Electric Currents

Charges in motion.

Creating Potential Difference.

• Alessandro Volta (1745 - 1827)– Ranked potentials created by

combing two metal discs.– Created the first continuous

source of charge using zinc and silver plates stacked in an acidic solution.

Sources of Continuous Current

• 1.) Electromagnetic - By rotating a conducting loop in a magnetic field. This forces free electrons to vibrate in the circuit. (A generator)

• 2.) Photoelectric Effect - Electrons are emitted from a conductor when illuminated by light with a short enough wavelength. – (mostly in the ultraviolet spectrum, but cesium and

potassium will emit electrons at the visible light frequency.)

• 3.) Thermoelectric - A “Thermocouple” can be created by heating a junction between an iron and a copper wire, and by submerging the other junction in an ice bath.

• 4.) Piezoelectric - certain crystals under stress will become charged.– The crystal needle of a record player.

(quartz watch)• 5.) Chemical - some reactions give off

electrons. – Primary cells: chemical process that can

be “used up.”– Storage cells: “rechargeable” reverses

reaction by introducing energy.– Fuel Cells: primary cell, but materials are

supplied continuously.

The Electric Battery

• Metals in a battery are called electrodes.

• The solution they are in is an electrolyte.

• The exposed parts of the metals are terminals.

• In diagrams, a battery is represented by:

+ -

How does it work?

• The electrolyte solution dissolves one of the electrodes, whose ions enter the electrolyte leaving the electrons on the terminal, which is now negative.

• The electrolyte (with ions) becomes positive.• The electrons from the other terminal enter

the electrolyte, leaving that terminal positive.

A Battery

Electric Current• The amount of charge to pass a point in a

circuit per unit of time.

• Units of Electric Current are C/s or Amperes.• Conventional current flows from the positive

terminal to the negative terminal at approximately 3 x 108 m/s.

• Electrons move opposite to conventional current at approximately 1 mm/s.

I =

VQVt

Ohm’s Law• Current (I) is proportional to the

potential difference (V).

• V = IR, – where R is the resistance met along the

way.• In a diagram, a resistor is represented

by:

Resistance is measured in Ohms (Ω)

Electric Power

• The rate of energy transferred, given by:P = IV P = I2R P = V2/R

• Unit of power is the Watt. (Joules/sec)– Usually Kilowatts are used.

• The Kilowatt-hour is the product of power transferred for a time period.– Unit of Energy = 3.6 x 106 J

Direct or Alternating Current

• Direct - One direction of current flow. – Produced by constant potential difference.

• Like a battery.

• Alternating - Current flows oscillates.– Produced by a changing electric potential.

• Like a generator.

The magnitude of voltage in A.C. circuits changes as a function of time and frequency of oscillation such that:

V =V 0 sin2π ft where V0 is the peak voltage.

Peak Current• Combining Ohm’s Law with the

equation for peak voltage,

• Thus the current changes as a function of frequency of oscillation and time. I0 = peak current

I =(V 0 sin2π ft)

R=I0 sin2π ft

Combine with equation for Power, P =I02Rsin2π ft

Some Practice

Electric Circuits

Series Circuits

• Electric current has a single path through the circuit.

• Because there is only one path through the circuit, the current through each source of resistance is the same.

• The total resistance to the current is the sum of the individual resistors in the circuit.

• I = V/RT (RT = R1 + R2 + …)• If one source of resistance breaks its

connection in the circuit, power is lost to all other devices in the circuit.

Voltage in Series

• The Voltage changes across EACH resistor in a series circuit. Use V=IR for each source of resistance.

• Total Voltage drop across all resistors equals the voltage across the circuit. If you start with 120 V, you can never lose more than 120 V. What changes is the current.

Example

2 Ω 4 Ω

6 V

I = V/RT

I = 6V/(4+2) ΩI = 6V/6 ΩI = 1 Amp

R1 R2

V1=1A x 2 Ω = 2V

V2=1A x 4 Ω = 4V

VT= 2V + 4V = 6V

Parallel Circuits

• Electric current has more than one path through the circuit.

In Parallel Circuits• Each device connects to the same two points

A and B of the circuit. The voltage across each device is the same.

• The total current divides evenly among the branches. Current passes easier through devices with low resistance, so the amount of current is inversely proportional to the resistance in the branch. I = V/R applies to each branch separately.

Example

6V

R1 = 2 Ω

R2 = 4 Ω

I1 = V/RI1 = 6V/2 ΩI1 = 3A

I2 = V/RI2 = 6V/4 ΩI2 = 1.5 A

IT = 4.5 A

NOTICE!

• As the number of parallel branches increases, the overall resistance DECREASES!

• This means that the overall resistance is less than the the resistance in any one of the branches.

Example

6V

R1 = 2 Ω

R2 = 4 Ω

To find total resistancein parallel circuits, you add the inverse of each resistor, and then take the inverse of your sum.

1/R1 + 1/R2 = 1/RT

1/2 Ω + 1/4 Ω = 1/RT

2/4 Ω + 1/4 Ω = 3/4 Ω

RT = 4/3 Ω

BOTH!?!?!?Upon encountering both series and parallel resistorsin a circuit, find the parallel resistance, and then combine it with the series resistance to find the total.

6V

1Ω

2 Ω 2 Ω

RT= R1+ (1/R2+ 1/R3)RT = 1Ω+(1/2Ω + 1/2Ω)RT = 1Ω+(2/2 Ω)RT = 1Ω+(1 Ω)RT = 2 Ω

Try one.

12 V

3 Ω6 Ω

6 Ω

IR1 = ?

IR2 = ?

IR3 = ?

RT = ?

VR1 = ?

VR2 = ?

VR3 = ?

R1R2

R3

Factors That Affect Resistance

• 1.) Temperature:– Most metals and alloys increase resistance when

heated.– Carbon, and semiconductors decrease resistance

• 2.) Length:– The resistance of a uniform conductor is directly

proportional to the length of the conductor.• 3.) Cross-Sectional Area:

– Resistance is inversely proportional to cross-section.

Superconductors• When temperature drops toward zero, some

materials exhibit a sudden dive in resistivity. (Transition temperature)

• Resistance in a wire can be found by – R = ( is resistivity)

• Resistivity is a property of conductors.– Copper = 1.7 x 10-6 cm @20– Iron = 1 x 10-5 cm @20– Gold = 2.4 x 10-6 cm @20

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