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notes about current electricity for physics


Current Electricity

How is current produced?

When a high potential is connected by a conductive material to a low potential. Flow of electrons is called electric current. Units: C/s = Amperes (Amps)

q I t

Low Potential Equal Potential

High Equal Potential Potential

Low PotentialElectron Pump

High Potential

When the potentials are equal, the current stops flowing To increase the potential of the electrons an electron pump must convert (do work) another form of energy into electrical potential energy.

Electron pump examples: Voltaic or galvanic cell (dry cell) Several cells connected (Battery) Photovoltaic cell (solar cell) Generator

Two types of current

Direct Current (D.C.) Flow of electrons in only one direction Battery

Alternating Current (A.C.) Flow of electrons at first in one direction and then the other direction. Generator In your House 60 Hertz

What affects current flow?

Potential Difference Voltage Resistance - slowing or reduction of the flow of current Resistance depends on the material, length of material, and the cross sectional area of the material through which the current is moving.


R is resistance, L is length,

is resistivity, A is the cross sectional area

Ohms Law

Ohms Law stated The ratio of V/I is a constant, where V is the voltage applied across a piece of material (such as wire) and I is the current through the material. R is the resistance of the piece of material. Units: Ohms (W) = Volt/amp Voltage Resistance Current


Electric Power

When there is current (I) in a circuit as a result of a voltage (V), the electric power (P) delivered to the circuit is: Power current voltage P I V Units: watts = amp x volt = J/s

Electric Power

Many electrical devices become hot when provided with sufficient electric power. Toasters, irons, space heaters, heating elements in electric stoves, and incandescent light bulbs. In such cases it is possible to obtain two equivalent power equations

PI R When electric energy is transferred into thermal energy in a resistor, the increase in thermal energy is:2

V2 P R

Q I Rt2

units: Kilowatt hour or Joules

Current in electric Circuits

An electric Circuit is a closed loop. Basic Circuits consist of three things: Electron pump (Battery) Device that reduces potential. (User) Conducting connections (Wires)

Direct Current Circuits:

Circuit closed loop in which current can flow Basic simple circuit Voltage source Voltage uses conducting connections (wire)

Diagrams for electric circuits Describe in words use photographs/drawings Use symbols (schematics)

Schematic Symbols

Drawing Schematic Circuits

Draw symbol for battery and indicate + and - terminals Draw wire from + to the voltage user and draw its symbol. If a point where there are two current paths, draw connection symbol and follow one path to the user. Return to the second path and draw to its user and continue until paths join. Follow path to the - terminal Check work Connect ammeter in series Connect voltmeter in parallel


Describe the configuration of a working circuit Interpret circuit diagrams Describe the characteristics of series connections Determine equivalent resistance of circuits having two or more resistors. Explain the cause of short circuits Homework: Read Pages 531-541 Pg 534 #1-3, Pg 537 # 6-9

Series Circuits

Only one path for current to follow User followed by another user (ie. Resistor followed by a light)

Vequ V1 V2 V3 ...

Series Circuit Rules

Current is the same for each user The effective resistance is the sum of all resistors in the seriesRequ R1 R2 R3 ...

The sum of the voltage drop is equal to the total voltage dropVequ V1 V2 V3 ...

Parallel Circuits

Two or more paths for current to follow

Parallel Circuit Rules

Total current in the circuit is the sum of the current in all its paths (branches) I tot I 1 I 2 I 3 ... The equivalent resistance decreases with more parallel resistors1 1 1 1 ... Requ R1 R2 R3

Voltage is the same in each path.




Describe the configuration of a working circuit Distinguish between series and parallel circuits Describe the characteristics of series connections and of parallel connections. Interpret circuit diagrams Determine equivalent resistance of circuits having two or more resistors. Explain the cause and prevention of overloading household circuits. Homework: Read Pages 531-541 Pg 534 #1-3, Pg 537 # 6-9