A. Electric Charge

1. Type of Charges

a. positive

b. negative

c. neutral (lack of charge)

2. Source of charge: The Atom

All matter is made up of atoms. Each atom contain a central "nucleus".

The nucleus contains protons and neutrons, and the nucleus is considered to be at rest. Electrons move around the nucleus in the empty space of the atom.

In a nuetral atom, the number of protons equal the number of electrons

a. Atomic Structure

Protons and electrons have equal and opposite electric charge. By convention, electrons are said to have a negative charge. Similarly, protons are said to have a positive charge. Neutrons have no charge.

Atoms are electrically neutral...not because they contain no overall charge...they have equal numbers of protons and electrons...their total charge adds up to zero.

Like energy and momentum, charge is neither created nor destroyed, it is conserved.

Opposite charges attract and like charges repel. As a result negatively charged electrons are attracted to the positive nucleus

An electron is said to have a charge of - and a proton a charge of +.

1 An atom in it's normal state has no charge. This is due to the fact that atoms

A have only neutrons

B have no protons or electrons

C have equal numbers of protons and electrons

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2 What object moves freely around the atom?

A Electron

B Neutron

C Proton

D Nucleus

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3 What symbol is used for the charge on a proton?

A +

B -

C o

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What the atom DOESNT look like:

This is NOT what an atom looks like!!!

If an atom was magnified so that the nucleus was the size of a dime on the 50 yard line of a football field, the first electrons would be at the edge of the stadium…nothing exists in between. The remaining electrons could be miles away (depending on the element/size of the atom)

And the electrons would STILL be too small to see. Atoms are almost all empty space.

Since everything (including us) is made of atoms, that means everything (including us) is mostly empty space.

More on Electrons

The nuclei of atoms are much more massive than electrons. Each proton or neutron is 2000 times more massive than an electron with each nucleus containing at least one proton.

That's one reason that when electric charge moves it's usually the result of electrons moving, not protons.

The other reason is that in solids, the nuclei are locked together so they can't move...regardless of their mass.

b. ions

Some atoms are charged because they have more protons or electrons-----

When that happens----the atom gains a charge and is called an “ion”

Ions are atoms with charges

b. ions

Some atoms are charged because they have more protons or electrons-----

When that happens----the atom gains a charge and is called an “ion”

Ions are atoms with charges

3. Build up of Charge

Solids are a form of matter whose nuclei form a fixed structure.

Nuclei, and their protons, are "locked" into position.Some electrons are bound more tightly to their atoms than other electrons.

It depends on the make up of the material. For example, human hair easily gives up electrons while plastic is good at “stealing” electrons.

Static electricity occurs when material obtains and holds onto a charge (either positive or negative)

4. Movement of Charge

A nucleus, and the protons, are "locked" into position. Protons are never removed or added to atoms (unless we are working in the field of nuclear physics—not this unit)

In solids….

Some materials have electrons that are bound more tightly to their atoms than other electrons. In other materials, electrons are free to move.

In conductors, some electrons are free to move through the solid. In insulators, no electrons are free to move

In conductors, electrons move freely inside the solid. Like charges repel, therefore the electrons tend to want to spread as far apart as possible.

a. Conductors

Insulators are materials that have strongly bound electrons which cannot move within the solid. Different insulators have varying levels of insulation capabilities.

However, electrons can still be picked up or removed from the material (just can’t move with in it)

b. Insulators

Some materials separate into ions when placed in water. The charges separate—and then can move. Electrolytes solutions are conductors. Pure water is an insulator.

c. Electrolyte

4 Free electrons in a conductor will:

A spread out as far as possible on the surface of the conductor.

B be located at the center of the conductor.

C have no organized distribution

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5. Methods of forming charge

a. Charging by friction

Electrons, carrying negative charge, move from one object to another. As a result, rubbed objects each gain a net chargethat is equal and opposite to the other.

without rubbing

after rubbing

...rub with a conductor

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b. Charging by Induction

Negatively Charged

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Neutral

(but polarized - meaning the charges are separated)

Charging by induction involves separation of charges within materials without them touching.

