Transcript CURRENT

OHM’S LAW
Charge-volt-current-resistance
Ohm’s law
Circuits
An electric circuit is an external path that
charges can follow between two
terminals using a conducting material.
For charge to flow, the path must be
complete and unbroken.
An example of a conductor used to form
a circuit is copper wire. You can think
of a wire as a pipe for charge to move
through.
Charge
 Charge- is measured in coulombs-(C)
 1 COULOMB -AMOUNT OF ENERGY CARRIED BY 6.25 X108
ELECTRONS
 Two kinds of charge positive and negative
 Like charges repel and opposite charges attract
 Charge is a basic property of matter
 Electrical forces are forces between charges ie: static
electricity
Circuits
So do positive or negative charges flow through the
circuit?
Historically, positive charges were identified as the ones
that flowed in the circuit. It was not until a later time
that we understood that it was the negative charge
(electrons) which were free to move.
In reality, electrons flow from the negative terminal of the
battery, through a conductor, to the positive terminal.
This is the actual current.
Current
While actual current is the flow of electrons from the negative terminal to the
positive terminal, we use conventional current to model electric circuits.
Conventional current is the flow of positive charges flowing from the positive
to the negative terminal.
Going forward, we will use conventional current to discuss electric circuits.
Both conventional current and actual current lead to the same predictions but
flow in opposite directions.
Current
The letter "I" is the symbol for current. It is defined as the amount of charge
that flows past a location in a conductor per unit time.
ΔQ is the amount of charge, and Δt is the time it flowed past the location.
ΔQ
I=
Δt
Current
I=
ΔQ
Δt
The current, has the units Coulombs per second
.
The units can be rewritten as Amperes (A).
1 A = 1 C/s
Amperes are often called "amps".
CURRENT
 Current(I) – measured in amperes or amps(A)
 Current is the flow of charge
 1 amp is the flow of 1 coulomb per second
 current flows from plus to minus or high to low
 charge is what actually flows in a wire not electrons
 current carries energy and does work
 a voltage difference (battery) supplies energy to make
charges flow

ALTERNATING CURRENT
 In a DIRECT CURRENT (dc)
circuit the current flows in one
direction only. Normally batteries

In an ALTERNATING CURRENT
(ac) circuit the direction of
current flow through the circuit
changes at a particular frequency
(f). Flows in both directions
Normally house
 The frequency used in the United
States is 60 cycles per second or
60Hz.
CURRENT
http://www.youtube.com/watch?feature=pla
yer_detailpage&v=8gvJzrjwjds
12 C of charge passes a location in a circuit
in 10 seconds. What is the current flowing
past the point?
answer
1
A circuit has 3 A of current. How long does it
take 45 C of charge to travel through the
circuit?
answer
2
Batteries
A terminal is a conductor that allows
for the movement of charge.
Positive Terminal
Batteries convert chemical energy to
electrical energy which maintains the
potential difference.
The chemical reaction acts like an
escalator, carrying charge up to a
higher voltage.
Negative Terminal
Click here for a Battery Voltage
Simulation from PhET
VOLTAGE
 Voltage (V)– measure in Volts(V)- potential
difference
 Measure of energy in a circuit
 Energy flows from high energy to low energy
 Batteries add energy or voltage to a circuit
 Voltage is reduced when energy is used
 Bulbs and electric devices use energy
VOLTAGE
 http://www.youtube.com/watch?v=g287MugJC9E
Reviewing Basic Circuits
The circuit cannot have gaps.
The bulb had to be between the wire and the terminal.
A voltage difference is needed to make the bulb light.
The bulb still lights regardless of which side of the battery you place it on.
What is going on in the circuit?
What is the role of the battery?
Batteries and Current
The battery pushes current through the circuit. A battery acts like a pump, pushing charge through the
circuit. It is the circuit's energy source.
Charges do not experience an electrical force unless there is a difference in electrical potential
(voltage).Therefore, batteries have a potential difference between their terminals. The positive
terminal is at a higher voltage than the negative terminal.
click here for a video from
Veritasium's Derek on current
How will voltage affect current?
Conductors
Some conductors "conduct" better or worse than others. Reminder: conducting
means a material allows for the free flow of electrons.
The flow of electrons is just another name for current. Another way to look at it is
that some conductors resist current to a greater or lesser extent.
We call this resistance, R. Resistance is measured in ohms which is noted by the
Greek symbol omega (Ω)
How will resistance affect current?
Click here to run another
PhET simulation
Current vs Resistance & Voltage
Raising resistance reduces current.
Raising voltage increases current.
We can combine these relationships in what we call "Ohm's Law".
I=
V
R
Another way to write this is that:
V OR
R= I
V = IR
You can see that one Ω =
V
click here for a Veritasium
music video on electricity
A
Resistance –Ohm’s law
 RESISTANCE – measure in Ohms (  )
 low resistance – a lot of current flows
 high resistance – a little current flows
 Devices that use electrical energy have resistance
 Light bulbs ,TV Etc.


RESISTANCE = VOLTAGE
CURRENT





R= V
 = V
I
A
RESISTANCE
 The resistance of
electrical devices
ranges from very small
(0.001 Ω) to very large
(10×106 Ω).
 Each device is designed
with a resistance that
allows the right amount
of current to flow when
connected to the
voltage the device was
designed for.
Resistance
OHM'S LAW
 The magnitude of the electric current that flows through a
closed circuit depends directly on the voltage between the
battery terminals and inversely to the circuit resistance. The
relationship that connects current, voltage and resistance is
known as OHM'S LAW and is written as follows:
OHM’ S LAW-
 http://www.youtube.com/w
atch?feature=player_detailp
age&v=J4Vq-xHqUo8
Current vs Resistance & Voltage
Raising resistance reduces current.
Raising voltage increases current.
We can combine these relationships in what we call "Ohm's Law".
V
I= R
Another way to write this is that:
V OR V = IR
R= I
You can see that one Ω = V
click here for a Veritasium
music video on electricity
A
 Ohm’s Law states – relationship between
 Current (I) measured in amperes (A)
 Voltage (V) measured in Volts(V) potential difference
 Resistance (R )measured in Ohms( )

 Ohm’s law


Current (amps)
I = V – voltage (volts)
R- resistance (Ohms)


 Can also be stated as V = I x R
A flashlight has a resistance of 25 Ω and is
connected by a wire to a 120 V source of
voltage. What is the current in the
flashlight?
answer
4
What is the current in a wire whose resistance
is 3 Ω if 1.5 V is applied to it?
answer
5
How much voltage is needed in order to
produce a 0.70 A current through a 490 Ω
resistor?
answer
6
What is the resistance of a rheostat coil, if
0.05 A of current flows through it when 6 V is
applied across it?
answer
7
 A BATTERY OPERATED CD PLAYER USES 12V FROM THE
WALL SOCKET AND DRAWS A CURRENT OF 2.5 AMPS
CALCULATE THE RESISTANCEOF THE CD PLAYER.
answer
 12V/2.5= 4.8A
 Work on problems