15.2 Electric Current, Resistance and Voltage pp Mr. Richter

Agenda  Warm-Up  Check HW  Introduction to Current  Notes:  Charge and Current  The Source of Current (Voltage)  Resistance (Conductors and Insulators)

Objectives: We Will Be Able To…  Describe the relationship between electrons and current.  Explain, at the atomic level, the difference between insulators, semi-conductors and conductors.  Identify how voltage and charge are related.

Warm-Up:  What would cause electrons to flow in a wire?  Discuss at your table and we will discuss as a class in a minute.

Current

 Electric current is the flow of charge.  Current is measured in amperes (A). Abbreviated: amps.  One amp of current is the equivalent of one coulomb of charge passing by in one second.  Or roughly 6.24 x10 18 electrons per second!  Current is a measure of how many electrons pass by a point every second.

The Direction of Current  Benjamin Franklin defined current as going from positive to negative (just like everything else).  This is before we discovered that electrons have a negative charge. Electrons actually flow away from negative to positive.

The Direction of Current  We still say current flows from positive to negative, even though it’s the electrons that really move.  It doesn’t matter if it is electrons or protons moving. Usually the direction of current does not matter.

The Source of Current Voltage

The Source of Current  In metals (and some other materials) electrons are shared between atoms.  Electrons can move freely from one atom to another, and they are always moving randomly.

The Source of Current (p. 347)  Because electrons move randomly, there is usually no current.  The average movements of electrons cancel each other out.

The Source of Current (p. 347)  Electrons will only flow in one direction if there is something to push them.  When a voltage source is applied across a wire, electrons drift toward the positive end, creating current.

Voltage (potential)  Voltage potential measures the energy per unit of charge.  How much energy does each electron have? How much work can each electron do?  Voltage is measured in volts (V).  One volt = 1 joule/coulomb

Video Wrap-Up! 

Putting Voltage and Current Together  A small light bulb that draws 3 A of current is connected to a 1.5 volt battery for 10 seconds. 1.How much charge passes through the light bulb? 1.3 A = 3 coulombs of charge per second 2.3 A * 10 seconds = 30 C of charge 2.How much energy does the light bulb use? volts = 1.5 joules for every coulomb of charge V * 30 C = 45 joules

Warm-Up: Day coulombs of charge pass through a light bulb in 2.5 seconds. What is the current passing through the light bulb? 2.How much total energy does a 12 V battery supply to 6 C of charge?

Where Current Comes From

 If the wire is connected to a battery, electrons still bounce randomly, but…  Also slightly in the direction of the positive terminal of the battery.  This happens very slowly (a few mm per second) and is called drift velocity.  see p347

Where Current Comes From  If electrons move so slowly, why doesn’t it take a long time for lights to turn on?  There are already electrons in the wires. The electrons from the battery just push those along.  This happens immediately, so the light turns on right away.

Resistance

 What does resistance mean?  Resistance is the measure of how strongly an object inhibits the flow of electric current.  High resistance = low current, few electrons flow  Low resistance = high current, many electrons flow.

Resistance of Certain Materials  Just like with heat energy, some materials are better at transferring electrical charge than others.  Most materials can be classified as:  conductors  insulators  semiconductors

Conductors  Conductors are materials that allow the flow of electrons freely.  Current flows through conductors easily.  Electrons in conductors are free to bounce around.  Examples:  metals: copper, gold, silver, etc…  liquids: water, salt water

Insulators  Insulators are materials that inhibit the flow of electrons.  Current does not flow through insulators. Excess charge remains on the surface.  Electrons in insulators are tightly bound and cannot move.  Examples:  air, wood, glass, rubber  mostly the same as thermal insulators

Semiconductors  Semiconductors are between conductors and insulators.  They transfer electrons better than insulators, but not as well as conductors.  Semiconductors can be use to control the flow of electrons. Mostly used in complex electronics like computers, cell phones and televisions.  Examples:  silicon, carbon, germanium

Wrap-Up: Did we meet our objectives?  Describe the relationship between electrons and current.  Explain, at the atomic level, the difference between insulators, semi-conductors and conductors.  Identify how voltage and charge are related.

Homework  p Reviewing Concepts  #16-21