Chapter 34 Notes

Electric Circuits

Electric Current Intro• Voltage is an “electric pressure”

• That can produce a flow of charge, or current,

within a conductor

• The flow is restrained by the resistance it

encounters

• When the flow takes place along one direction,

it’s called direct current (DC)

• When it flows to and fro, it is called alternating

current (AC)

• The rate at which energy is transferred by electric

current is power

Flow of Charge and Electric Current

• Charge flows through a conductor until there is a common potential, i.e. a common voltage.

• Voltage provides the “electric pressure” to move electrons between the 2 ends of a wire.

• Charges will only flow through a circuit if there is a voltage acrossthe circuit.

• Sources of continuous voltage

• A battery

• Electromagnetic induction part 1 part 2

• Chemical cells

• Solar cells

• Thermocouple

• Piezoelectric cell

Current and Voltage Sources

• To maintain a continuous current there must be a voltage source (an “electric pump”) to provide a sustained potential difference.

• Current is the flow of electric charge.

• In solids, only electrons are free to move.

• In fluids, + and - ions may move as well (like the electrolyte in a car battery).

• Examples of energy suppliers (providing a steady flow) were discussed on the previous slide

• Electric current is measured in amperes (A)

• the flow of 1 coulomb of charge per second

Current (I) and Voltage (V)

sources• An alternating potential difference of 120 volts is supplied to

each outlet in your home.

• Remember: Electric Potential (Volts) = Electric Potential Energy

(Joules)/Amount of Charge (Coulombs)

• 120 Joules of energy is supplied to each Coulomb of charge that is

made to flow in the circuit.

Electric Resistance (R)• The amount of current depends on both voltage and

resistance. (Ohm’s Law)

• Resistance is a measure of how “easily” the electrons can flow

through a conductor.

• The resistance of a wire depends on:• The conductivity of the material its made from (how well it

conducts)

• Its thickness (thick have less resistance than thin)

• Its length (long wires have more resistance than short (optical vs coaxial

audio cables))

• And, its temperature.• The higher the temperature, the more resistance (why there’s fans in

our electronics or our phones get hot when playing games).

• Resistance can become zero at very low temp, near absolute zero

Measuring Resistance

• Electric resistance is measured in Ohms (Ω ).

• Current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit

• Ohm’s Law:

Current = Voltage/Resistance• Current is measured in Amperes (A).

• An older symbol for ampere is the “amp.”

• 1 ampere = 1 volt/ohm

• The greater the resistance, the less the current

• Resistors are used to regulate current inside electric devices, such as radios.

Ohm’s Law Practice

• What is the resistance of an electric griddle that draws 12

amps of current when connected to a 120 V circuit?

• How much current is drawn by a lamp that has a resistance of

100 ohms when a voltage of 50 V is impressed across it?

Fill in the blankVoltage (V) Current (A) Resistance (Ώ)

1.5

0.30

0.05

30

1.5

15

3.0

0.10

3.0

0.15

0.2

30

Electrical Transmission

• DC, direct current, current in one direction (a battery)

• AC, alternating current, electrons move in both directions

• Electrical transmission using AC is more efficient than DC.

• High voltage electricity is more efficiently transmitted than low

voltage electricity.

• Easier to increase/decrease AC voltage using transformers than

DC voltage.

Power

• The rate at which electric energy is converted into another

form is called electric power.

• Power = current x voltage

• P = I x V

• Power measured in Watts (W)

• A 60 watt device uses 60 joules of energy each second

• Easy to do calculations

Calculating Electrical Energy

usage• The electric power rating (in watts) of an appliance is usually

listed on the appliance

• Use the power rating to calculate how much energy it uses for

the time that it is on

• Example

• A toaster has a power rating of 1200W. If it takes 1 minute to

toast a slice of bread, how much energy does the toaster use?

• Energy = power (watts) x time (seconds)

• 1200W x 60s

• 72,000 watt-seconds or joules

• This energy could run a 60W light bulb for more than 20 minutes

• 72,000J is a huge number and can scare consumers so electric

companies use something different

Calculating Electrical Energy

Usage (cont)• Electric companies use the kilo-watt hour (kWh)

• Still calculated by multiplying power by time

• But power rating is expressed in kWh and time of usage in hours

• You can convert a power rating in watts to kilowatts by

dividing by 1000W/kilowatt

• The 1200W toaster uses 1.2 kW

• Measure time in hours

• The toaster takes 1 minute/60min = 0.017 hours

• Calculate electrical energy used as follows

• Energy = Power x time

• 1.2 kW x 0.017 h

• = 0.02 kWh

Power examples

• How much power is used by a calculator that operates on 8V

and 0.1A? If it is used for one hour, how much energy does it

use?

• Will a 1200W hair dryer operate on a 120V line if the current

is limited to 15A by a fuse? Can two hair dryers operate on

this line?

• Assignment

Electricity Formulas

Formulas: V = I R I = V/R R= V/I P = IV E = Pt

Variable Unit (abbreviation)

• Voltage (V) Volts (V)

• Current (I) Amps (A)

• Resistance (R) Ohms (Ω)

• Power (P) Watts (W)

Electric Bills

• The electric company charges you for the energy you use each

month.

• If they used the standard unit, Joules, they would have to

charge you for millions of them. This would scare people.

• So they invented a larger unit of energy, the kilowatt-hour,

that is 3,600,000 times larger than a Joule.

Kilowatt-hour

• Joules are watts x seconds. If you use a 100-W bulb for 30

hours (108,000 s) you use 10,800,000 Joules of energy.

• E = Pt = 100 x 108,000 = 10,800,000 J

• Kilowatt-hours are kilowatts x hours. If you use a 100-W bulb

(0.100 kW) for 30 hours you use 3 kilowatt-hours of energy.

• E = Pt = 0.1 x 30 = 3 kWh

Power in appliances

61” Flat screen

2 hr 30 min/day

Power in appliances

Blu Ray

2 hr 23 min/day

(length of

Skyfall)

Power in appliances

Receiver for DTS

surround sound

2 hr 23 min/day

(length of Skyfall)

Power in appliances Nintendo Wii

45 min/ day

Power in appliances Microwave

1 min 30 sec/ day