1. Electric Charge and Coulomb's Law
Electricity
Electric Charge and Coulomb's Law

2. The basic components of an atom are:
Protons
Electrons
Neutrons
Atoms are held together by electric force.
Electric force is one of the most powerful fundamental forces. Much more powerful than gravity!

3. Protons and electrons possess a quality called electric charge.
Electric charge gives them an attractive force.
Protons have a positive charge.
Electrons have a negative charge.
A positive particle always attracts a negative.
Particles with the same charge always repel.

4. Matter can develop a charge.
This happens when an imbalance of protons and electrons exists.
The process by which matter becomes charged is ionization.
This occurs when electrons are added or removed.
Removing electrons  Positive Charge
Adding Electrons  Negative Charge

5. The magnitude of charge on an electron or proton is denoted as e.
The charge of a particle is denoted as q.
So, when the phrase “a charge” is used, it means that a particle with charge of #e electrons or protons.
The SI Unit of charge is the Coulomb, C.
The charge of an electron OR proton is 1.6 x C.
One Coulomb is a MASSSIVE amount of charge!

6. So, the net electric charge is always conserved.
Suppose two objects come in contact. Object A has a charge of -5. Object B has a charge of +1.
The objects at touched, and charge flows between them.
Both objects will have a charge of -2 afterward.
So, the net electric charge is always conserved.

7. ε0 – Electric Constant = 8.85 x 10-12 q1, q2 - charges
Coulomb’s Law describes the interaction between charged particles as an electric force.
FE – Electric Force
ε0 – Electric Constant = 8.85 x 10-12
q1, q2 - charges
r – distance between charges
Coulomb’s Law

8. SOME TERMINOLOGY:
SOURCE Charge – one that has a significant amount of charge compared to its surroundings. A charged object will act as a source charge.
POINT Charge – a source charge that is considered, for simplicity, to exist at a single point.
TEST Charge – an imaginary charge with a value that is insignificant when compared to a source charge. In problems test charge is often placed at various points within an electric field.

9. SOURCE CHARGE
TEST CHARGE
POINT CHARGE

11. Finding the net force is simply a matter of applying vector concepts.
Electric forces are subject to superposition.
If two or more charges are near each other, the electric force from each will overlap and add together.
Finding the net force is simply a matter of applying vector concepts.

12. An electric field exists around all charged particles
An electric field exists around all charged particles. Electric force can be felt by other charged particles while within this field.
This field reaches extends in all directions.

13. Electric field vectors may be added like any other vectors.
Suppose the fields from two source charges overlapped.
Their fields would then add together through superposition.
Take a moment to sketch the diagram.
Explain why the test charge experiences the forces shown.

14. This same concept is valid for Charge!
Let’s make an analogy:
Suppose you have an object of mass M that is on Earth’s surface.
You move the mass upward some height, and it gains GPE.
Because it was lifted, the object went against what it would naturally do. This required WORK.
This same concept is valid for Charge!

15. Electric PE is dependent on:
Suppose you have an object of some charge, +Q.
This charge is moved closer to another positive charge.
It gains Electrical Potential Energy.
Because it was forced to move in a manner that would not naturally occur, WORK was done on it.
Electric PE is dependent on:
Charge Magnitude
Distance from the source charge (location in E-field)

16. Electric Potential is defined as the Electric Potential Energy per Charge.
Also called “potential difference” or “voltage”.
The unit is Joules/Coulomb
Also called the “volt”.
ΔV = ΔUc/q

17. Any charge at a given location would have the same electric potential…
Electric potential is dependent only on the location of a charge within a field.
Any charge at a given location would have the same electric potential…
But different amount of potential energy (depending on the amount of charge).
It’s like gh with GPE. At a given height, the product of gh is always the same.
But objects with different mass would have different amounts of GPE!

18. Two equal, but opposite charges form a pair called an electric dipole.
Electric field lines point away from positive source charges, and toward negative source charges.
Two equal, but opposite charges form a pair called an electric dipole.
-Di meaning two.
Note that electric field lines NEVER CROSS.

19. Suppose a positive test charge were placed into the field.
The force it experiences would be tangent to the field line that crosses through it.
The force vector would point in the same direction as the field line.

