Transcript Ohm`s Law - science1d

1)
2)
Remove wire A from the circuit. What
happens?
The outside bulb will go out while the
inside bulb will remain lit.
Why does this happen?
The circuit is only broken at the outside
bulb, whereas the inside bulb still has an
unbroken path.
1)
Now put back wire A. Remove wire B
from the circuit. What happens?
Same as question #1
2)
Why does this happen?
Same as question #2
Put back wire B. Now remove wire C. What
happens?
Both bulbs go out.
2) Why does this happen?
The current cannot reach either bulb; the
circuit is broken/incomplete.
3) How is this circuit wired – in series or
parallel? How do you know?
It’s wired in parallel because there is more
than one path for the electrons to follow.
1)
1)
2)
3)
Close the switch. What happens?
Both bulbs turn on.
Remove one of the lights from the
circuit. What happens?
Both bulbs go out. OR
One bulb is still lit and glows brighter.
Why does this happen?
The circuit is broken/incomplete. OR
All voltage/energy is going to one bulb
only and is not divided between both.
1)
2)
Put the light bulb back on. Now remove
the battery from the circuit. What
happens?
Both bulbs go out.
Why does this happen?
There is no source to supply electricity
to bulbs.
1)
2)
3)
Disconnect wire A. What happens? Put
back wire A and disconnect wire B.
What happens?
In both cases, both bulbs go out.
Why does this happen?
The circuit is incomplete/broken.
How is this circuit wired – in series or
parallel? How do you know?
It is wired in series because there is only
one path to follow.
Direct Current
•Flows in only one
direction
•Eg: battery
Alternating Current
•Flows back and
forth at regular
intervals called
cycles
•Eg: wall socket
What is the major difference between static electricity and current
electricity????
Static electricity builds up on an insulator, while current electricity is the
flow of electrons through a conductor.
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http://www.youtube.com/watch?v=oSc
VYw5WaFA
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http://www.youtube.com/watch?v=aXr
cdvWQYew
Georg Ohm described how voltage and
current are affected when one of the
values is changed.
 Ohm’s Law states that as long as the
temperature stays the same,

V = IR
 This
means:
› The resistance, R, remains
constant
› The current, I, is directly
proportional to the voltage, V

We can rearrange this equation depending
on what we don’t know.
› If we are looking for the voltage, V, what would
the equation be?
 V = IR
› If we are looking for the current, I, what would the
equation be?
 I = V/R
› If we are looking for the resistance, R, what would
the equation be?
 R = V/I

A pneumonic for remembering Ohm’s
Law:

A current of 1.5 A flows through a 30-Ω resistor
that is connected across a battery. What is
the battery’s voltage?
› Use the GRASP method.
› G – Given: current I = 1.5 A; resistance R = 30 Ω
› R – Required: voltage V = ?
› A and S – Analysis and Solution:
 The correct equation is V = IR
 Substitute the values & their units; solve the problem
 V = IR
= (1.5A)(30 Ω)
= 45 V
› P – Paraphrase: The voltage in the circuit is 45 V.

A fire truck has a searchlight with a resistance
of 60 Ω that is placed across a 24-V battery.
What is the current in this circuit?
› Use the GRASP method.
› G – Given: resistance R = 60 Ω; voltage V = 24 V
› R – Required: current I = ?
› A and S – Analysis and Solution:
 The correct equation is I = V/R
 Substitute the values & their units; solve the problem
 I = V/R
= (24V)/(60 Ω)
= 0.4 A
› P – Paraphrase: The current in this circuit is 0.4 A.

A current of 625 mA runs through a bulb that
is connected to a 120-V power supply. What is
the resistance of the bulb?
› Use the GRASP method.
› G – Given: current I = 625 mA = 0.625 A;
voltage V = 120 V
› R – Required: resistance R = ?
› A and S – Analysis and Solution:
 The correct equation is R = V/I
 Substitute the values & their units; solve the problem
 R = V/I
= (120V)/(0.625A)
= 192 Ω
› P – Paraphrase: The resistance in the circuit is 192Ω.
When a problem develops within a wire that
allows electrons to flow through a device along
a different path than the one intended.
 A short circuit is an accidental low-resistance
connection between two points in a circuit,
often causing excess current flow.
 These can be dangerous – how??

› When a transmission line’s knocked down in a storm

However, it can also be helpful:
› Using a wire to go across a load – current flows more
easily through the wire path than through a bulb, for
example – allowing work to be done on a device while
not interrupting the circuit.