AP Physics – Electric Fields – 1 Ans
1. A 455 g block is positioned as shown on a smooth ramp. A 655 g block rests on
a smooth table top. The 455 g block is released, slides down the ramp and
undergoes an inelastic collision with the other block. The two blocks slide off
the table and fall to the deck, striking it a horizontal distance x from the edge of
the table. Okay, find the following things: (a) the potential energy of the 455 g
block before it is released, (b) the speed of the 455 g block at the bottom of the
ramp, (c) the speed of the two blocks after the collision, (d) the distance x the
two blocks travel before they hit the deck, and (e) the kinetic energy of the two
blocks just before they hit the deck.
(a) U  mgy
 mg sin   l
m

 0.455 kg  9.8 2  sin 44.0o  0.750 m  
s 

(b) U  K U  1 mv 2 v  2U
m
2
(c) m v   m  m  v ' v ' 
1 1
1
2
(d)
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
m1v1
m
 1  m2 


 0.455 kg   3.19 ms 


0.455
0.655
kg
kg



3.19

75.0 cm
655 g
44.0
98.0 cm
x
2.32 J
m
s
1.31
m
s
m
2  0.98 m 
x  vt  1.31  0.4472 s  
0.586 m
 0.4472 s
s
m
9.8 2
s
What is the fundamental rule in static electricity? Like Charges repel, opposites attract
One vigorously rubs a balloon with a bit of fur or cloth. After this has been done, the balloon can then be "hung"
on the wall or on a person's clothing. What happened to make all this occur? The balloon was charged with
electrons.
What are things that allow electricity to flow through them and what are things that do not allow any flow of
electricity? What type of materials act in these ways? Conductors=metals , insulators=rubber, plastics
How does one go about charging an object by conduction? A charged object touches an uncharged object and the
charge spreads out to the second object.(now they are both charged)
How could one go about charging an object by induction? A negatively charged object comes close to an
uncharged object. The electrons in the uncharged object rush to the other side to avoid the negatively charged
obect.The uncharged object becomes polarized and the electrons on the far side are then touched by a third object
and removed from the uncharged object. The uncharged object is now positively charged because electrons were
removed.
Where on an object does the charge reside? On the outside (to get as far away from each other as they can.)
How did the Physics Kahuna so magically move an entire two by four without touching it? Using the electric field
of a charged object.
Explain how an electroscope works. It determines whether an object has a charge or not. It works by bringing the
charged object near the electroscope and something visual will indicate a charge (ie. Two pieces of foil will move
apart from each other)
The electrophorus was looked upon as a magical device - it seemed to be an endless source of charge. How does it
do it? It didn’t lose its charged, it used induction and pushed electrons in a metal object to the top of the metal
where the observer could touch and feel electrons travel into their finger.
A uniform 4.00 m long rod weighs 235 N. One end of it is attached to a wall and a cable supports the other end.
The cable makes an angle of 41.0 with the rod. Find the tension in the cable and the components of the force
exerted by the wall on the rod.
T
1
y  at 2
2
t
2y
a
d
 T sin   d  0
2
 235 N  
T
2 sin 41.0
FB
kg  m  m 

2  2.32

s2



0.455 kg
455 g

T sin   d
179 N
 FB
d
2
T
FB
2sin 
R
41.0
(b) Sum forces:
FB
X: RX  T cos   0
Y:
RX  T cos 
RY  FB  T sin   0
  235 N  cos 41.00
RY  FB  T sin 

177 N
RY  235 N  179 N  sin 41.00

118 N
12. What is the wavelength in nanometers of an electromagnetic wave that has a frequency of 2.35 x 1016 Hz?
m
3.0 x 108
v
s
v f  
12.8 nm

  1.28 x 108 m 
f
16 1
2.35 x 10
s