Download Electricity Review ANSWERS - hhs-snc1d

Electricity Review ANSWERS
This is your review for Monday’s test. This does not cover everything, but it’s a good start. I will
ask everything about everything, so be prepared! I will post the answers on the Wikispace this
weekend – hopefully by noon Sunday. To get to the wikispace, simply Google “Hillcrest” and go
to the hillcrest webpage. Follow the links to “Academics” and “Science” and finally to the course.
You do not have to hand in. This will not be marked.
1. Given the equation V = I R, solve for I.
I = V/R
2. Given the equation P = E/t, solve for E. E = P t
3. Given the equation I = Q/t, solve for Q. Q = I t
4. Given the equation P = IV, solve for V.
V = P/I
5. Use the equations above to determine which of the following is FALSE:
a)
b)
c)
d)
One Volt is equal to one Ampere-Ohm. True V = IR
One Watt is equal to one Joule per second. True P = E/t
One Ampere is equal to one Coulomb-second. False I = Q/t not I = Q t
One Watt is equal to one Ampere-Volt. True P = IV
6. Complete the sentence:
a)
b)
c)
d)
e)
f)
The symbol for charge is:
The unit for charge is the:
The unit symbol for charge is:
The symbol for power is:
The unit for power is the:
The unit symbol for power is:
Q
Coulomb
C
P
Watt
W
7. Given the equation I = Q/t, solve for t.
Remember: variables move diagonally when then move to other side.
I = Q/t
start with equation, move t to left side. It goes next to I
It = Q
now move I to right side. It goes under Q
t = Q/I
Final answer.
8. A 30W light bulb runs for 4 hours. How many Watt-hours of energy are used?
30W x 4h = 120 Watt-hours (120Wh)
In kiloWatthours: 0.120 kWh
9. A 30W light bulb runs for 4 hours. How many Joules of energy are used?
For Joules, must convert hours to seconds:
30W x 4h x 60min/h x 60s/min = 432000J
10. When a radio is plugged into a 120V wall socket, 0.5A of current flow through the radio. What is
the resistance of the radio?
R = V/I
R = 120V/0.5A
R = 240Ω
11. A flashlight uses two 1.5V batteries and allow a current of 2.5A to flow. What is the power of the
flashlight?
P = IV
P = 2.5A(3.0V)
P = 7.5W
12. Refering to the electrostatic series from your notes, what charge would a plastic ruler obtain
when rubbed against a cotton shirt?
Plastic has a stronger hold on electrons than cotton. So the shirt will lose electrons to the ruler.
The ruler will be negatively charged. The shirt will be positively charged.
13. When a charged object is brought in contact with a neutral object, the neutral object becomes
charged. Is the charge of the neutral object the same as or opposite to the charged object?
This is charging by contact. The electrons will jump from one object to the other.
For example, if a negative object touches a neutral object, electrons jump from the negative
object to the neutral object. Now the neutral object is negative.
If a positive object touches a neutral object, the electrons jump from the neutral object to the
positive object. The neutral object then becomes positive.
So the answer is, the neutral object gets the same charge as the charged object.
14. When a charged object is brought near a neutral object that is grounded, and the ground is
removed, the neutral object becomes charged. Is the charge of the neutral object the same as or
opposite to the charged object?
This is charging by induction. In this case, electrons are induced to leave or enter a grounded
neutral object by the presence of a charged object.
For example, if the charged object is positive, then electrons will be induced to enter the neutral
object, making it negative.
If the charged object is negative, then electrons will be induced to leave the neutral obejct,
making it positive.
So the answer is, the neutral object always takes the opposite charge of the charged object.
15. Question #12 is an example of charging by __Friction ____.
16. Question #13 is an example of charging by __Contact____.
17. Question #14 is an example of charging by __Induction____.
18. What are the four basic components of a circuit? Power source, connectors, loads, switches.
The power source can be a battery or outlet, the connectors are usually wires, the loads are
anything that uses the energy (resistor, lamp, toaster, radio...) .
19. A circuit has one resistor. A voltmeter is attached in parallel to the resistor. What happens to the
voltage reading on the meter when I add another resistor in series (i.e. does the reading go up or
down or stay the same)? Not an easy question. When a resistor is added, the current drops. A
lower current means a lower voltage reading, since V = IR
20. A circuit has one resistor. A voltmeter is attached in parallel to the resistor. What happens to the
voltage reading on the meter when I add another resistor in parallel (i.e. does the reading go up
or down or stay the same)? Stays the same.
21. A circuit has one resistor. An ammeter is attached in series with the resistor. What happens to
the meter reading when I add another resistor in series? Current reading goes down.
22. A circuit has one resistor. An ammeter is attached in series with the resistor. What happens to
the meter reading when I add another resistor in parallel? Depends where the meter is. If the
meter is next to the battery, the current will go up. If the meter is next to the resistor (along one
of the branches) the current will stay the same.
23. Two resistors, each 24 Ohms, are attached in series to a 9V battery. What is the current flowing
through the resistors?
I = V/R
I0 = V0/R0
Io = 9V/48Ω
Io = 0.1875A
This is the current leaving the battery. Since there is only one route for
it to go, this must be the same as I1 and I2.
I1= I2= 0.1875A
24. Two resistors, each 24 Ohms, are attached in parallel to a 9V battery. What is the current:
a) leaving the battery?
Io = Vo/Ro
Remember Ro = Req and 1/Req = 1/24 + 1/24 so Req = 24/2 = 12Ω
Io = 9V/12Ω
Io = 0.75A
b) re-entering the battery?
Always the same current leaving the battery as entering: 0.75A
c) flowing through one resistor?
I1 = V1/R1
I1 = 9V/24Ω
I1 = 0.375A
Each branch has the same voltage, so V1= Vo= 9V
25. Give two advantages of electromagnets over permanent magnets.
Electromagnets can be extremely strong
Electromagnets can be turned on and off
Electromagnets can be turned up or down (stronger or weaker)
Electromagnets can be reversed (North becomes South, South becomes North) by switching the
direction of the current through the coil of wire.
Electromagnets can be fairly lightweight
26. Give two advantages of permanent magnets over electromagnets.
Permanent magnets don’t need electricity
Permanent magnets can be very small
Permanent magnets work virtually for ever