Electric Current

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Transcript Electric Current

22.1 Objectives
 Students define an electric current and the ampere.
 Students describe conditions that create current in
an electric circuit.
 Students draw circuits and recognize they are
closed loops.
 Students define power in electric circuits.
 Students define resistance and describe Ohm’s
law.
Bill Nye Electric Current
What is Electric Current?
Electric Current
 Electric current is simply the flow of charge
 Electrons flowing in a wire constitute a current
 Measured in Coulombs per second, or Amperes
 Colloquially, Amp (A)
 refers to amount of charge crossing through cross-sectional area
per unit time
 Electrons have a charge of –1.610-19 Coulombs
 so (negative) one Coulomb is 61018 electrons
 one amp is 61018 electrons per second
 subtle gotcha: electrons flow in direction opposite to current,
since current is implicitly positive charge flow, but electrons are
negative
Electric Current
1C 6 10 e
1A

s
second
18

What is an Electric Circuit?
Would This Work?
Would This Work?
Would This Work?
The Central Concept:
Closed Circuit
Circuit in Diagram Form
battery
_
+
current
bulb
In a closed circuit, current
flows around the loop
electrons flow opposite the
indicated current direction!
(repelled by negative terminal)
Current flowing through the
filament makes it glow.
No Loop  No Current  No Light
Current is the
Flowrate
It sometimes helps to think of current as
flow of water, which is more familiar to
us. High current means lots of water
flow per unit time. Low current is more
like a trickle.
In electronics, it is the flow of charge,
not water, that is described by the word
current. And it’s always electrons
doing the flowing (thus electronics)
Voltage is the
Pressure
It sometimes helps to think of voltage as
water pressure. High voltage means lots
of water pressure. If the water faucet
valve is closed, no water flows no matte
how high a pressure there is. No current
is present if no electrons flow.
In electronics, it is the flow of charge,
not water, that is described by the word
current. And it’s always electrons
doing the flowing (thus electronics)
Resistance is the
Valve
It helps to think of resistance as the
valve. High resistance means the valve
is closed and little water flows through.
Low resistance means the valve is open
and a lot of water flows through.
Currents Divide and
Merge at Junctions

How much would the current
through the battery change if
I unscrewed one of the 2
bulbs?
+
A
B
How would the brightness of
“A” change if I unscrewed
“B”?
Answer
 The battery is supplying an equal amount of
current to each of the two bulbs. If one of
the bulbs is disconnected, the current
through the battery will be halved.
 Unscrewing “B” would not affect the current
through “A” so it will stay the same
brightness.
 Why wouldn’t more current flow through A?
 The battery does not supply constant current (is
there current even when the battery is
disconnected? NO. But there is voltage!)
Paul Hewitt Video #98
Bill Nye Electric Current
Electric
Circuit
Symbols
Series Circuits
 A connection or arrangement of devices that provides
only one path for a current.
 The same current flows through each device
(resistance).
Parallel Circuits
 A connection or arrangement of
two or more devices that provides
more than one path for a
current.
 The current is divided through
each path and merges again after
each device.
 Each device gets that same
voltage.
Series and Parallel Circuits
Parallel
Series
Series and Parallel Combo
Series
Parallel
Meters in a Circuit – Ammeter
 The ammeter measures current (I) in amperes or amps or
Coulombs/second
 It must be connected in series. Why?
 The voltmeter measures potential difference (ΔV) in volts or
Joules/Coulomb
 It must be connected in parallel. So what is I(ΔV)?
Circuit Diagram
This is useful for measuring resistance, flow of charge is
impeded by fixed atoms in conductors
R = ΔV/I measured as volts/amp or ohms (Ω)
An iron has a 6.9 A current when connected to a 120 V
outlet, what is its resistance? How is this a circuit?
Resistanc
e
 Increasing ΔV is like increasing the slope of a water
run, it increases current (I), they are directly
related
 The constant of proportionality is the resistance of
the conductor
 Obstructions such as rocks act as the resistance,
decreasing the current
 Resistance in a circuit is due to collisions between
the electrons carrying the current with the fixed
atoms inside the conductor
Georg Simon Ohm
 1787 – 1854
 Formulated the concept of
resistance
 Discovered the proportionality
between current and voltages
 For many materials, including most
metals, resistance remains constant
over a wide range of applied
voltages or currents
 This statement has become known
as Ohm’s Law, ΔV = IR
30
Paul Hewitt Video #100
Ohmic Devices
 Ohm’s Law is an empirical




relationship that is valid only
for certain materials
Materials that obey Ohm’s
Law are said to be ohmic
The resistance is constant
over a wide range of voltages
The relationship between
current and voltage is linear
How do light dimmers work?
Non-ohmic Materials
 The resistance increases as
the voltage increases
 Notice that current is small
and resistance is high when
reversed
 It therefore acts as a one
way valve for current
 A diode is a common
example of a non-ohmic
device
Paul Hewitt Video #102
Paul Hewitt Video #99
22.2 Objectives
 Students explain how electric energy is converted
into thermal energy.
 Students determine why high-voltage transmission
lines are used to carry electric energy over long
distances.
 Students define kilowatt-hour in electric energy
delivery.
Power
 The POWER dissipated in a resistor (or resisting
appliance) is proportional to the square of the
current that passes through it and to the resistance.
P  IV  I(IR)  I R
2
Energy
 POWER is Electric ENERGY per unit
time.
E  Pt  IVt  I(IR)t  I Rt
2
Resistance Becomes Heat
Electric energy becomes thermal energy when the
friction of electrons moving in a wire dissipates
heat. Resistors are coils of wire that can dissipate
heat.
P  IV  I(IR)  I R
2
Alternating Current
 Current comes to your house as a sine wave that alternates
between positive and negative current. The is called
alternating current (AC).
 This oscillating current has to be converted to direct
current (DC = one direction current like a battery).
 Wonder how we get direct current?
Rectification of AC Current
 We must get the down humps to flip to up-humps by
rectification!
Let’s see if Mr. Mosher can Draw!
 Mr. Mosher will now explain how diodes can be used
to separated the up-humps and down-humps of a
sine wave.
 A capacitor is used to flip (rectify) the down-humps
to up-humps.
 The combination is DC!
 P.S. - Please be patient with Mr. Mosher!
AC/DC Converter
 A diode is used to capture current in ONE DIRECTION ONLY.
Use another diode to get the OTHER DIRECTION.
 Put them together and what have you got? Direct Current!
 Now go and play!!!!!!!!
Power Plants and You
Power Plants and You
Electric energy becomes thermal energy
due to the friction of electrons. By
keeping the current very low and wire
resistance minimized.
Some long distance lines use
500,000 volts with low
amperes. Current can be
increase in your home with
transformers.
Power Costs and You
Power at your house is measured
in Kilowatt (1,000 Watts) - hours.
E  Pt  kilowatt  hr