Transcript Current and Resistance

Current and
Resistance
Current and Resistance
Voltage and Current
• Electrical potential energy – potential energy
of a charged object due to its position in an
electric field
• Potential difference – the change in the
electrical potential energy per unit charge
– Voltage
– Measured in volts (V)
• Measured with a voltmeter, which has high
resistance and is set parallel to a circuit
element
Current and Charge Movement
• Current – the rate at which electric charges
move through a given area
– Movement of electric charge
– Symbolized by I and measured in amperes (A)
• 1 A = 1 C/s
• Electric current = (charge passing through a given
area) / (time interval)
• I=ΔQ/Δt
Current and Charge Movement
• Current is based on the movement of charge carriers
– Electrons (negative charge) and protons (positive charge)
• However, the general convention for current assumes
motion of positive charge
– Because current was defined by Ben Franklin before the structure
of the atom was known
• Materials that have free electrons are good electrical
conductors and current easily passes through
– Metals
– Ions in solution (electrolytes)
Sources and Types of Current
• Batteries supply chemical energy to be
converted to electrical potential energy
which is then converted into kinetic energy
by the electronic device
• Generators convert different forms of
mechanical energy (such as kinetic in
hydroelectric dams) into electrical potential
energy which is then transformed into
kinetic energy by electrical devices
Sources and Types of Current
• Two types of current: alternating current (ac) and
direct current (dc)
– Classified on whether or not the direction of the
current changes
– Direct - current does not change direction
• Batteries
– Have positive and negative terminals
– Alternating – current continually reverses direction
• Wall outlets
• No net motion of charge carriers
– Vibrate back and forth
– Must change direction very rapidly to avoid flickering
– In the US, the ac frequency is 60Hz
» Different in other countries
» Must have an adaptor
Voltage and Current
• Cell – a device that is a source of electric current
because of a potential difference or voltage
between the terminals
• Parts of a cell
– Terminals – the ends of the cell
• One is positive
• One is negative
– Electrolyte – a solution that conducts electricity
– Electrodes – conducting materials
Voltage and Current
• Cells
– Several types:
• Electrochemical – batteries
• Photoelectric and photovoltaic – satellites,
calculators, streetlights
• Thermoelectric – thermostats
• Piezoelectric – microphones, headsets, computer
keypads, record stylus
Voltage and Current
• Electrochemical
– Two types: wet cells and dry cells
– Dry cells – contain a paste-like electrolyte
• Common batteries
– Wet cells – contain a liquid electrolyte
• Car batteries
Behaviors of Resistors
• Resistance – the opposition to the flow of current in a
conductor
– Symbolized by R
– Measured in Ohms (Ω) = 1V/A
• Resistance = (potential difference) / current
• R = V / I
• Ohm’s law - resistance is constant for a range of potential
differences
– Does not hold for all materials
Behaviors of Resistors
• Materials that obey Ohm’s law are classified as ohmic
– Current versus potential difference graph is linear
• Materials that do not obey Ohm’s law are classified
nonohmic
– Current versus potential difference graph is nonlinear
– Diode – a common nonohmic material
• High resistance in one direction and low resistance in the other
– Treat all resistors as ohmic unless specifically told otherwise
Behaviors of Resistors
• Resistance is based on length, crosssectional area, material, and temperature
• Resistance is caused by electron collisions
with the material they are traveling through
and other electrons
– With each collision, the electrons loose energy
– Factors that contribute to electron collisions
increase resistance
Behaviors of Resistors
• All else being equal, shorter wires have
less resistance than longer wires
• All else being equal, wires with greater
cross-sectional areas have less resistance
than those with less cross-sectional area
• Colder temperatures are typically
associated with reduced resistance
Behaviors of Resistors
Behaviors of Resistors
• Resistors are used to control the amount of current
in a conductor
– Too much current can overload a circuit
• Moisture, particularly salt water (including sweat)
decreases the resistance of the human body
• Normal dry resistance is around 500,000
• Soaked with salt water, the body’s resistance
decreases to about 100
• Currents of less than .01A are felt as a slight
tingling if they are perceived at all
• Currents above .15A through the chest cavity are
fatal
Electrical Resistance
• Conductors have low resistance
• Superconductors – a material whose resistance is
zero at or below some critical temperature
– Varies with each material
• Usually very cold
– Some new high temperature superconductors
» Superconduct at 150K
– Best conductors such as copper do not exhibit
superconductivity
• Insulators have high resistance
– Grounding – providing a pathway for current to
leave a charged object
• Semiconductors have intermediate resistance
Electrical Resistance