  Physics 12.  Minutephysics…. 

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 Physics 12

 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

 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 move o Insulator – objects that restrict the movement of electric charge

 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.

 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)

 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 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 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

 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

 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.

 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

 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 carriers o INSULATORS oppose the flow of electric current and have a high resistance. o CONDUCTORS have a low resistance.  This leads to Ohm’s Law….

 Named after the German scientist George Ohm.  Ohm figured out a basic law for electricity I electricity….

V = Volts (electrical potential/voltage) I = Amperes (current) R = Ohms (resistance) And……..I = V / R R = V / I

 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 (↓).

If a water heater has a voltage drop of 240V and a resistance of 12.8 Ω, what is the current? I = V / R I = 240V / 12.8 Ω I = 18.75A The water heater has a current of 18.75 amps.

 Voltage o Page 692 1-3  Current o Page 696 4-11  Ohms Law o Page 714 21-26

1. The thickness of wires 2. The length of wire 3. The material of wire 4. The temperature of the wire

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