1. ELECTRICITY UNIT REVIEW

2. 1. What is electric force? What is static electricity? Electric force is the force exerted by an object with an electric charge; this can be a force of attraction or repulsion Ex: after rubbing a comb in your hair, it becomes negatively charged. When placed next to a thin stream of water falling from a tap, the comb can attract the positive water molecules to bend the stream Static Electricity is an imbalance of electric charge on the surface of an object. The charges are at rest on the surface of the object.

3. 2. What electric charge do protons, neutrons, and electrons carry? Which of these 3 particles can be added or removed from atoms? Protons (p + )  + positive Neutrons (n 0 )  neutral (no charge) Electrons (e - )  - negative Protons and Neutrons are found in the nucleus and are held there by very strong forces. Electrons move around the space surrounding the nucleus. Electrons are the only particles which can be added or removed from atoms.

4. 3. What is a neutral object? Negatively charged object? Positively charged object? A neutral object has the same amount of protons and electrons A negatively charged object has more electrons than it does protons. Ex: a negative ion (atom that picked up an electron) A positively charged object has more protons than it does electrons. Ex: a positive ion (atom that lost an electron)

5. 4. What is induced charge separation? How can charge be induced? Induced charge separation is a shift in the position of electrons in a neutral object that occurs when a charged object is brought near it

6. 5. Law of Electric Charges Objects with like charges repel each other. Object with opposite charges attract each other.

7. 6. What are the two most common methods of charging by contact? Charging by Friction Charging by Conduction

8. 7. Explain charging by friction. How does charging by friction allow you to touch someone and shock them? Charging by Friction – the transfer of electrons between two neutral objects (made from different materials) that occurs when they are rubbed together or come in contact (touch) Ex: your hair is neutral, your comb is neutral, but when rubbed together, your hair becomes positively charged, and your comb becomes negatively charged

9. 8. Explain the electrostatic series (and relate it to charging by friction). The electrostatic series (or triboelectric series) is a list of materials arranged in order of their tendency to gain electrons. Lower on the list = stronger tendency to gain electrons If charging by friction occurs between an object higher on the list with an object lower on the list, the lower object will gain the electrons and become negatively charged…the object higher on the list lost its electrons and is therefore positively charged.

10. 9. Explain charging by conduction. How does grounding relate to charging by conduction? Charging by Conduction – charging an object by contact with a charged object The two objects have different amounts of charge, and when they come in contact, the electrons move from one object to the other. Grounding – connecting an object to a large body, like Earth, that is capable of effectively removing an electric charge that the object might have – removing excess charge

11. 11. What is a conductor? Name some examples of good conductors. Conductors – a material that lets electrons move easily through it Ex: Copper Silver Gold Aluminum Graphite

12. 12. What is an insulator? Name some examples of good insulators. Insulator – a material that does not easily allow the movement of electrons through it Ex: Rubber Wool Plastic Wood Ebonite Fur

13. 13. What is a semiconductor? Semiconductors – have special properties so that their electrical conductivity is between that of conductors and insulators Example of a special property: rising temperature actually decreases resistance in semiconductors; this can be useful. Semiconductors are used in LED (light emitting diodes).

14. 14. Explain charging by induction. How is it different from charging by friction and conduction? Charging by Induction – charging a neutral object by bringing another charged object close to, but not touching, the neutral object Charging by induction always results in 2 objects with opposite charges.

15. 15. How is an object temporarily charged by induction? How can an object permanently be charged by induction? Temporarily charged by induction: As the charged object is brought close to the neutral object, the induced charged will only be temporary, as the electrons will move back to their original positions once the charged object is taken away Permanently charged by induction: By grounding the object, it can be permanently charged by induction. Ex: a negatively charged rod can attract protons in a pith ball and will repel the electrons in the pith ball. By grounding the pith ball, the excess electrons can leave to the ground. The ground is removed first, and the negatively charged rod is removed second. The pith ball is now permanently positive.

16. 16. What is electric discharge? Electric Discharge – the rapid transfer of electric charge from one object to another Ex: lightning, or zapping someone J j

17. 17. In what way is lightning similar to a battery, and why does the electric discharge from the cloud occur?

18. 18. What is current electricity? How is it different than static electricity? Static Electricity is an imbalance of electric charge on the surface of an object. The charges are at rest on the surface of the object. vs Current Electricity is the controlled flow of electrons through a conductor. The electrons are always moving/flowing through the conductor.

19. 19. What is electrical energy? Electrical energy – the energy provided by current electricity (the flow of electrons in an electrical circuit) Like all forms of energy, it is measured in Joules (J) Like all forms of energy, it has the capacity to do work (ie. provide Power to loads).

20. 20. Current electricity moves easily through ___conductors___and poorly through ___insulators___.

