Download Physics_Chapter 6

Biology – Premed
Windsor University School of Medicine and
Health Sciences
J.C. Rowe
Course Instructor.
Pre Med – Physics Chapter 6
Electricity
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Chapter 6 Electricity
 Electric Circuits
 Current and Voltage
 Electrical Resistance and Ohm’s Law
Chapter 6 Objectives
1. Describe the difference between current and voltage.
2. Describe the connection between voltage, current,
energy, and power.
3. Describe the function of a battery in a circuit.
4. Calculate the current in a circuit using Ohm’s law.
5. Draw and interpret a circuit diagram with wires,
battery, bulb, and switch.
6. Measure current, voltage, and resistance with a
multimeter.
7. Give examples and applications of conductors,
insulators, and semiconductors.
Chapter 6 Vocabulary Terms
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electricity
electric current
voltage
resistance
Ohm’s law
battery
open circuit
closed circuit
short circuit
 switch
 circuit diagram
 electrical
conductivity
 potentiometer
 wire
 volt
 electrical symbols
 amperes (amps)
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multimeter
ohm
resistor
ammeter
electrical
insulator
 semiconductor
 conductor
 electric circuit
Electric Circuits
Key Question:
What is an electric circuit?
Electric Circuits
 Electricity refers to the
presence of electric current
in wires, motors, light bulbs,
and other devices.
 Electric current is similar to
a current of water, but
electric current flows in solid
metal wires so it is not
visible.
 Electric current can carry a
lot of power.
Electric Circuits
 An electric circuit is something that provides a
complete path through which electricity travels.
 Wires in electric circuits are similar in some ways to
pipes and hoses that carry water.
Electric Circuits
 When drawing a circuit diagram, symbols are used to
represent each part of the circuit.
 These electrical symbols are quicker and easier to draw
than realistic pictures of the components.
Current and Voltage
Key Question:
How does current move
through a circuit?
Current and voltage
 Electric current is measured in
units called amperes, or amps
(A) for short.
 One amp is a flow of a certain
quantity of electricity in one
second.
 The amount of electric current
entering a circuit always equals
the amount exiting the circuit.
Current and voltage
 Conventional current was
proposed by Ben Franklin in the
1700’s.
 Scientists later discovered that
the particles that carry electricity
in a wire actually travel from
negative to positive.
 Today, we still use Franklin’s
definition.
Voltage
 Voltage is a measure of electric
potential energy, just like height
is a measure of gravitational
potential energy.
 Voltage is measured in volts (V).
 A voltage difference of 1 volt
means 1 amp of current does 1
joule of work in 1 second.
Voltage
 Since 1 joule per second
is a watt (power), you
can interpret voltage as
measuring the available
electrical power per amp
of current that flows.
Voltage
 The positive end of a 1.5 volt
battery is 1.5 volts higher than
the negative end.
 If you connect batteries
positive-to-negative, each
battery adds 1.5 volts to the
total.
 Three batteries make 4.5 volts.
 Each unit of current coming
out of the positive end of the
three-battery stack has 4.5
joules of energy.
Measuring voltage of a cell
 Set the meter to DC volts.
 Touch the red (+) lead of
the meter to the (+) battery
terminal.
 Touch the black (-) lead of
the meter to the (-) battery
terminal.
 Adjust the meter dial as
necessary.
Measuring voltage in a circuit
 Measure the voltage
across the battery
exactly as before.
 DO NOT DISCONNECT
THE CIRCUIT.
NOTE: Since voltage is measured from
one point to another, we usually assign the
negative terminal of a battery to be zero
volts (0 V).
Current and voltage
 A battery uses chemical
energy to create a voltage
difference between its two
terminals.
 In a battery, chemical
reactions provide the
energy to pump the current
from low voltage to high
voltage.
 A fully charged battery adds
energy proportional to its
voltage.
What does a battery do?
 A battery uses chemical energy to move charges.
 If you connect a circuit with a battery the charges
flow out of the battery carrying energy.
How do these batteries differ?
 Some are smaller and don't store as much energy.
 Other batteries made with Ni and Cd can be recharged.
 Which battery above has the greatest voltage capacity?
Measuring Current
 In practical electricity,
we still label current
flowing from plus to
minus or HIGH voltage
to LOW voltage.
 Current can't be
measured unless the
charges flow through
the meter.
Current is a flow of charge
Electrical Resistance and Ohm’s Law
Key Question:
How are voltage, current, and resistance related?
Electrical resistance
 Resistance measures how difficult it is for
current to flow.
Electrical Resistance
 The total amount of electrical resistance in a circuit
determines the amount of current that in the circuit for
a given voltage.
 The more resistance the circuit has, the less current
that flows.
Measuring resistance
 Set the meter to
measure resistance (W).
 Set the black and red
leads on opposite ends
of the objects.
The ohm
 Resistance is
measured in ohms
(W).
 One ohm is the
resistance when a
voltage of 1 volt is
applied with a
current of 1 amp.
Ohm's law
 German physicist Georg Ohm
experimented with circuits to
find an exact mathematical
relationship between voltage,
current and resistance.
 Ohm's law can be used to
predict any one of the three
variable if given the other
two.
Calculate current
 A light bulb with a resistance of 2 ohms is
connected in a circuit that has a single 1.5-volt
battery.
 Calculate the current that flows in the circuit.
 Assume the wires have zero resistance.
The resistance of electrical devices
 The resistance of
electrical devices ranges
from very small (0.001 Ω)
to very large (10×106 Ω).
 Each device is designed
with a resistance that
allows the right amount
of current to flow when
connected to the voltage
the device was designed
for.
Changing resistance
 The resistance of many
materials, including those
in light bulbs, increases
as temperature increases.
 A graph of current versus
voltage for a light bulb
shows a curve.
 A device with constant
resistance would show a
straight line on this
graph.
Electrical Conductivity
 The electrical conductivity describes a material’s
ability to pass electric current.
Conductors and insulators
 A material such as copper is
called a conductor because it
can conduct, or carry, electric
current.
 Materials that insulate against
(or block) the flow of current
are classified as electrical
insulators.
 Some materials are neither
conductors nor insulators.
 These materials are named
semiconductors.
Resistors
 Electrical components called
resistors can be used to
control current.
 Resistors have striped color
codes to record their "values"
(writing on them is difficult).
Potentiometers
 Potentiometers are a type of "variable" resistor that
can change from low to high.
 They are wired so that as you turn the knob, it
changes the distance the current has to flow.
Application: Hybrid Gas/Electric Cars