Electric Currents AP Physics Chapter 18 Electric Currents

Electric Currents AP Physics Chapter 18

Electric Currents 18. 1 The Electric Battery

The Electric Battery Alessandro Volta – produces the first battery made of zinc, silver, and cloth soaked in salt solution Electric Cell – two plates or rods (electrodes) of dissimilar metal placed a solution (like dilute acid) Battery – several cells connected together 18. 1

The Electric Battery The potential difference (voltage) between the terminals of a cell depend on what the electrodes are made of. Remember half-cell reactions from chemistry When two cells are placed end to end, or in series, their voltages add up 18. 1

Electric Currents 18. 2 Electric Current

Electric Current Circuit – continuous conducting path between terminals of a battery (or other source of EMF) Electric Current – flow of charge (electrons) I – current (amperes) Q – charge (coulomb) T – time 18. 2

Electric Current Ampere (for Andre’ Ampere) Usually called an amp Open Circuit – break in the circuit, no current flow 18. 2

Electric Current Short Circuit – when the load is bypassed Current increase Ground – allows for a continuous path for charge flow 18. 2

Electric Current For historical reasons, current is defined as being in the direction that positive charge flows 18. 2

Electric Currents 18. 3 Ohm’s Law

Ohm’s Law George Simon Ohm The actual values depend on the resistance of the conductor Called Ohm’s Law R – resistance measured in Ohms (W) 18. 3

Ohm’s Law Only true for Ohmic materials Vacuum Tubes, Transistors, Diodes are nonohmic 18. 3

Ohm’s Law A graph of current vs. potential difference The metallic conductor is ohmic The diode and filament are not 18. 3

Ohm’s Law Resistor – anything that uses electric energy Resistor – device used to control current The symbol for a resistor is 18. 3

Ohm’s Law The resistance value of a resistor is indicated by the colored bands on the resistor 18. 3

Ohm’s Law Misconceptions 1. Cells (batteries) do not put out a constant current. They maintain a constant potential difference. 2. Current passes through a wire and depends on the resistance of the wire. Voltage is across the ends of the wire. 3. Current is not a vector, it is always parallel to the conductor. The direction is from + to -. 18. 3

Ohm’s Law Misconceptions 4. Current or charge do not increase or decrease. The amount of charge in one end of the wire comes out of the other end. 18. 3

Electric Currents 18. 4 Resistivity

Resistivity Resistance is found to be directly proportional to its length and inversely proportional to its cross sectional area. ρ is called the resistivity (Wm) Longer extension cords must be thicker to keep resistance low 18. 4

Resistivity Some common resistivity values Material Silver Copper Gold Aluminum Tungsten Platinum Nichrome Resistivity Temperature Coefficient (Wm) (Co-1) 1. 59 x 10 -8 0. 0061 1. 68 x 10 -8 0. 0068 2. 44 x 10 -8 0. 0034 2. 65 x 10 -8 0. 00429 5. 6 x 10 -8 0. 0045 10. 6 x 10 -8 0. 00651 100 x 10 -8 0. 0009 18. 4

Resistivity Best Conductor is Silver, but Copper is close and much cheaper Tungsten is used in filaments Nichrome Apparently an Anime character 18. 4

Electric Currents 18. 5 Electric Power

Electric Power The rate of energy flow for an electric circuit That is more commonly written as Combining with Ohm’s Law it can also be written 18. 5

Electric Power The power company charges by the kilowatthour (k. Wh) Just a cool picture 18. 5

Electric Currents 18. 6 Power in Household Circuits

Power in Household Circuits Household circuits – wires will heat up as current increases In a 20 A household circuit In a 15 A household circuit Circuits are typically designed to run at 80% of the rated power output Different circuits have different gauge wires (diameter) 18. 6

Power in Household Circuits Circuit Breakers and Fuses Break the circuit 18. 6