BackgroundBackground

• Circuit- a connection including a power source, wiring, and one or more resistors

• Resistor – something that uses electrical energy (or “lowers the potential energy”) (ie: bulb, motor, buzzer, appliance, etc.)

• Power Source – a battery or outlet

SchematicsSchematics

Wire

Battery

Resistor

Light Bulb

Ammeter

Voltmeter

Switch

Types of CircuitsTypes of Circuits

SeriesSeries ParallelParallel CombinedCombined

Electricity has one path to follow

Electricity has more than 1 path to follow

Parts of the circuit are series; parts are parallel

When one bulb goes out, they all go out

When one bulb goes out, the others stay lit

Depends on where in the circuit the bulb goes out

Ohm’s Law : Ohm’s Law : V = IRV = IRVoltage (V)Voltage (V) Current (I)Current (I) Resistance Resistance

(R)(R)Measured in Volts

(V) ( J/C) (or N.m /

C)

Measured in Amps (A) which are C/s

Measured in Ohms () which is a J.s /

C2

“Electric Pressure”

Rate of Flow of e-

Opposition to e-flow

-power company’s generator provides 120 V to your home outlets;-the two holes in an outlet have a p.d. of 120 V between them (120 J of energy is supplied to each C of charge)-named for Allesandro Volta (Italian physicist)-meas. by voltmeters

“the flow of e-‘s from high potential to low potential”-Named for French physicist Andre Ampere- is maintained by e-‘s pumping back (we say a “circuit is complete”)-ammeters measure current; -- galvanometers measure v. weak currents

Power and ChargePower and ChargePower (P)Power (P) Charge (q)Charge (q)

Measured in Watts (W)

Measured in Coulombs (C)

Energy expended by a current in 1 second

When a substance has gained or lost electrons

1 W = 1 amp x 1 1 W = 1 amp x 1 voltvolt

1 C is the charge 1 C is the charge of 6.25 x 10of 6.25 x 101818 e-’s. (lightning e-’s. (lightning ~10C) and 1 e- ~10C) and 1 e- = 1.60 x 10= 1.60 x 10-19-19 C C

The Water Analogy - The Water Analogy - CURRENT CURRENT

(read, don’t copy)(read, don’t copy)

• When a hose is attached to a faucet and the valve is opened, water flows from the faucet through the hose. If you used a stopwatch and a graduated container, you could measure the rate of flow of the water in, for example, liters per second.

• Similarly, we can measure the number of e-’s flowing past a given point in a unit of time.

• Current is expressed as e-’s per second and is measured in amps.

• 1 amp = 1 C of electrons per second• 1C = 6.25 E 18 e-’s

Factoids About AmpsFactoids About Amps• “It’s not the volts that kill you, it’s the amps!”

• Plug: ground (removes excess charges from an a.c. object

Current in Amps Effect

0.001 can be felt

0.005 painful

0.010 spasms

0.015 lose muscle control

0.070 fatal if >1 sec.

The Water Analogy - The Water Analogy - VOLTAGE VOLTAGE

(read, don’t copy)(read, don’t copy)

• Water flows through a hose because there is a driving force behind the water. We can increase the rate of flow of the water by increasing the driving force. (Add pumps to the line)

• The driving force in a circuit is called the “electromotive force” (emf) and is a “difference in potential” that causes e-’s to move.

• EMF is also known as voltage and is measured in volts

• Think of “potential” the way we say something has a lot of “potential energy” when raised to a great height

The Water Analogy – The Water Analogy – RESISTANCE (read, don’t RESISTANCE (read, don’t

copy)copy)

• Water molecules, moving through a pipe, rub against the walls of the pipe and slow down. The walls of the pipe oppose or offer resistance to the flow of water. Longer and narrower pipes offer more resistance to the flow of water than shorter and wider pipes.

Factoids About OhmsFactoids About Ohms

Typical Resistances: OhmsCommon Wires 0.03

Household wiring (per m)

0.004

Body, soaked in salt water

100

Body, dry 500,000

What Impedes What Impedes Electricity?Electricity?

