Final

Question 1

TRUE or FALSE: An object that is positively charged contains all protons and no electrons.

Answer 1

False.

Positively charged objects have electrons; they simply possess more protons than electrons.

Question 2

TRUE or FALSE: An object that is negatively charged could contain only electrons with no accompanying protons.

Answer 2

False.

Negatively charged objects have protons; it’s just their number of electrons is greater than their number of protons.

Question 3

TRUE or FALSE: An object that is electrically neutral contains only neutrons.

Answer 3

False.
Electrically neutral atoms simply possess the same number of electrons as protons. This gives the objects a balance of both type of charge.

Question 4

Upon entering the room, you observe two balloons suspended from the ceiling. You notice that instead of hanging straight down vertically, the balloons seems to be repelling each other. You can conclusively say …

Answer 4

both balloons are charged with the same type of charge.

Observing a repulsive interaction is sufficient evidence to conclude that both balloons are charged. However, further testing or additional information would be required to determine the type of charge the balloons have.

Question 5

Two objects are shown in the image below. One is neutral and the other is negative. Object X will ____ object Y.

Answer 5

repel

Question 6

Which of the diagrams below best represents the charge distribution on a metal sphere when a positively charged plastic tube is placed nearby?

Answer 6

D. In each case, the tube (in pink) is charged. Its charge should polarize the metal sphere. This is not shown in A; so A can be ruled out. In B, C and D the sphere is polarized. But in B and C, the diagram shows the separation of charge in a manner that would indicate an attraction between like charges. On the other hand, D shows that the negative charge on the sphere is attracted to the positive tube.

Question 7

If an electric circuit could be compared to a water circuit at a water park, then the battery would be analogous to the ____.

Answer 7

water pump

Question 8

If an electric circuit could be compared to a water circuit at a water park, then the positive terminal of the battery would be analogous to the ____.

Answer 8

top of the slide

Question 9

If an electric circuit could be compared to a water circuit at a water park, then the current would be analogous to the ____.

Answer 9

gallons of water flowing down the slide per minute

Question 10

If an electric circuit could be compared to a water circuit at a water park, then the charge would be analogous to the ____.

Answer 10

water

Question 11

If an electric circuit could be compared to a water circuit at a water park, then the electric potential difference would be analogous to the ____.

Answer 11

water pressure

Question 12

A. Utilize your understanding of the requirements of an electric circuit to state whether charge would flow through the following arrangements of cells, bulbs, wires and switches.

Answer 12

No

The switch is in the open position, so there is no closed conducting loop

Question 13

B. Utilize your understanding of the requirements of an electric circuit to state whether charge would flow through the following arrangements of cells, bulbs, wires and switches.

Answer 13

No

There is no closed conducting loop from + to - terminal.

Question 14

In the movie Tango and Cash, Kurt Russell and Sylvester Stallone escape from a prison by jumping off the top of a tall wall through the air and onto a high-voltage power line. Before the jump, Stallone objects to the idea, telling Russell “We’re going to fry.” Russell responds with “You didn’t take high school Physics did you. As long as you’re only touching one wire and you’re feet aren’t touching the ground, you don’t get electrocuted.” Is this a correct statement?

Answer 14

Yes. In order for there to be a sustained flow of charge from one location to another, there must be a difference in electric potential. In this case, there would be a momentary flow of charge between the wire and the actor until they reach the same electric potential. Once at the same potential, charge flow would cease and there would be no electrocution. However, if the actor’s feet touched the ground (an electric potential of 0) or another wire of a different potential, then there would be a sustained charge flow which likely would lead to electrocution.

Question 15

What is the common unit of measure for electric ENERGY?

Answer 15

watt-hours (kWh for residential)

Question 16

What is the common unit of measure of electric POWER?

Answer 16

watts

Question 17

Imagine you are lighting a library with a solar suitcase. How much energy will 6 5-W lamps use per night if run them for 4 hours per night?

Answer 17

6*5 watts x 4 hr/night =

120 wh/night

Question 18

Image that a battery needs to supply all of that energy. What is the minimum capacity 12-V battery (in Amp-hours) that would be needed to supply 120wh? (For now assume that you can drain the battery completely. We will correct that assumption later when we talk about specific types of batteries.)

Answer 18

120wh/12v = 10 Ah

used P=IV => Wh = Ah x V (divide by V)

Question 19

120Wh
If you are in a location that gets 6 sun-hours per day, what capacity of PV panels (in watts) would you need to supply the batteries? (For now assume there are no battery losses, and assume that the panels simply generate electricity at full power output for those 6 hours and none for the rest of the time.)

Answer 19

120Wh/6h = 20W

to supply the energy to be used in the night

Question 20

If you use the 35-watt panels we have in the classroom, how many of them would it take?

Answer 20

only one supplies more than you need based on this analysis

Question 21

If a one-gigawatt power plant generates power non-stop for one year, how much electricity (i.e. electric energy) will it generate?

Answer 21

1GW x (24x365)h/yr = 8760 GWh or 8.76 billion kWh

Question 22

If a home use 10 kWh/day of energy on average, how many homes could that power plant supply with 8.76 billion kWh?

Answer 22

8.76 x 10^9kWh/yr x yr*homes/3650kWh = 2.4x10^6

Question 23

A 5 MW wind turbine generates energy 40% of the time. How much electricity (i.e. electric energy does it generate in one year?

Answer 23

5 MW x 0.4 (365x24)hr/yr = 17520 MWh/yr = 17.5 GWh/yr

Question 24

You want to run three two-watt lights (3 2-W lights) on a 12-V solar electric system, how much current will that system draw when operating those lamps

Answer 24

I = P/V => 3x2W/12V = 0.5 A

Question 25

Three resistors with resistance values of 2-Ω , 4-Ω , and 6-Ω are placed in series. These would provide a resistance that is equivalent to one _____-Ω resistor.

Answer 25

12

Question 26

Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.

Answer 26

• decrease the voltage
• increase the resistance
  The current in a circuit is directly proportional to the electric potential difference impressed across the circuit and inversely proportional to the resistance of the circuit. Reducing the current can be done by reducing the voltage (choice A) or by increasing the resistance (choice D).

Question 27

You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause ____.

Answer 27

your resistance to be lower.
Wet hands have less resistance and thus less hindrance to the flow of charge; the current would thus be increased. Touching an outlet with wet hands increases the risk of charge flowing through you and causing electrical shock or even electrocution.

Question 28

3. Calculate the current of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).
   Give your answer in Amps.

Answer 28

ΔV = 120 V (true for US household outlets)
P = 1500 W
 P = ΔV • I 
I = P / ΔV = (1500 W) / (120 V)
I = 12.5 Amp

Question 29

3. Calculate the resistance of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).
   Give your answer in Ohms.

Answer 29

ΔV = 120 V (true for US household outlets)
P = 1500 W
ΔV = I • R
R = ΔV / I = (120 V) / (12.5 Amp)
R = 9.6 Ω

Question 30

As more and more resistors are added in parallel to a circuit, the equivalent resistance of the circuit ____________ .

Answer 30

decreases

Question 31

As more and more resistors are added in parallel to a circuit, the total current of the circuit ____________ .

Answer 31

increases

Question 32

An electric circuit is diagrammed below. For the circuit, indicate which two devices are connected in series.

Answer 32

Ammeter and Resistor are in series

Question 33

An electric circuit is diagrammed below. For the circuit, indicate which two devices are connected in parallel.

Answer 33

Speaker and Bulb are in parallel (in separate branches)