21. 21. What is an electric circuit? What are the 4 basic components of electric circuits? Electric Circuit – a continuous path in which electrons can flow 1.Conducting Wires – conductor through which electrons flow 2.Load – converts electrical energy into another form of energy 3.Switch – controls the flow of electrons by opening/closing the circuit 4.Source – motivates electron flow, determines initial voltage

22. 22. What is an electric cell? What energy conversion takes place in electric cells? Electric Cell – a source for electricity – it is a device that converts chemical energy into electrical energy

23. 24. Explain the difference between Alternating Current (AC) and Direct Current (DC). Direct Current (DC) – the flow of electrons is in one direction through the circuit Alternating Current (AC) – the flow of electrons alternates in direction through the circuit

24. Electricity does not exist as a primary form of energy, it is produced when one type of energy is converted into electrical energy. Electrical generating plants convert mechanical energy from falling water into electrical energy.

25. 26. What is electrical power? Electrical Power is a measure of the rate at which electrical energy is being produced or used. The symbol for electrical power is P. Power = Energy/time = J/s or kWh/h The SI unit for electrical power is the WATT, and the symbol for this is W  used when calculating energy usage

26. 28. What is efficiency a measure of when dealing with electrical devices? EFFICIENCY – comparison of the energy output of a device with the energy supplied How much useful energy an electrical device produces compared with the amount of energy that was supplied to the device. % efficiency = (energy out / energy in) x 100% Tip: % efficiency = (smaller #/ larger #) x 100%

27. 30. What is the formula for calculating the cost of electricity? Cost to operate = power used x time x cost of electricity Cost of electricity is usually given in ₵/kWh, so make sure to convert power used into kWh, and time into h Converting to h (hours) If time is in s (seconds) – divide number of seconds by 3600 If time is in min (minutes) – divide number of minutes by 60 Converting to kWh (kilowatt hours) If power used is in W (Watts) - divide number of watts by 1000

28. 31. What is a circuit diagram? Circuit Diagram – a way of drawing an electric circuit using standard symbols.

29. 32. What is the difference between series and parallel circuits in terms of how the loads are connected and the path electrons flow? Series + Parallel – terms used to describe how loads are connected Series – loads are connected end to end so that electrons have only one path to flow through Parallel – loads are connected in branches so that there are 2 or more paths for electrons to flow

30. 33. What is electric current? What is its symbol? What is the unit for electric current? Current (I) - the measure of the rate of electrons that flow past a given point in a circuit Measured in amperes (A) Ammeters are used to measure current – must be connected in series with a load to measure the current flowing through that load

31. 34. What device is used to measure electric current? How must this device be connected in a circuit? Why is it connected in this way?

32. 35. Circuit breakers and fuses protect electrical devices from excess current Circuit breakers and fuses protect electrical devices from excess current. When drawn into a circuit diagram, these devices would be labelled as switches and fuses for example would receive the following symbol

33. 36. What is potential difference? What is another word for potential difference? What is its symbol? What is the unit for potential difference? Voltage/Potential Difference (V) - the difference in electric potential energy per unit charge measure at two different points in a circuit Voltage drop occurs across loads as electrical energy is converted into other forms of energy. The electrons lose some electrical potential by powering a load. Measured in volts (V)

34. Voltage/Potential Difference(V) Electrons leave the negative terminal to power the loads, and when they return to the positive terminal, they have less electric potential than they started with (because some energy was used to run the motor). Once back in the cell, chemical reactions “re-energize” the electrons and send them out the negative terminal again.

35. 37. What device is used to measure potential difference? How must this device be connected in a circuit? Why is it connected in this way? Voltmeters are used to measure voltage (potential difference) – must be connected in parallel with either a load or a power source to measure the drop in voltage across the load or power source (this drop is the voltage/potential difference)

36. 38. What is resistance? What is its symbol? What is the unit for resistance? Resistance (R) - the opposition to the movement of electrons as they flow through a circuit. Measured in ohms (Ω) All materials have some internal resistance. Greater resistance means the flow of electrons (current) will decrease

37. 39. What device is used to measure resistance? How must this device be connected in a circuit? Why should a circuit be powered off for this device to get accurate readings? Ohmmeters are used to measure resistance – must be connected in parallel with a load, but since they have their own power source to provide current through the load, the circuit cannot be powered up (it must be off).

38. 40. What are the 4 factors that affect resistance? Explain each. 4 factors that affect internal resistance: – Type of material Insulators have a high internal resistance, and conductors have a very low internal resistance. – Cross-sectional area Thicker wires have less resistance than thinner ones – more room to move freely for electron flow – Length Longer wires have increased internal resistance – more distance for electrons to travel – Temperature Electrons hitting the atoms of the material produce the heat, as a result, the atoms of the material gain energy and vibrate faster, causing them to bump into more flowing electrons.

39. 41. Using symbols, write down the 3 variations on the equation for Ohm’s Law. What does each equation calculate? Calculating Voltage  V = I x R Calculating Resistance  R = V / I Calculating Current  I = V / R

40. 42. How do series and parallel circuits differ? In series circuits: The voltage across loads add up The current across loads is the same In parallel circuits: The voltage across loads is the same The current across loads add up

42. 44. What mathematical formula is used to show how the current splits in parallel circuits? I load = I source / # loads * Only if loads are identical

43. 46. What mathematical formula is used to show how the voltage drop across each load decreases in series circuits? V load = V source / # loads * Only if loads are identical

44. 47. How does the total resistance in the circuit differ for series and parallel circuits In a parallel circuit, the total resistance is less than if the loads were connected in series