Transcript PHYSICAL SCIENCE

PHYSICAL SCIENCE
Electricity
Part 2: Current
13.2 Current Objectives
Describe how batteries are sources of
voltage.
Explain how a potential difference produces a
current.
Define resistance.
Calculate the resistance, current, or voltage,
given the other two quantities.
Distinguish between conductors,
superconductors, semiconductors, and
insulators.
Voltage and Current
Electrical potential energy is the potential
energy of a charged object due to its position
in an electric field.
A negative charge will move away from
another negative charge.
The electrical potential energy of the moving
charge decreases because the electric field
does work on the charge.
A force can push a charge in the opposite
direction of the electric force which acts to
increase the electrical potential energy.
Voltage and Current
Potential difference is the change in the
electrical potential energy per unit charge.
The SI unit for potential difference is the volt
(V), which is equivalent to 1 joule per
coulomb.
The potential difference of batteries can
range from 1.5 V to 12 V.
Most common batteries are an electric cells
that convert chemical energy into electrical
energy.
Voltage and Current
Cells contain an electrolyte, a solution that
conducts electricity, and two electrodes, each
a different conducting material.
Wet cells, like a car battery, contain a wet
electrolyte.
Dry cells, like those in a flashlight, contain a
paste-like electrolyte.
The average cell has a potential difference of
1.5 V between the positive and negative
terminals.
Dry Cell
Wet Cell
Voltage and Current
In a flashlight, electrons move through the
light bulb from the negative terminal to the
positive terminal.
An electric current is produced when charges
are accelerated by an electric field to move to
a position of lower potential energy.
Current is the rate at which electric charges
move through a conductor.
The SI unit for current is the ampere (amp).
Voltage and Current
One amp equals one coulomb of charge
moving past a point in one second.
The charges always move from one terminal
to the other in the same direction for direct
charge.
In metals, moving electrons make up the
current, in gases and most solutions, current
is the result of the movement of positive and
negative charges.
Voltage and Current
Conventional current is defined as
movement of positive charge so the
direction of current in a wire is opposite
the direction that electrons move in that
wire.
Nerve signals are in the form of a
changing voltage across the nerve cell
membrane.
Sodium-Potassium Pump
Electrical Resistance
Resistance is the ratio of the voltage across a
conductor to the current it carries.
Resistance is caused by internal friction,
which slows the movement of charges
through a conducting material.
The resistance of a dim 40 W light bulb has a
higher resistance than the filament of a bright
100 W light bulb.
In most materials, some of the kinetic energy
of electrons is lost as heat.
Resistance in Light Bulbs
Electrical Resistance
Resistance = voltage/current or R=V/I
The SI unit of resistance is the ohm
(), which is equal to volts per amp.
A resistor is a special type of conductor
used to control current.
Every resistor is designed to have a
specific resistance.
Georg Simon Ohm and Ohm’s
Law
Electrical Resistance
A good conductor is any material in which
electrons can flow easily under the influence
of an electric field.
Metals tend to make excellent conductors
because electrons can move freely through
them.
Superconductors are metals and compounds
that have zero resistance when their
temperature falls below the critical
temperature.
The critical temperature of materials varies.
Electrical Resistance
Insulators have high resistance to charge
movement.
Insulating materials are used to prevent
electric current from leaking.
One may need to provide a pathway for
current to leave a charged object and do so
by means of grounding.
Semiconductors have properties between
those of conductors and insulators.