Electrostatics and the Electric Field

1. When the balloon is rubbed on the sweater, what might happen?

A. Some positive charges in the sweater will move onto the balloon

B. Some negative charges in the sweater will move onto the balloon

1. When the balloon is rubbed on the sweater, what might happen?

2. What do you think will happen when the balloon is moved closer to the wall?

Neutral wall

Negatively charged balloon

A. Some positive charges in the wall will move towards the balloon

B. Some negative charges in the wall will move towards the balloon

C. Some positive charges in the wall will go onto the balloon

D. Some negative charges on the balloon will go to the wall

2. What do you think will happen when the balloon is moved closer to the wall?

3. What do you think the balloons will do?

Negatively charged balloon

Negatively charged balloon

A. The balloons will move towards each other

B. The balloons will move away from each other

C. The balloons will not move.

3. What do you think the balloons will do?

4. What might happen to the charge on the man when he touches the door knob?

A. Most electrons will go into the knob and down to the earth.

B. Some electrons will go from the earth through the knob and into the man.

4. What might happen to the charge on the man when he touches the door knob?

Charges create a force The force causes objects to either repel or attract

Electric force is a non-contact force . . . it causes action between charges without the charges coming in contact

Non-contact forces are often called “field” forces (like the gravitational force)

Electric Force

We can call the space around a charged object the electric field

The electric field exerts a force on objects within the field

The size of the force is based on the amount of charge present and the distance between objects

Electric Field

The electric field is a vector quantity

It has a magnitude and a directionThe direction of the field can be measured using a positive test charge (as a standard)

http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html

Electric Field

To show direction, electric field lines are drawn around the charges within an electric field

The electric field is stronger where the field lines are closer together

Arrows move away from a positive charge (because a positive charge repels the positive test charge)

Electric Field Lines

Electric field lines point toward a negative charge (because a negative charge attracts the positive test charge)

Electric field lines are also called lines of force

Electric Field Lines

Electric Field Direction

 

Electric Field Magnitude

Note: If you substitute “F” from the Coulomb’s law equation into the “F” of the electric field equation, you get the following:

 

There is an electric field in the atmosphere surrounding the Earth which has a magnitude of roughly 150 N/C and a direction pointing towards the center of the Earth. Determine the magnitude and the direction of the force experienced by a dust particle having a negative charge of 9.2x10-16 C.

Example 1

Review . . . Potential energy is due to an objects position

All energy is measured in joules For an object to gain gravitational potential energy, work must be done on the object

Potential Energy

Example: a book is lifted into the air and gains gravitational potential energy

The energy gained by an electric charge when work is done on the charge is called electric potential energy

Work is done on the charge when it is moved against an electric field

Example: A negatively charged balloon is pushed toward a Van de Graff generator

Once the charge is released, the electric potential energy becomes kinetic energy

Electric Potential Energy

Electric potential is a comparison of the electric potential energy and the number of charges present

A balloon charged by rubbing it on your hair does not have many charges (only about 1 millionth of a Coulomb)

This is why a balloon doesn’t shock you . . .it can have high voltage, but not enough charge to harm you

Electric Potential

 

Electric Potential

q

Work

q

PEV

How much energy is stored when 7.2 C of charge is moved through a potential difference of 1.5 V?

Example 2

1. What is the space surrounding a charged particle called?

2. How does a charged particle gain electric potential energy?

3. Is the strength of an electric field effected by distance? If so, how?

4. What is a volt?

Review

Electric field

Work must be done on the charge.

The field strength increases as the distance decreases.

1 Joule/1 Coulomb (unit for electrical potential)

A high voltage transmitter Allows us to enjoy television, automobiles,

and just about every other electronic device Wireless electrical transmission X-rays, radio, wireless telegraphy all

attributed to Nikola Tesla http://www.youtube.com/watch?v=pBx2wkg

9nhk

Tesla Coil

Lightning

LIGHTNING

Lightning Lightning can travel up to 140,000

mph Produce temperatures of 54,000°F Can form fulgurites (sand into

glass) Transfers 15 C of electric charge

A natural capacitor Charges are polarized within a cloud The top of the cloud becomes positively charged

The bottom of the cloud becomes negatively charged

The electric field induces a movement of electrons from the cloud down toward Earth

What is lightning?

http://www.teachersdomain.org/asset/phy03_vid_lightning/

Make a lightning strike http://environment.nationalgeographic.com/

environment/natural-disasters/lightning-interactive/

Lightning Strikes

The electric field causes ionization of the air surrounding the cloud

This results in a conductive substance called plasma

The lightning bolt begins as a step leader or flow of electrons from the cloud toward the ground

A positive streamer rises up from the ground When the step leader and the streamer meet,

a conductive pathway allows the excess electrons to move to the earth

A lightning strike

A lightning rod is a protective measure for tall buildings, farm houses, etc.

Used to direct lightning strikes to the ground

This allows the excess electrons to flow into the ground and not into the house or building

Lightning Rod

You are safe in a vehicle during a lightning storm.

http://www.youtube.com/watch?v=C8xt9AP41-A

True or False

Electrical energy can be stored in a device called a capacitor

Capacitors are found in nearly all electronic devices◦Televisions, camera flash, computer

A capacitor must be charged A capacitor is discharged when a conducting path is provided between the plates

Capacitance

A device that stores charge

Made of 2 conductors separated by an insulator

Capacitors

The charge stored will SHOCK you if you are in the conducting pathway!!

Found in many electrical circuits- computers, televisions, camera flashes