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, cross-sectional 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