Download Current #3 - Southgate Schools

Targets:
I can explain
resistance
and its function in a
circuit.
I can calculate resistance
in series and parallel
circuits.
I can relate current to
electric potential
difference and resistance.
Electrical
Resistance
Physics A
Current #3
Resistance
 An
electron traveling through the wires and loads
of the external circuit encounters resistance.
 Resistance

is the hindrance to the flow of charge.
While the electric potential difference established
between the two terminals encourages the
movement of charge, it is resistance that
discourages it.
 The
rate at which charge flows from terminal to
terminal is the result of the combined effect of
these two quantities.
Factors That Affect Resistance
 The
flow of charge through wires is often compared
to the flow of water through pipes.
 The
resistance to the flow of charge in an electric
circuit is analogous to the frictional effects between
water and the pipe surfaces as well as the
resistance offered by obstacles that are present in
its path.
 It
is this resistance that hinders the water flow and
reduces both its flow rate.
 Like
the resistance to water flow, the total amount
of resistance to charge flow within a wire of an
electric circuit is affected by some clearly
identifiable variables.
Factors That Affect Resistance
 The
total length of the wires will affect the amount of
resistance.
 The
longer the wire, the more resistance that there will
be.
 There
is a direct relationship between the amount of
resistance encountered by charge and the length of
wire it must traverse.
 After
all, if resistance occurs as the result of collisions
between charge carriers and the atoms of the wire,
then there is likely to be more collisions in a longer
wire.
 More
collisions mean more resistance.
Factors That Affect Resistance

The cross-sectional area of the wires will affect the
amount of resistance.

Wider wires have a greater cross-sectional area.

Water will flow through a wider pipe at a higher rate than it
will flow through a narrow pipe.

This can be attributed to the lower amount of resistance
that is present in the wider pipe.

In the same manner, the wider the wire, the less
resistance that there will be to the flow of electric charge.

When all other variables are the same, charge will flow at
higher rates through wider wires with greater crosssectional areas than through thinner wires.
Factors That Affect Resistance
 A third
variable that affects the resistance to charge
flow is the material that a wire is made of.
 Some
materials are better conductors than others
and offer less resistance to the flow of charge.
 Silver
is one of the best conductors but is never used
in wires of household circuits due to its cost.
 Copper
and aluminum are among the least
expensive materials with suitable conducting ability
to permit their use in wires of household circuits.
Factors That Affect Resistance

The conducting ability of a
material is often indicated by
its resistivity.

The resistivity of a material is
dependent upon the
material's electronic structure
and its temperature.

For most (but not all)
materials, resistivity increases
with increasing temperature.

The table lists resistivity
values for various materials at
temperatures of 20 degrees
Celsius.
Ohm’s Law
ΔV = I • R
This
equation indicates the two variables
that would affect the amount of current in a
circuit.
Ohm’s Law
I = ΔV / R
How
are current and electric potential
difference related?
The
greater the battery voltage (i.e., electric
potential difference), the ___________ the
current.
Ohm’s Law
I = ΔV / R
How
are current and total resistance
related?
The
greater the resistance, the ________
the current.
Charge
flows at the greatest rates when the
battery voltage is increased and the
resistance is decreased.
Check for Understanding
Which circuit has the greatest resistance?
Check for Understanding
1. Which of the following will cause the
current through an electrical circuit to
decrease? Choose all that apply.
a. decrease the voltage
b. decrease the resistance
c. increase the voltage
d. increase the resistance
Check for Understanding
2. A certain electrical circuit contains a
battery with three cells, wires and a light bulb.
Which of the following would cause the bulb
to shine less brightly? Choose all that apply.
a. increase the voltage of the battery
(add another cell)
b. decrease the voltage of the battery
(remove a cell)
c. decrease the resistance of the circuit
d. increase the resistance of the circuit
Check for Understanding
3. You have likely been warned to avoid contact
with electrical appliances or even electrical
outlets when your hands are wet. Such contact is
more dangerous when your hands are wet (vs.
dry) because wet hands cause ____.
a. the voltage of the circuit to be higher
b. the voltage of the circuit to be lower
c. your resistance to be higher
d. your resistance to be lower
e. the current through you to be lower
Check for Understanding
4. If the resistance of a circuit were tripled, then
the current through the circuit would be ____.
a. one-third as much
b. three times as much
c. unchanged
d. ... nonsense! There would be no way to
make such a prediction.
Check for Understanding
5. If the voltage across a circuit is quadrupled,
then the current through the circuit would be
____.
a. one-fourth as much
b. four times as much
c. unchanged
d. ... nonsense! There would be no way to
make such a prediction.
Check for Understanding
6. A circuit is wired with a power supply, a resistor
and an ammeter (for measuring current). The
ammeter reads a current of 24 mA (milliAmps).
Determine the new current if the voltage of the
power supply was ...
a. ... increased by a factor of 2 and the
resistance was held constant.
b. ... increased by a factor of 3 and the
resistance was held constant.
c. ... decreased by a factor of 2 and the
resistance was held constant.
Check for Understanding
6. A circuit is wired with a power supply, a resistor
and an ammeter. The ammeter reads a current of
24 mA. Determine the new current if the voltage
of the power supply was ...
d. ... held constant and the resistance was
increased by a factor of 2.
e. ... held constant and the resistance was
increased by a factor of 4.
f. ... held constant and the resistance was
decreased by a factor of 2.
Check for Understanding
6. A circuit is wired with a power supply, a resistor
and an ammeter. The ammeter reads a current of
24 mA. Determine the new current if the voltage
of the power supply was ...
g. ... increased by a factor of 2 and the
resistance was increased by a factor of 2.
h. ... increased by a factor of 3 and the
resistance was decreased by a factor of 2.
i. ... decreased by a factor of 2 and the
resistance was increased by a factor of 2.
Check for Understanding
7. An electrical device with a resistance of
3.0 Ω will allow a current of 4.0 amps to flow
through it if a voltage drop of ________ Volts is
impressed across the device.
Check for Understanding
8. When a voltage of 120 V is impressed across
an electric heater, a current of 10.0 amps will
flow through the heater if the resistance is
________ Ω.
Check for Understanding
9. A flashlight that is powered by 3 Volts and
uses a bulb with a resistance of 60 Ω will have a
current of ________ Amps.
10. Use the Ohm's law equation to determine the
missing values in the following circuits.