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Circuits
QOD:
When building a circuit with the least
resistance, what type of wires would be
used?
Electric Current
• is the flow of electric charge
• In solids such as copper wire, electrons are free
to flow through the substance but protons can’t
• In fluids such as a car battery, both positive and
negative ions are free to flow
• Current is measured in amperes(A)
– 1 amp is the flow of 1 C of charge per second or
6.24 billion electrons per second
Voltage
• is potential difference, which you get when you
connect a positive terminal to a negative
terminal and form a circuit
• Measured in Volts(V)
• Charges will not flow without this potential
difference, which provides the “electric
pressure” to move the electric field
Electric Resistance
• the resistance to current(flow of electric field
energy) by the conducting material itself & is
measured in ohms (Ω)
• More resistance = less flow of electrons
Affecting Electrical Resistance
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Depends on the conductivity of the wire,
Thickness -thick wires have less resistance
Length – long wires have more resistance
Temperature - higher temp means more
resistance
• Superconductors
– no resistance to current at low temperatures
Ohm’s Law
• current = voltage/resistance or I = V/R
• states that the current through a conductor
between two points is directly proportional to
the potential difference across the two points.
• I = current measured in amperes
• V = voltage measured in volts
• R = resistance measured in ohms
1. While cooking dinner,
Mariah’s oven uses a 220.V line
and draws 8.00 A of current
when heated to its maximum
temperature. What is the
resistance of the oven when it is
fully heated?
2. What is the resistance of the
heating element in a car lock
de-icer that contains a 1.5-V
battery supplying a current of
0.5 A to the circuit?
Ohm’s Law and it’s Effect on You
• Normally, the human body has a resistance of about
100,000 Ω, but if you are soaked with saltwater, it can
be lowered to just 100 Ω, increasing current
dramatically
• Current in Amps
Effect
0.001
Can be felt
0.005
Painful
0.010
Involuntary Muscle Spasms
0.015
Loss of muscle control
0.070
Fatal if it lasts for more than 1s
DC vs AC
• Electric current may be AC or DC
• DC – Direct Current – flow of charge is always
in one direction, such as a battery(- to +)
• AC – Alternating Current – Electrons in the
circuit move first in 1 direction and then the
opposite direction.
– Here in the US, AC circuits alternate back & forth at
60 Hz or 60 times per second and outlets are at
120V
Electric Power
• the rate at which electrical energy is
converted into another form such as
mechanical energy, heat, or light
• P=IXV
• Power is measured in Watts(W)
• I = current measured in amps
• V = volts
3. The lighter in Jordan's car has a
resistance of 4.0 Ω. a) How much
current does the lighter draw when it
is run off the car's 12- V battery? b)
How much power does the lighter
use?
4. A 120.-V outlet in Ashtin's
college dorm room is wired with a
circuit breaker on a 5-A line so that
students cannot overload the
circuit, a) If Ashtin tries to iron a
blouse for class with her 700-W
iron, will she trip the circuit
breaker? b) What is the resistance
of the iron?
Circuits
• 2 types:
– Series – A circuit which has only one
pathway for the electrons to flow between the
terminals of the power source
– Parallel – A circuit which has multiple
separate paths that the electrons can take
Series Circuit
• Only 1 pathway
• Current is the same throughout the circuit
• Total Resistance is the sum of all resistances
– The same applies to current and voltage
• If one bulb burns out, they all go out
because the circuit is broken
Parallel Circuit
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•
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Multiple pathways for current
One bulb goes out, the others stay lit
Voltage is the same across each bulb
Total resistance decreases
as more branches are added
Circuit Schematics
• Circuits are described using simple diagrams
called schematic diagrams