Physics 30 - Electrostatics: Conduction, Friction, Induction, Field lines, Charge Distribution in Conductors (copy)

What are the rules that charged objects are bound by?

1. objects and particles with opposite polarities (ie. Positive/negative) are attracted to each other

2. Objects and particles with the same polarity (ie. positive/positive or negative/negative) are repelled by each other

3. Neutral objects - but not particles - are attracted to charged objects

• a neutrally charged object contains equal amounts of protons and electrons

• the amount of protons in an object will remain constant

• positively charged objects have more protons than electrons

• negatively charged objects have more electrons than protons

Charging by conduction

Method of charging an object via contact with another object.

Steps required to charge a neutral object by conduction

1. have two objects with different charges contact each other

2. the electrons will move from the object that is more negatively charged to less negatively charged

3. the electrons will stop moving when the objects become equally charged or when the contact between the objects is broken

***When charging a neutral object by conduction, it becomes the same charge as the charged object. gain like charges.

Charging by friction

when two insulating materials are rubbed together and electrons move from one material to the other. One object gains electrons and becomes negatively charged, while the other loses electrons and becomes positively charged. This happens due to the different affinities for electrons between the two materials.

• electron affinity is the measure of an atom’s or molecule’s ability to attract and hold onto electrons. When two materials with different electron affinities come into contact and are rubbed together, one material may have a stronger tendency to attract electrons than the other.

• higher electron affinity—strongly attracts electrons

• Low electron affinity—weakly attracts electrons

gain equal but opposite charges

insulators

• materials that do not allow electrons to flow are called insulators (ex. wood, plastic, rubber)

Conductors

• materials that do allow electrons to flow are called conductors (ex. metals, ionized water)

• Conductors share charges

Charging by induction

1. bring the charged object near the neutral object

2. attach a ground to the neutral object (a ground is an object with an enormous supply of electrons—it could be a wire attached to the earth, your finger, etc.

3. Remove the ground

4. take the charged object away

5. note: when charging a neutral object by induction, it becomes the opposite charge as the charged object

gain opposite charges

Charge induced separation

neutral objects are attracted to both positively and negatively objects. this is called charge induced separation

• when a negatively charged object is brought near a neutral object, the electrons in the neutral object are repelled to the far side of the object. considering that the protons remain in their original positions, this results in the negatively charged object being close to a relatively positive area. thus, the objects are attracted to each other.

• When a positively charged object is brought near a neutral object, the electrons in the neutral object are attracted to the side of the object close to the charged object. Considering that the protons remain in their original positions, this results in the positively charged object being close to a relatively negative area. thus, the objects are attracted to each other.

Note: the neutral object remains neutral in this special scenario as the electrons in it only change their position in the object, causing them to be attracted to each other. in order to actually become and remain charged, the objects need to contact each other (ie. charge by conduction) or a ground needs to be used (charge by induction)

whats an electroscope

• device that demonstrates the presence of an electrical charge.

• When the electroscope is charged - either positively or negatively - the thin pieces of metal in the electroscope become the same charge. Considering that objects with the same polarity repel each other, the thin pieces of metal separate from each other, visually demonstrating that the object is charged.

when drawing field lines, there are three general rules that must be followed

1. field lines always enter and leave a surface perpendicularly

2. The density of field lines (ie. how close the field lines are to each other) is proportional to the magnitude of the strength of the field

3. When drawing a field produced from more than one source, the field lines never cross and are determined through the vector addition of each field

What is Charge distribution in conductors

electric fields enter or leave a surface perpendicularly. The reason this occurs is due to the charge distribution of the excess charges in a conductor. Electrons feel a force of repulsion from each other and thus move as far away from each other as possible until they are at rest (i.e., are in a state of equilibrium).

Solid conducting sphere

When a solid conducting sphere has an excess of electrons, the furthest these excess electrons can be from each other is on the surface of the conducting sphere. The electrons spread out equally around the outside of the sphere, resulting in an electric field that is the same magnitude anywhere around the sphere (assuming the same distance away from the surface of the sphere).

hollow conducting sphere

When a hollow conducting sphere has an excess of electrons, the furthest these excess electrons can be from each other is on the surface of the conducting sphere. The electrons spread out equally around the outside of the sphere, resulting in an electric field that is the same magnitude anywhere around the sphere (assuming the same distance away from the surface of the sphere).

Interestingly, this results in the electric field being equal to zero anywhere inside the hollow sphere. For this reason, if your car is to ever be struck by lightning (which causes it to gain a dangerous amount of excess electrons), you will be safe inside your car due to the electric field still being zero. If you were to open your car door, however, those excess electrons would then have a path to travel, so do not this! Instead, wait for the fire department to ground your car, removing the excess electrons safely.

Irregularly shaped conductors

When an irregularly shaped conductor has an excess of electrons, the electrons end up being closer together in areas that are more 'pointy' and further apart in the 'flatter' areas of the conductor. This results in the electric field being stronger in the 'pointy' areas of these conductors.

Flat conductors

On a flat conductor, excess electrons spread out evenly across the conductor. This results in the electric field being uniform.