There are 4 factors that affect resistance in wires:

1. Length – longer conductors offer greater resistance

2. Temperature – R as T (for most metals)

However, R as T for C and semiconductors)

Superconductors are certain metals which become excellent conductors at extremely low temperatures

3. Type of Material – every substance has its own electrical resistance (ie: Cu conducts better than Al)

4. Diameter of Wires – thick wires have less resistance than thin wires; resistance varies inversely with the cross-sectional area of the conductor

The Water AnalogyThe Water Analogy - - SUMMARYSUMMARY

Voltage (V)Voltage (V) Current (I)Current (I) Resistance Resistance (R)(R)

Measured in Volts (V)

Measured in Amps (A)

Measured in Ohms ()

“Electric Pressure”

Rate of Flow of e-

Opposition to Flow of electrons

Increase the pressure at the pump increases voltage

Amount of water flowing through a hose in 1s

Water has a harder time going through a long, thin hose than a short, wide hose

Why Does Electricity Why Does Electricity Flow?Flow?Electricity flows whenever there is an electric

“potential difference”

No flow – No potential difference b/w bird’s feet

No flow – No potential difference b/w person’s hands

Zap!

One wire with one V is connected by balloon fabric to another wire with another voltage

Types of CurrentTypes of Current

• DC – “Direct Current”DC – “Direct Current”– Batteries are a source of DC currentBatteries are a source of DC current– Electricity travels in one direction (- to Electricity travels in one direction (- to

+)+)– All energy is stored (chemical) and can All energy is stored (chemical) and can

be used up quicklybe used up quickly• AC – “Alternating Current”AC – “Alternating Current”

– Current coming from outletsCurrent coming from outlets– First goes in one direction, then in the First goes in one direction, then in the

opposite at a rate of 60 cycles per opposite at a rate of 60 cycles per second (aka: 60 Hz)second (aka: 60 Hz)

Remember:Remember:

• Electrons don’t actually travel through the wire, they wiggle and their energy transfers

• The power company sells you energy; you supply the electrons

• Can be transmitted long distances with voltage step-ups

• AC has a lower heat loss in wires

Safety FactoidSafety Factoid

•Electricians work “with one hand in their pocket” when there’s danger of a hot wire since if they used 2 hands the current can travel across their chest (the current doesn’t necessarily want to travel through the person; it just wants to get to the ground since the ground has relatively few e-‘s)

Calculations Involving Calculations Involving CircuitsCircuits

Series Parallel

Voltage Vtot = V1+V2+V3… Same in each resistor

Current Same in each resistor

Itot =I1 +I2 +I3…

Resistance

Rtot = R1 +R2 +R3…

Example 1 - SeriesExample 1 - SeriesIf VT = V1 + V2…then

100V = V1 + 60V

So…V1 = 40 VV1 = 40 V

40 V

If V = IR,……..then

40V = I1 ( 20

So…I1 = 2 A

I1 = 2 A

2 A

(and since it’s a series circuit and I is the same everywhere, then…)To solve for R2, V = IR, and

60V = 2A(R2)

So…R2 = 30

30

Example 2 - ParallelExample 2 - ParallelYou have both R’s.

If 1/RT = 1/R1 + 1/R2

Then 1/RT = 1/3 + 1/6

So RT = 6/3 or 2Now you have a V and a T. If V = IR, then

6V = I(2)

So I = 3A

2

RT = 2

3A

IT = 3A

Since, for a Parallel Circuit, the voltage is the same in each branch, V1 must be 6V and V2 must be 6V

V1 = 6 V

V2 = 6 V

In a Parallel Circuit, the current is not (necessarily) the same in each branch. V1 = I1R1; 6 = (I)3; I1 = 2A

V2 = I2R2; 6 = (I)6; I2 = 1A

I1+I2=IT

6V 2A 1A

6V

Example 3 - CombinedExample 3 - CombinedYou have two parallel resistors and you know their values. Start there.

1/RT = 1/5 + 1/20

1/RT = 4/20 + 1/20 = 5/20 so

R2,3 = 4

So it is as if…

R2,3 = 4

And to find RT, simply add R1 and R2,3

So you get 9

9Since we know that the IT is 4A but we know that in a parallel branch the current splits up, it would only be true that at I1 the current would be 4A.

1

Kirchoff’s RulesKirchoff’s Rules

• For complex circuitsFor complex circuits