physics 12. minutephysics…
TRANSCRIPT
Clip of the day: Minutephysics…. http://www.youtube.com/watch?v=jN-FfJKgis8
Review: Electric Potential
Electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent.
Electric potential is the potential energy per charge.
Unit = J/C or V
Q
EV Q
r
kqV
Conductors vs. Insulators
In 1729 Stephen Gray observed the movement of electric charge on some materials while not on others
As a result he classified materials as either:o Conductors – objects that allow electric charge to moveo Insulator – objects that restrict the movement of electric
charge
Beyond Electrostatics: As a result of Gray’s discovery,
scientists began to move beyond electrostatics and start to consider moving electric charges
In 1800, Alessandro Volta added to this new branch of charge theory when he invented the electrochemical cell
His cell was the first voltaic pile (or battery) and consisted of layers of silver, zinc and salt water soaked paper
When the top and bottom contacts were connected by a wire, an electric current flowed through the voltaic pile and the connecting wire.
Voltaic Cell One of the silver, zinc and salt water soaked discs
became known as a voltaic cell It derives electrical energy from spontaneous
redox reaction taking place within the cell. A voltaic cell requires:
o Cathode (positive end)o Anode (negative end)o Electrolyte (to allow the movement of charge)
Again…..ELECTRIC POTENTIAL or VOLTAGE (V)
The energy each electron has is called the
electric potential. Electric potential is commonly referred to as
voltage. A voltmeter is used to measure voltage.
Electric Current Electric current describes the flow of
charge carriers through a conductor Analogy: If we were discussing the flow of
water, it is likely we would consider a flow rate in litres per second; when it comes to charge, we consider charge per second (one ampere is equal to one coulomb per second)
Electric Current: Electric current is a measure of the rate at which
electric charges move past a given point in a circuit.
Symbol for current is (I). Unit for current is Ampere (A) = one coulomb per
second
Electric Circuits A circuit is simply a closed loop through which
charges can continuously move. An electric circuit exists anytime a closed loop
exists that involves a battery and some type of load (light bulb, etc.)
Charge carriers will flow from the battery with high electric potential, give off their electric potential as they cross the load and then return to the battery to regain electric potential
Requirements for a circuit:
There are two requirements that must be met in order to establish an electric circuit:1. There must be an energy supply capable doing work on
charge to move it from a low energy location to a high energy location and thus establish an electric potential difference across the two ends of the external circuit . This is the battery or power supply!
2. There must be a closed conducting loop in the external circuit that stretches from the high potential, positive terminal to the low potential, negative terminal.
Current vs. Electron Flow
By convention, current is assumed to be the flow of positive particles (from cathode to anode)
However, we now know that electrons are the charge carriers and have a negative charge (flow from anode to cathode)
So the direction of current flow and electron flow are in opposite directions
Resistance In a way that is similar to friction opposing motion,
electrical resistance opposes the flow of charge carriers
The ability to impede the flow of electrons in a material is called electrical resistance (R).
Resistance is measured in ohms (Ω) and is a function of the material that is conducting charge carrierso INSULATORS oppose the flow of electric current and have a
high resistance.o CONDUCTORS have a low resistance.
This leads to Ohm’s Law…. IRV
Ohm’s Law Named after the German scientist George Ohm. Ohm figured out a basic law for electricity
IRV
I electricity….
Ohm’s LawV = Volts (electrical potential/voltage) I = Amperes (current)R = Ohms (resistance)
And…….. I = V / RR = V / I
IRV
Ohm’s Law Current is directly proportional to voltage,
therefore, increasing (↑) the voltage increases (↑) the current.
The current is inversely proportional to resistance; therefore, as resistance increases (↑), current decreases (↓).
Sample problem:If a water heater has a voltage drop of 240V
and a resistance of 12.8Ω, what is the current?
I = V / RI = 240V / 12.8ΩI = 18.75A
The water heater has a current of 18.75 amps.