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. Electric charge is always conserved:  Suppose two objects come in contact. Object A develops a charge of -5. o It gained 5 electrons.  Object B would have a charge of +5. o It lost 5 electrons. So, the net electric charge is always conserved.

6. 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 10 -19 C.  One Coulomb is a MASSSIVE amount of charge!

7. Coulomb’s Law describes the interaction between charged particles as an electric force. F E – Electric Force ε 0 – Electric Constant = 8.85 x 10 -12 q 1, q 2 - charges r – distance between charges Coulomb’s Law

9. 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.

10. A field of electric force exists around a charged particle – known as an electric field.  This field reaches extends in all directions.

11. If a (test) charge (q) is placed in an electric field (E), it experiences electric force (F E ). Electric Field Strength The field of a positive charge points outward. The field of a negative charge points inward. The force acting on q.

12. The field strength weakens as the distance from the source charge increases.  The farther away, the weaker the charge. The closer together the field lines (more dense), the stronger the field. Note that the lines are farther apart as the distance from the charge increases.

13. Suppose a test charge, q1, experiences electric force from a point charge, q2. To find the strength of the electric field: E = F/q1. Substitute the electric force equation into the electric field equation.

14. 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.

16. 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.

17. 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. In order to bring two like charges together, work must be done on the charge.  In order to separate two opposite charges, work must be done as well. Consider two positive charges:  In order to move a positive closer to a positive charge, work must be done, of course. What would happen if the monkey let go of the charge?

22. By forcing the two like charges together, the monkey did work.  The small charge GAINED ENERGY by being moved closer to the source charge. The work that the monkey did was stored by the charge as Electric Potential Energy.  The closer the test charge came to the source charge, the pore potential it gained.

23. The Electric Potential Energy depends on:  The magnitudes of the charges.  The distance that a charge moves. Because it is a form of energy, the unit is the Joule. Electric Potential Energy

24. A decrease in U E is caused by moving WITH a charge’s natural response. For example:  If you move the positive charge from A to B, it decreases its potential energy because it had the tendency to do that anyway. Consider moving a bowling ball from a high shelf to a lower one. You decreased its PE… ++++++++++++ ---------- ++ AB

25. An increase in U E is caused by going AGAINST a charge’s natural response. For example:  If you move the positive charge from B to A, its PE increases because you had to exert work on it move it toward the like charge.  The work you used is stored as PE. Consider moving a bowling ball from a low shelf to a higher one. It wouldn’t have done that on its own. You stored energy in the ball by moving it. ++++++++++++ ---------- ++ AB

26. A change of electric potential energy ONLY occurs if the charge is moved ALONG the field lines. The change of potential energy is the same for both charges because r is the same. ++++++++++++ ---------- ++ AB + +

27. What if the monkey brought three charges instead of just one? How would it affect the work needed?  He would have to do more work, of course. Electric potential energy changes depending on the number of charges being moved. It is useful to describe the amount of electric potential energy per charge…the electric potential.

28. Electric potential is usually called VOLTAGE (V). Voltage is the Electric potential energy divided by the amount of charge moved. The unit for Voltage is the Volt, V. Electric Potential / Voltage

29. Areas within electric fields that have equal voltage are known as equipotential surfaces.  That does not mean that every line has the same value…just that the value of each line remains constant on that line. The dotted lines represent equipotential surfaces.

30. If a charge moves along an equipotential surface, there is no change in voltage, so ΔV = 0. Voltage is only present when there is a difference in charge.

31. If an electric field is uniform, then the voltage between two points can be found with: E is the strength of the electric field. d is the distance between the two points in the field. Voltage Across a Uniform Electric Field V = Ed

32. A capacitor is a device that stores electric potential energy, or electric charge.  Capacitors are usually composed of two conductors that hold equal & opposite charges, and are separated somehow. The parallel plate capacitor is most commonly used in explanations. It is composed of two conducting plates that are separated by an insulator.

33. The capacity for A capacitor to hold a charge is measured by its…capacitance, C.  It is measured in Farads, F. The higher the capacitance, the more charge the capacitor can hold. Capacitance for ANY Capacitor Not all capacitors are made with parallel plates that are separated.

34. The capacitance for a parallel plate capacitor can be found with the following equation: Where “d” is distance between the plates. “A” is the area of the plates. If Area increases, or if Distance decreases, the capacitance increases. Capacitance for a Parallel Plate

35. The amount of energy stored in a capacitor can be found if we know the amount of charge, and the change in voltage from one plate to the other. Potential Energy of a Capacitor

37. Electric current is the flow of electric charge through a conductor.  Recall that 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.

38. However, 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.

39. Electric current is a measure of the amount of charge passing a point each second. The unit for current is the ampere, or amp (A).  One ampere is equal to one Coulomb of charge per second. The flow of electric charge will be assumed to move from POSITIVE to NEGATIVE. Electric Current

40. 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)

41. Resistance depends on two factors: the type of material, and its shape. Every material has an intrinsic resistivity, or resistance to the flow of charge.  It is represented by the symbol rho: ρ. For a wire length L, and area A, it provides a resistance of: Resistance (Resistivity)

42. 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.

43. 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

44. 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)

45. A resistor provides some amount of resistance in a circuit.  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.

46. 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)

47. 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)

48. 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. o Incandescent bulbs work on this principle.

49. 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.

50. 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

51. 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.

52. 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. o At this point, current stops flowing.

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