Electric Circuits and Current Types

Direct Current

• Regular pencil cells produce direct current (DC)
• DC is a constant magnitude current flowing in one direction

Kinds of Cells

Primary Cell

• Chemical reaction is irreversible
• Converts chemical energy into electrical energy
• Cannot be recharged
• Provides weak current
• Examples: Simple voltaic cell, Daniel cell

Secondary Cell

• Chemical reaction is reversible
• Converts electrical energy to chemical energy when current is passed
• Converts chemical energy to electrical energy when current is drawn
• Can be recharged
• Provides small or large currents
• Examples: Lead accumulator, Ni–Fe accumulator

Electric Current

• Charge (Q)
• Current (I) = Charge(Q) / Time(t)
• Scalar quantity
• SI unit is ampere (A)
• Conventionally, current direction is the flow of positive charge (opposite to electron flow)

Symbols in Circuit Diagrams

• Circuits have various components
• Current sources include AC or DC:
  • AC: from mains or AC generator
  • DC: from cell or battery
• A cell is represented by two vertical lines of unequal lengths
• When multiple cells are connected in series, they form a battery (cathode of one to the anode of the next)

Resistance Wire, Resistance Box, Rheostat, or Variable Resistance

• Resistance wire is made of an alloy (e.g., nichrome)
• Fixed resistance value (depends on length and thickness)
• Resistance box: has several standard resistances in series
• Rheostat: adjusts current magnitude (variable resistance)
• Fixed resistance is represented by R
• Rheostat (Variable resistance) represented by Rh

Ammeter

• Measures current magnitude
• Connected in series

Voltmeter

• Measures potential difference
• Connected in parallel

Galvanometer

• Measures small currents
• Measures direction of current flow
• Connected in series

Load

• Appliance connected in a circuit
• Represented by symbol L (e.g., bulb, heater)

Connecting Wires

• Connect electrical components
• Made of conductive materials (e.g., copper, aluminum)

Simple Electric Circuit

• Components: cell, key, bulb, ammeter, voltmeter, rheostat
• Key: completes/breaks circuit
• Bulb lights when key is closed, ammeter and voltmeter show readings
• Resistance changes with rheostat jockey, affect bulb brightness and readings

Insulators

• Do not allow current flow
• Low free electrons
• High resistance

Conductors

• Allow current flow
• Many free electrons
• Low resistance

Flow of Electrons

• Positively charged conductor to uncharged conductor: electron flow from uncharged to positive
• Negatively charged conductor to uncharged conductor: electron flow from negative to uncharged
• Positively charged conductor to negatively charged conductor: electron flow from negative to positive

Direction of Current Flow

• Conventional current: from higher to lower potential
• Electronic current: opposite to conventional current (electron flow)

Potential Difference (Voltage)

• Work done in transferring a unit positive charge between two conductors
• Scalar quantity, SI unit is volt (V)
• 1 volt: 1 joule of work to transfer 1 coulomb of charge

Electrical Resistance

• Obstruction to current flow in a conductor
• Ohm's Law: Resistance = Potential difference / Current (R = V/I)

Factors Affecting Resistance

• Length (R α l)
• Cross-sectional area (R α 1/A)
• Material (depends on atom arrangement and concentration)
• Temperature (ions vibrate more at higher temps, increasing resistance)

Efficient Use of Energy

• Reduce energy consumption and cost
• Examples: CFLs, better insulation, energy-efficient appliances, vehicle improvements, building design, etc.