Spring 2011 Midterm Review Answers

1. Spring 2011 Midterm Review Answers CP PHYSICS Ms. Morrison

2. 1. How are positive ions formed? How are negative ions formed? Positive ions are formed by atoms that lose electrons Negative ions are formed by atoms that gain electrons

3. 2. What is the fundamental rule regarding charge interactions? Like charges repel and opposite charges attract.

4. 3. Can charges be created or destroyed? NO – they can only be separated. This is done when electrons are transferred from one object to another resulting in a negatively charged object and a positively charged object.

5. 4. How are conductors different from insulators? Conductors have electrons that can move easily from one atom the next while insulators hold their electrons tightly, so that they do not move.

6. 5. Explain how charges can be moved by: a. Conduction: a neutral body is charged when touched by a charged object, it takes on the same charge as the charged object, is permanent, repels b. Induction: a neutral object becomes temporarily charged when a charged object is brought near it, takes on the opposite charge, attracts c. Friction: two neutral objects become charged when they are rubbed together and electrons are transferred from one object to the other, results in one object having a positive charge and the other object having a negative charge so that they become attracted to each other d. Grounding: a charged object’s excess charge moves from it to through to the ground when a pathway from it to the ground is provided

7. 6. What happens to the electrical force experienced by two charged particles separated some distance if: a. One of the charges doubles: 2 F b. The distance is doubled: ¼ F c. One of the charges doubles and the distance is doubled: ½ F

8. 7. Two identical charges of 3.8 x 10-6 C are separated by a distance of 0.025 m. What electrical force do they exert on each other? Q1 = Q2 = 3.8 x 10-6 C d = 0.025 m k = 9 x 109 Nm2/C2 F = ? Fel = kQ1Q2 d2 = (9 x 109)(3.8 x 10-6)(3.8 x 10-6) (0.025)2 Fel = 207.94 N

9. 8. How far apart are two negative charges if they experience a force of 5.6 N on each other? One has a charge of -1.5 x 10-5 C, and the other has a charge of -7.2 x 10-4 C. Fel = 5.6 N Q1 = -1.5 x 10-5 C Q2 = -7.2 x 10-4 C k = 9 x 109 Nm2/C2 d = ? Fel = kQ1Q2 d2 5.6 = (9 x 109)(-1.5 x 10-5)(-7.2 x 10-4) d2 (5.6)d2= (9 x 109)(-1.5 x 10-5)(-7.2 x 10-4) 5.6 5.6 d2 = 17.36 d = 4.17 m

10. 9. Draw the electric fields for the following: a. Weak positive charge b. Strong negative charge

11. 9. Draw electric fields for the following: c. Two positive charges d. A negative charge and a positive charge

12. 10. What are the relationships between current and voltage and resistance according to Ohm’s Law? Current and voltage are directly proportional to each other. If the voltage increases, then the current will also increase and vice versa. (Assume resistance is constant.) Current and resistance are inversely proportional to each other. If the resistance increases, then the current will decrease and vice versa. (Assume voltage is constant.)

13. 11. Do you buy electrons from the power company? No, you buy energy from the power company. The electrons are already in your electrical devices and home wiring, and they flow because of the energy received from the power company.

14. 12. What are two safety devices used in circuits? How do they work? Fuses: small tube with metal ends and wire through middle, wire melts if the current increases beyond a specific value, must be replaced if used Circuit breakers: breaker pops open and in turns opens up the circuit when the current exceeds a specific value, can be reused, the breaker simply needs to be reset

15. 13. What is the difference between direct current and alternating current? Direct current is current that flows only in one direction. Alternating current is current that moves back and forth in two directions – results from the way it is produced.

16. 14. What is a source of direct current? Of alternating current? Batteries provide direct current. Generators (including power companies) provide alternating current.

17. 15. What is the current through an electrical device that is plugged into a 120 V outlet, if it experiences a resistance of 540 Ω? V = 120 V R = 540 Ω I = ? V = IR 120 = I (540) 540 540 I = 0.22 A

18. 16. What is the voltage across a device that provides 1200 Ω of resistance and has a current of 0.40 A through it? R = 1200 Ω I = 0.40 A V = ? V = IR = (0.40)(1200) V = 480 V

19. 17. What is the resistance of an electric frying pan which draws 5.8 amps when connected to a 120 V circuit? I = 5.8 A V = 120 V R = ? V = IR 120 = 5.8 R 5.8 5.8 R = 20.69 Ω

20. 18. What is the power of the frying pan in #17? I = 5.8 A V = 120 V P = ? P = VI = (120)(5.8) P = 696 W

21. 19. An electrical appliance uses 6.0 kWh in a month. If the power company charges $0.08/kWh, what is the cost to use this electrical appliance? E = 6.0 kWh rate = $0.08/kWh cost = ? cost = E x rate = (6.0)($0.08) cost = $0.48

22. 20. What is a series circuit? A circuit in which there is only one pathway for current to move. It does not matter how many electrical devices (resistances) are plugged into the circuit, there is only one way for the current to flow – it will flow through all the devices in the circuit.

