Download L43-DynamicElec-Jan10

ELECTRICAL CIRCUITS
All you need to be an inventor is a good
imagination and a pile of junk.
-Thomas Edison
Would This Work?
Would This Work?
Would This Work?
The Central Concept: Closed
Circuit
circuit diagram
Scientists usually draw electric circuits using symbols;
cell
lamp
switch
wires
Simple Circuits
• Series circuit
– All in a row
– 1 path for electricity
– 1 light goes out and
the circuit is broken
• Parallel circuit
– Many paths for electricity
– 1 light goes out and the
others stay on
measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
measuring current
This is how we draw an ammeter in a circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
measuring voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
measuring voltage
This is how we draw a voltmeter in a circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
measuring current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
fill in the missing ammeter readings.
3A
?
4A
?
3A
1A
?
4A
?
4A
1A
1A
?
How you should be thinking
about electric circuits:
Current: the actual
“substance” that is
flowing through the
wires of the circuit
(electrons!)
• Current Intensity: The number of charges
that flow past a given point in an electrical
circuit every second.
I = q/ Δt
I = current intensity (in A)
q= the charge (in C)
Δt = the time interval (in s)
• Question
What is the charge needed for 1 minute of
operation when a car headlight requires a
current of 15 A
What do we know?
I = 15A
Δt = 1 min = 60 sec
q=?
I = q/Δt
 q = I x Δt
q = 15A x 60sec = 900 C
Answer: the charge needed is 900 C
How you should be thinking
about electric circuits:
Voltage: a force that
pushes the current
through the circuit (in
this picture it would be
equivalent to gravity)
Potential Difference: Is the amount of energy
transferred between 2 points in an
electrical circuit.
V = E/q
V = is the potential difference (in V)
E = is the energy transferred (in J)
q = is the charge (in C)
• Question
What is the amount of energy provided by a
charge of 200 C and a potential difference
of 120 V ?
What do we know?
V = 120 V
q = 200 C
E=?
V = E/q
 E=Vxq
E = 120 x 200 = 24000 J
Answer: the amount of energy is 24000 J (Joules)
How you should be thinking
about electric circuits:
Resistance: friction that
impedes flow of current
through the circuit
(rocks in the river)
Resistance = is the ability of a material to
hinder the flow of electric current
4 factors that affect the resistance of a
substance to flow
Factor
Description
Nature of the
substance
Poor conductors resist current flow more than good conductors
Length
The longer an element or wire, the greater its resistance to current flow
Diameter
Smaller diameter resists current flow more than a larger diameter
Temperature
Warm element usually resists current flow more than a cold element
Ohm’s Law
V=IxR
Georg Simon Ohm (1787-1854)
I
= Current (Amperes) (amps)
V
= Voltage (Volts)
R
= Resistance (ohms)
Question
What is the Voltage if the resistance is 30
Ohms and the current intensity is 15 A?
R = 30 Ohms
I = 15 A
V=?
V = R x I = 30 x 15 = 450 V
Answer is 450 V
Question
An automobile headlight has an average
resistance of 44 ohms. Car batteries
provide a potential difference of 22 volts.
What amount of current passes
through the headlight?
Answer
Resistance = 44 Ohms
Voltage (potential difference) = 22 V
Current (I) = ?
V=RxI
I = V / R = 22V / 44 Ohms = 0.5 Amps
Class Work
1) p. 173 of the textbook
Complete questions 10 – 18
2) Website Ohm’s law questions and
answers
3) Finish formal lab