Electrons are induced (or forced) to move through the material.

The material is overall neutral, but “polarized” – which means the charges are separated

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Charging by Induction

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Neutral rod(very far away)

Neutral

Negatively Charged

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Charging by Induction

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Neutral rod brought to the neutral sphere....

Negatively Charged

Neutral

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Charging by Induction

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+ ---

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This sphere stays negatively charged

The sphere now becomes positively charged

Rod becomes negatively charged...

e- from sphere travel onto rod.

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Charging by Induction

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negatively charged

positively charged

negatively charged

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Charging by Induction

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separatethe spheres

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Charging by Induction

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5 A neutral plastic rod is rubbed by a piece of animal fur. Describe the charge on each item.

A Both items will be neutral.

B The fur and the rod will both have a negative net charge.

C The rod will have a negative net charge and the fur will have a positive net charge.

D The rod will have a positive net charge and the fur will have a negative net charge.

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c. Charging by Conduction

Negatively Charged(charge = -4e)

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Neutral Charge(charge = 0)

(identical spheres very far apart)

Charging by conduction involves objects touching

and transferring the charge between them.

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Charging by Conduction

Overall Charge = -4e

If the spheres are brought together to touch, their

electrons push as far apart as they can.

(remember, similar charges repel)

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Charging by Conduction

Negatively Charged(charge = -2e)

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Negatively Charged(charge = -2e)

(very far apart)

Once they are moved apart again, the charges cannot get back to where they came from. This results in an equal

distribution of charge.

6 If a conductor carrying a net charge of 8 electrons is brought into contact with an identical conductor with no net charge, what will be the charge on each conductor after they are separated?

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7 Metal sphere A has a charge of -2 and an identical metal sphere B has a charge of -4. If they are brought into contact with each other and then separated, what is the final charge on sphere B?

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8 Compared to insulators, metals are better conductors of electricity because metals contain more free _____.

A positive ions

B negative ions

C protons

D electrons

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e. Grounding

The earth is an enormous conductor.

When a wire is attached between the earth and another conductor, excess electrons will flow to the earth leaving the conductor neutral. This is "grounding".

Electrons flow to and from us to the earth all the time.

When you touch an object with a net charge, you may get a shock. This is because the conductor wants to get rid of its excess electrons. To do this, electrons flow through you to the ground.

Grounding

(symbol for "ground")

Electrical circuits and devices are usually grounded to protect from accumulating a net charge that could shock you.

To ground an electrical device a conductor must run from the device into the ground.

Plugs for many electrical devices have a third grounding pin that connects to a wire in the outlet which goes to the ground.

11 Sphere A carries a net positive charge, and sphere B is neutral. They are placed near each other on an insulated table. Sphere B is briefly touched with a wire that is grounded. Which statement is correct?

A Sphere B remains neutral.

B Sphere B is now positively charged.

C Sphere B is now negatively charged.

D The charge on sphere B cannot be determined without additional information.

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Return toTable ofContents

Electric Force

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Rod A: Neutral RodConductor

Rod B: Stationary,Negatively

Charged

If opposite charges attract and like charges repel...

far apart

Charged Objects

What will happen to the charges on Rod A if it is moved towards Rod B?

When A is brought towards B the electrons in A will be repelled.

Electrons in A will move to the left side of the rod. This makes the left and right sides of the rod have a different net charge.

A positive net charge on the right side of A will cause A to move towards B.

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Net NegativeCharge

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Net PositiveCharge

Charged Objects

Rod A: Neutral RodConductor

Rod B: Stationary,Negatively

Charged

13 What will happen when a neutral rod is brought near a negatively charged rod?

A The rods will move towards each other.

B The rods will move away from each other.

C Nothing; the rods will remain at rest.

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14 What happens to the electrons on a neutral insulator that is brought near a positively charged rod?

A All electrons move to the side of the insulator farthest from the rod.

B Each electron moves to the side of their atom closest to the rod.

C Nothing happens.

