Static and Current Electricity

Advanced Physical Science

Static Electricity

• Static Electricity = electric charge at REST on an object – remember static means not moving

• If something has static electricity, it means there an EXCESS or LACK of electrons (e-)

• Examples:– Clothes clinging when removed from dryer– Lightning (before the strike)– Your charged finger after walking on carpet

The Atom• Normally, atoms are electrically neutral• Each atom has the same number of electrons (negative)

as protons (positive)

• Electrons are whirling about, far away from the nucleus and can be removed from the atom by FRICTION!

Charge by FRICTION

• When certain materials are rubbed together, electrons are transferred from 1 surface to another.

• The blue balloon rubbed against sweater is charged

• Excess e- = Negative Charge• Lack of e- = Positive Charge• Like charges repel• Unlike charges attract

Charge by Conduction• Conduction =e- move from 1 object to another by DIRECT

CONTACT• Charging an electroscope shows conduction

• Electroscope• After charging, the metal leaves have same charge and repel

Charge by induction• Charges in an uncharged object are rearranged

without direct contact with the object• The negative charges on the yellow balloon

make a section of the wall have a positive charge.

• Electrons in the wall are repelled by and move away from the balloon

Induction Example 2

• Lightning

• The negative charge at the bottom of the cloud INDUCES a positive charge on the ground

Static Discharge

• When a surface has acquired a strong negative charge, the extra e- may jump to a neutral or positive object

• The “jump” of e- gives a spark• Spark= a rapid movement of a # of e- through

the air• Ex: lightning

Current Electricity

Current electricity is the flow of electric charges through a wire or other conductor.

Current flows from high voltage to low voltage. Current only occurs when there is a difference in voltage.

Current flows from positive to negative.

More about Current

Current: Current: the flow of electrons through a conductor.Current is measured in amperes (amps or A)Symbol used for current in equations is I.

Why “I”? Originally current was referred to as

electrical Intensity by French scientist, André-Marie Ampère.

Measured using an ammeter

Notes - continued

Two types of electric current: AC & DC – No, this is not the rock band!!AC – alternating current – the electrons are changing direction rapidly. (household current – in U.S. current alternates 120 times per second)DC – direct current – the electrons flow in only one direction (ex. = battery)

Important Terms

Current: Current: the flow of electrons through a conductor.Conductor: a material through which electric charges can flow.Resistance: opposition to the flow of electrons

Notes about Resistance

Resistance (R) is measured in ohms.Named for Georg Ohm – German scientist who

developed mathematical descriptions of electrical circuits.

The symbol for ohms = Ω (Greek letter omega)

A light bulb offers resistance to flow of e-Your skin also offers resistance to flow of e- Wet skin offers less resistance than dry

OR

More on Resistance

Resistance depends on:• Thickness and/or length of the wire– Thicker wire = less resistance

• How well the material conducts electric current

• Temperature – In general for metals, as temperature increases,

resistance increases (direct relationship)

Voltage

Voltage: Push that causes electric charges to flow

Also known as potential differenceElectrons flow from high potential to low

potentialThe difference in voltage is what drives the

current through a resistor

More about Voltage

Without voltage, there will be no electric current.

Voltage is measured in volts (V)Measured using a voltmeter

Ohm’s Law

I = Current (amps or A)V = Voltage (volts of V)R = Resistance (ohms or Ω)Instead of learning different formulas for current,

voltage and electricity, you can use this single formula to find ANY of them

Use Ohm’s Law Triangle to solve

R

VI

Reminders about CircuitsThere are 2 Types of Circuits:Series – one path for the electrons to travelIf one light goes out, they all go out.

Circuits (Cont)Parallel – more than one path for the electrons

to travelIf one light goes out, the remainder of the

lights remain lit. Don’t you hope your house is wired this way?

Play with Circuits

• http://www.andythelwell.com/blobz/

• http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Use Ohm’s Law in a Circuit• Given that you have a 12 volt battery and a

lightbulb that offers 3 ohms (Ω) of resistance, what is the amount of current (I) going through the wire?

• Solve…• 12V = I*3 Ω

• I = 12V/3 Ω

• I = 4 amps

Electrical Power

Rate at which electrical energy is converted to other forms of energy.

Measured in watts. Formula: P = PowerV = VoltageI = Current

IVP

Electric Energy

Electric energy =P = power (in kilowatts)T = time in hours Electric energy is measured in kilowatt hours.1 kWh = 1000 watts of power for 1 hour of timeConvert watts to kilowatts by dividing by 1000

(dimensional analysis)Ex: 220 watts * 1 kilowatt/1000 watts = 0.22 kW

tP

Calculating Electrical Energy Cost• From the HW the other night…• A room was lighted with three 100 watt bulbs for 5 hrs

per day. If the cost of electricity was $0.09 per kWh, how much would be saved by switching to 60 watt bulbs?

• Step 1: You are using 3 bulbs, so figure out how many watts in 3 bulbs

• 100 watts*3 = 300 watts• 60 watts*3= 180 watts

Calculating Energy Cost (cont)

• Comparing 300 watts vs 180 watts

• Step 2: Convert 300 watt and 180 watt to kilowatt (divide by 1000)

• 300 watt * 1 kilowatt/1000 watt = 0.3 kW• 180 w * 1 kw/1000 w = 0.18 kW

• Step 3: Find the difference in kW used • 0.3 kW -0.18 kW = 0.12 kW

Calculating Energy Cost (cont)

• Step 4: determine energy by multiplying by the hours used

• 0.12 kW * 5 h= 0.6 kWh

• Step 5: What is the cost? $0.09 per kWh• 0.6 kWh * $0.09/kWh= $0.054 or 5.4 (5) cents