20. Suppose a negative test charge were placed into the field.
The force it experiences would be tangent to the field line that crosses through it.
The force vector would point in the opposite direction of the field line.

21. Repelling Field Lines

22. Repelling Field Lines

23. A uniform electric field exerts constant, equal influence over a charge at all points in the field.
In other words, the field is equally strong throughout.
It is similar to a gravitational field, so kinematics may be used!

24. Current, Resistance, and Circuits
Electricity
Current, Resistance, and Circuits

25. Electric current is the flow of electric charge through a conductor.
Electric charge, when stationary, moves to the outside of a conductor.
When electric charge begins moving, the electric field is sustained within the conductor.
Inside the conductor, electrons are randomly moving…very slowly.

26. The motion of electrons inside a conductor is electric current.
If a potential difference (voltage) exists across the conductor, the electrons experience an electric force.
The electric force causes them to begin moving orderly in the same direction.
The motion of electrons inside a conductor is electric current.

27. Ohm’s Law (Resistance)
Whenever electric charge flows, it encounters resistance.
Think of electric resistance as opposition to the flow of electric charge.
The relationship between voltage, current, and resistance is known as Ohm’s Law:
The unit for electric resistance is the Ohm, Ω.
Ohm’s Law (Resistance)

28. An electric circuit is a pathway for electric charge to flow.
A current can be maintained if the voltage source is connected by such a pathway.
A battery connected by wires, for example, is a circuit.
Voltage exists because the positive terminal has a higher potential than the negative terminal.
This produces an electric field in the conductor (wire), which exerts electric force on electrons.

29. The force does work on the electrons, and current is produced as charge flows through the conductor.
By the time a charge reaches the negative terminal, it has lost all of its electric potential energy!
The rate at which electric energy is transferred through this process is known as POWER.
It is essentially the transfer of energy over time.
Power

30. Within a circuit, there are three main components:
Resistors:
Batteries: OR
The longer line is POSITIVE, the shorter one NEGATIVE.
Wires (They’re just straight lines)

31. Here is a basic circuit diagram.
A resistor provides some amount of resistance in a circuit. Those that follow Ohms Law are OHMIC.
They are not insulators (which provided nearly 100% resistance).
The resistance provided by an ordinary wire is NOT 0, but it is negligible for our studies.
Here is a basic circuit diagram.
Ohm’s Law can be applied to find the voltage, current, or resistance within a circuit.

32. When combined in series, resistors are side by side.
Resistors are often combined in circuits. The way they are combined affects the total resistance that exists in the circuit.
When combined in series, resistors are side by side.
Add the resistances of each resistor in series to find the total resistance.
Resistance (Series)

33. Resistance (Parallel)
When combined in parallel, electric current has multiple pathways to travel.
The reciprocal of the total resistance is equal to the sum of the reciprocals of each resistor.
Resistance (Parallel) 1

34. The voltage drop results in a loss of energy.
When electric current traverses a resistor, it experiences a voltage drop.
This can be calculated using Ohm’s Law.
The voltage drop results in a loss of energy.
Energy “absorbed” by a resistor is converted into thermal energy.
In other words, resistors become hot as current passes through them.
Incandescent bulbs work on this principle.

35. There are a couple of additional rules that can be applied to resistors and circuits
Kirchkoff’s Rules:
The Loop Rule – the sum of the voltages (positive and negative) in a closed circuit must be zero)
In other words, when a charge gets back to the point where it started, its potential energy is the same.

36. Going against the flow causes a gain in potential (energy).
Going across a resistor in the same direction as the flow of current causes a drop in potential (energy).
Going against the flow causes a gain in potential (energy).
When going from positive to negative, the Voltage decreases, and vice-versa.
Voltage Drop
Voltage Gain

37. The Junction Rule:
The total current entering the circuit must equal the total current leaving the circuit.
Essentially, this is the conservation of electric charge.

38. Capacitors are usually charged by placing them inside a circuit.
Electrons accumulate on the negative side of the capacitor, and leave the positive side.
As the capacitor charges, it acts as a wire.
The capacitor will charge until the voltage across the plates equals the voltage of the battery.
At this point, current stops flowing.