Transcript Ch 22 Electric Current and Circuits

Ohm’s Law
From Static to Electric current
You cannot rub a balloon on
your head and then plug in
your radio to run off the charge
on your hair…darn!!!! 
 Electric current is the flow of
electric charge through a
conductor either in one
direction (DC) or alternating
directions (AC).

Pushing charge along
Just as water will not flow in a pipe unless one
end is at a different gravitational potential than
the other, electric charge will not “flow” unless
there is a difference in Electric potential across a
conducting path.
 Electric potential= electrical potential energy /
charge.
 1volt = 1 joule / 1 Coulomb
 A coulomb is the unit of charge.

How do you flow?
In order to keep charge moving, there
must continue to be a difference in
electric potential.
 Several sources provide this difference:

 Generator (produces AC)
 Battery (produces DC)
 Photovoltaic cell (solar cell)

Voltage provides the electric pressure
that moves the charges. Voltage may
also be called electromotive force (emf).
Electric Current
Recall Static Electricity is a buildup of charge.
 Electric charge that flows from one place to
another (sustainable) is called electric
current.
 Conventional current is the flow of “positive”
charge from high potential difference to low
potential difference.
 The unit of current is the ampere or amp (A).
One amp means one coulomb of charge flows
per second.

Effects of Various Electric
Current on the Body
Current in Amps
Effect
0.001 A
Can be felt (tingle)
0.005 A
Painful
0.010 A
Muscle Spasms
0.015 A
Loss of Muscle Control
0.070 A
Fatal if longer than 1 sec
Obstacles
The flow of electric current can be interrupted by
the object it passes through.
 Resistors dissipate energy by heat. The material of
the resistor is conductive, but not as conductive as
the wire used to connect circuit components. The
symbol for a resistor is:
15 W


This is called resistance. Resistance is measured
in ohms (W).
 High resistance harder for charge to pass through.
 Low resistance easier for charge to pass through.
Factors affecting resistance
Resistance of an object depends on the
material it is made of. Copper is a better
conductor than iron.
 If your skin gets wet, your resistance is
lower and the currents can rise to
dangerous levels of tens of milliamps.
 (Lie detector test?)
 Length of a wire affects resistance as
well as diameter.

Look inside a Resistor
Resistor in a Circuit
i
Resistance and Ohm’s Law
Any device that uses current or controls how
much current will flow is called a resistor.
 Resistance is the ratio of potential difference,
V, to current, I.

V
I

R
V
R
I
1volt
1amp 
1ohm
Resistance is measured in Ohms, Ω.
Power
A charge moving in a circuit expends energy.
 This might result in heating the circuit, or
turning a motor.
 The rate at which electrical energy is
converted to another form, such as
mechanical, heat or light, is called Electric
Power.

Current and Power
The rate of charge flow is called
current, I, and is measured in coulombs
/ sec or amperes, A.
 Power is the rate energy is transferred.
 1J/1s = 1 watt
 Power = I * V
 (3.0 C/s) * (120 J/C) = 360 W

Example
A 6 volt battery delivers 0.5 A of current
to an electric motor connected across
its terminals. What is the power of the
motor?
A. 3 w
B. 12 w
C. 5.5 w
D. 6.5 w

Electricity
Capacitors

Capacitors store energy as an electric field
bound between the plates of the capacitor.
The charge polarity of the capacitor will
match the polarity of the circuit power
source. Once a capacitor is charged, no
additional current will flow through. The
symbol for a capacitor is:
50 F
Capacitor in a Circuit
E
E
E
E
Electric Circuits
When charge flows through a conductor
in a continuous loop from a voltage
source (pump) and back to it, we have
an electric circuit.
 The total amount of charge in the circuit
does not change as it moves around.
 Electric energy can then be converted to
different forms like light, thermal energy,
mechanical energy, etc…

Drawing Circuit Diagrams
Light up a bulb with one wire and one
battery.
 Draw 4 DIFFERENT ways this can
happen.
 Next let’s draw your “circuit” in
schematic terms.

Electric Circuit Symbols
Your turn to Practice

Homework: Pg. 526 #’s 3, 4, 9, 11-13,
21, 27, 33, 38, 47