each of those paths have
the same load the current splits in two

What do you think will happen in figure 5 (b)?
The electrons will have three possible paths to follow
and the current will split in three.
As a
result:



The total resistance in a parallel circuit will be
less than if the loads were connected in series
since the electrons that flow through one branch
go through only one load
Since the electrons flow through one branch and
go through one load than ALL the electric
potential energy that the electron receives from
the battery will be converted into light and heat
From this, the voltage drop across each parallel
load will be the SAME as the voltage drop across
the battery




Again, the brightness of each lamp in a series
circuit decreases as you connect more lamps,
let’s examine electric potential energy of the
circuit to explain this. RECALL BELOW:
The battery converts chemical energy into
electric potential energy.
This potential energy gets transferred to all the
electrons that leave the battery and creates the
current flow in the circuit
As electrons move through the circuit the electric
potential energy gets converted to kinetic energy
Circuit having one load:
 When electron passes through the load the electric
potential energy that the electron received from battery
gets converted into light and heat.

Once the electron completes one loop, it returns to
the battery only to be given more electric potential
energy which allows the electron to continue moving
through the loop of the circuit over and over again

Since voltage is related to electric potential
energy the voltage drop across the one
load will be the SAME as the voltage across
the battery
Circuits having two identical loads:
 An electron leaving the battery will have the same amount of
electric potential energy as if there were one load



Reason: the battery can only supply so much potential
energy.
For electron to go through each lamp only half of the
electric potential energy gets converted into light and
heat.
Since only half gets converted though each lamp, the
voltage drop across each lamp is half the voltage drop
across the battery


The more the identical lamps connected in series means
less of an electrons electric potential energy gets
converted to heat and light.
The voltage drop across each load decrease
Vsource
___________
An
accidental low resistance
connection between two points in a
circuit, causing excess current
(recall: decrease resistance-increase
current)
Electrical device will not work and
could be dangerous. Wire becomes
hot and could start a fire
(supplement p. 462)
If
there is too much current
flowing through a circuit, it will
become overloaded.
This
can be dangerous because
it can cause the wires to heat
up and start a fire.
A
fuse prevents too much current from
flowing through a circuit.
If
the current is too high, the wire
inside it melts.
This
opens the circuit
so no current can flow.
Once
a fuse has melted, it needs to be
replaced.
A fuse box holds all the fuses for all the
circuits in a house or apartment.
Like
a fuse, a circuit
breaker stops electric
current from flowing
in an overloaded
circuit.
It
works like a switch.
If
the circuit is overloaded, the
circuit breaker trips, opening the
circuit.
After
the problem is fixed, the
circuit breaker can be reset to close
the circuit.
Unlike
a fuse, a circuit breaker can
be used over and over again.
A
circuit panel has a circuit breaker for each
circuit in a house or building.

3 different kinds of connections
1.
Hot or “live” wires
– carry current to the appliance
- red or black
2. Neutral wire
- carries current away from the load
- white
3.
Ground wire
- carries unsafe current to the ground
- green or bare copper wire
Connects
to the ground.
It
gets rid of electric current by
letting it flow to the Earth.
It
is a safety feature that
ensures unsafe current flows to
the ground rather than through
you.
Appliances
used near
water are a danger
because water conducts
electricity.
Parts
of your home that
are close to water often
have a GFI outlet.
A
sensor measures the current
flowing in the hot wire and the
neutral wire.
If
there is a difference in
current, the circuit breaker
opens the circuit.
If
you push
the “test”
button, the
power to the
outlet is cut.
The
“reset”
button
restores the
power.
Homework:
 Read
11.3 and supplement your notes
 Complete p. 460-461 ALL practice problems
(#1-3 x 3boxes) p. 467 #1,3,7-10,12
 Chapter 11 review p. 470-471 #1-4,6,8-10,13
videos
http://www.youtube.com/watch?v=5laTkjIN
Hrg ELECTRIC CURRENT
http://www.youtube.com/watch?v=YGvu9iq
jJq4&feature=related RESISTANCE
http://www.youtube.com/watch?v=F1p3fgb
DnkY&feature=related VOLTAGE