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B. Electrical Force

Newton's law of inertia says that objects at rest tend to stay at rest unless a force acts on the object.

The neutral pith ball accelerates towards the stationary charged rod so there must be a force present. We call this the electrical force.

B. Electrical Force1. defined

Just like any force, electrical force is the push and pull between two objects. This is one of the three forces that acts at a distance and is also measured in newtons.

Like charges repel (push away from each other)

Opposite charges attract (pull towards each other)

Electrical force is the force of attraction or repulsion that exists between charged particles. It is a vector quantity—meaning it is measured by both magnitude (how strong the force is) and the direction (repel or attract)

The lined shown in the image are called “electrical field lines”. The indicate the direction and size of the force.

How could the force between two charged particles be increased?

Electrical Force

In 1760, Benjamin Franklin and Joseph Priestly reasoned that the force between two charged objects must decrease as the inverse square of the distance between them:

1r2FE α

In the 1780's, Charles Coulomb showed that the electrical forcewas proportional to the charge (q) on each object.

B. Electrical Force2. Coulomb’s Law

Thus, the size of the electrical force is: k|q1||q2| r12

2FE =

k = a constant that equals 9.0x109 Nm2/C2

|q1| = the absolute value of the net charge on one object

|q2| = the absolute value of the net charge on the other object

r12 = the distance between the objects if they are point charges, or between the centers of the objects if they are spherical. Said as "the distance between 1 and 2".

Measurement of Charge

Charge (q) is measured in Coulombs (C).

Because small amounts of charge can generate large amounts of force, charge is often measured in:

milli-Coulombs (mC) = 10-3 C

micro-Coulombs (μC) = 10-6 C

nano-Coulombs (nC) = 10-9 C

15 A +20 μC point charge is located 20 cm away from a -40 μC point charge. What is the force on each due to the other?

Coulomb's Law

Coulomb's Law finds the size of the force.

Both objects feel the same force (Newton's third law).

The full solution included direction. To determine this look at the sign of both charges (like charges repel & opposite charges attract). For instance, the full answer to the previous problem would be: FE = 180 N toward each other.

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16 Two charges of +2.1 μC and +20 μC are separated by a distance of 18 cm. Find the direction and magnitude of electrostatic force between the charges.

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17 Two charges of 5.0 nC and -3.6 nC are located 7 cm from each other. Find the direction and magnitude of the electrostatic force between them.

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18 Two positive charges of 1 mC and 10 mC are separated by a distance of 10 m. Find the direction and the magnitude of electrostatic force between the charges.

F = 1.2 x 105 N away towards each other

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19 A particle with a charge of +7.4 µC is separated from another charged particle with a charge of -3.6 µC by a distance of 1.4 m. Find the direction and the magnitude of electrostatic force between the particles.

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20 What is the distance between two charges +7.8 μC and +9.2 μC if they exert a force of 4.5 mN on each other?

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21 A -4.2 µC charge exerts an attractive force of 1.8 mN on a second charge which is a distance of 2.4 m away. What is the magnitude and sign of the second charge?

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Two equal negatively charged objects repel each other with a force of 18 mN. What is the charge on each object if the distance between them is 9 cm? How many extra electrons are on each object?

Two charged conducting spheres have net charges of +4 μC and -8 μC and attract each other with a force of 16 mN. The spheres are brought into contact and then moved apart to the initial distance. What is the new force between the spheres? Is this force attractive or repulsive?

+4 μC -8 μC

r12

Before Touching

q q

r12

After Touching

When the two spheres touch, there is conduction and charge is shared.

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Two charged conducting spheres have net charges of +4 μC and -8 μC and attract each other with a force of 16 mN. The spheres are brought into contact and then moved apart to the initial distance. What is the new force between the spheres? Is this force attractive or repulsive?

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22 Two conducting spheres have a net charge of 5.0 mC and -9.0 mC and attract each other with a force of 4.05x103 N. The spheres are brought into contact and then moved apart to the initial distance. What is the new force between the spheres? Is the force attractive or repulsive?