23. 21. What is a parallel circuit? A circuit in which there are two or more pathways for current to move.

24. 22. If one light goes out in a series circuit, do the remaining lights go out? Yes, the circuit is essentially open, so no current can flow through it. This interrupts the flow of current through every other device in the circuit.

25. 23. If one light goes out in a parallel circuit, do the remaining lights go out? No, the remaining lights will stay on if they are in different branches of the circuit. This is because the different branches provide alternate pathways for the current to flow.

26. 24. What happens to the equivalent (total) resistance in a series circuit when you add another resistor to it? The total resistance will increase every time another resistor is added to the circuit. This is because the total resistance is calculated by adding up all the individual resistances in the circuit.

27. 25. What happens to the equivalent resistance in a parallel circuit when you add another resistor to it? The total resistance decreases every time another resistor is added into the circuit in parallel. This is because the total resistance is less than the lowest resistor in the circuit. (This is also why circuit breakers are tripped when too many resistors are plugged into the circuit – think too many hair dryers.)

28. 26. What does an ammeter measure? How is it connected into a circuit? An ammeter measures the current flow through the circuit. It is connected in series, because the current must flow through it to get an accurate measurement.

29. 27. What does a voltmeter measure? How is it connected into a circuit? A voltmeter measures the voltage across an electrical device. It is connected into a circuit in parallel because it provides another pathway though which current can flow. Remember – voltage is also known as potential difference, so to get a reading of the potential difference, the meter must be connected into the circuit before and after the electrical device.

30. 28. If there are three lamps connected in series, how many paths can the current take? One – a series circuit has only one pathway through which the current can flow.

31. 29. If there are three lamps connected in parallel, how many paths can the current take? If each lamp is in its own separate branch of the circuit, then the current has three pathways it can take through the circuit.

32. 30. Determine the equivalent resistance for the following circuits: R = 15 Ω R = 11 Ω

33. 31. Calculate the values represented in the missing blanks for the following circuit diagrams. Also, identify whether the circuit is a series or parallel circuit. Series V1 = 40 V, IT = 2 A, R2 = 30 Ω Series RT = 110 Ω, V1 = 40 V, V2 = 120 V, V3 = 60 V, VT = 220 V

34. 31. Calculate the values represented in the missing blanks for the following circuit diagrams. Also, identify whether the circuit is a series or parallel circuit. Parallel V1 = 40 V, V2 = 40 V, I1 = 2 A, I2 = 2 A, R2 = 20 Ω, RT = 10 Ω Parallel RT = 15 Ω, VT = 60 V

35. 32. How are magnetic poles similar to electric charges? Like charges/poles repel each other. Opposite charges/poles attract each other. Both exert forces that can act over a distance – the charges/poles do not have to be touching each other for the force to act between them.

36. 33. What is the major difference between magnetic poles and electric charges? Electric charges can be separated from each other – electrons can be removed so that there is a negatively charged object and a positively charged object. Magnetic poles cannot be separated – you cannot separate the north pole from the south pole. If you break the magnet in half, all you get is two smaller magnets each with a north and south pole.

37. 34. How is the magnetic field oriented around a magnet? The magnetic field lines leave the north pole and curve around the magnet and enter the south pole.

38. 35. Where is the magnet field the strongest? The magnetic field of a magnet is strongest at its poles.

39. 36. What is the smallest magnet? The smallest magnet in the universe is an electron. This is because magnetic fields are created by moving charges, and electrons spin on their axes just like Earth spins on its axis.

40. 37. What happens to the magnetic field around a current-carrying wire if the current through it is reversed? If the current flow is reversed, then the direction of the magnetic field will also be reversed.

41. 38. How does an electric motor work? A coil of wire is placed inside a permanent magnet so that when electricity goes through the wire it becomes an electromagnet and interacts with the permanent magnet’s magnetic field. The wire then spins and every 180o the current reverses direction so that the poles of the electromagnet flip so that the wire will keep spinning and spinning. Motors convert electrical energy into mechanical energy.

42. 39. What are two ways to generate current in a wire? Move a wire in and out of a magnetic field (magnet). Move a magnet in and out of a coil of wire.

43. 40. How does an electric generator work? A coil of wire is located inside permanent magnet – just like a motor. When you turn the coil of wire (steam, wind, water), then electricity is generated because the wire is moving within the permanent magnet’s magnetic field. The electricity generated is alternating current. Generators convert mechanical energy into electrical energy – opposite of a motor.

44. 41. What three factors does the strength of an electromagnet depend on? The number of loops in the wire – more loops = stronger electromagnet The amount of current flow through the coil of wire – more current = stronger electromagnet Presence of an iron core – if iron core is present, then the electromagnet will be stronger

45. 42. What three factors does the voltage (and current) produced by a generator depend on? The number of loops in the wire – more loops = more voltage generated The strength of the permanent magnet’s magnetic field – stronger magnet = more voltage generated The speed of rotation of the coil of wire – the faster the coil of wire is rotated = more voltage generated