Download Unit 2 Electric Forces And Fields Review 2015

Physics 30 Unit 2 Review
Electrical Forces and Fields
Electrical Forces & Fields
Electrostatics:
Law of Charges:
repel and unlike charges
Like charges _______
attract
__________.
Movement of Charges:
electrons move within insulators
Only _________
and conductors.
Grounding:
earth is both an unlimited
The _______
electrons
depository or source of ___________.
end
Charging by Friction:
insulators together will
Rubbing two ___________
force the movement of electrons
________ from
one object into the other.
Charging by Conduction:
Touching two charged objects will cause
transfer of charge.
a ___________
Charging by Induction:
charge separation
Causing a ____________
____within an
touching it.
object without ___________
end
Examples:
There are three equal sized spheres. Sphere A has a charge of +2.0
mC, sphere B has a charge of -4.0 mC, and sphere C has a charge of
+8.0 mC. If sphere C touches A, then touches B, what are the
charges on all spheres?
A
2.0
B
-4.0
A
5.0
B
0.5
A
5.0
B
0.5
C
8.0
C
5.0
C
0.5
C
0.5
end
Examples:
Explain how to charge an electroscope negatively through
induction. You have a rod that when rubbed with fur becomes
positively charged.
•Ground the top of the electroscope.
•Rub the rod with fur so it becomes positively charged through friction.
•Bring a positively charged object near the electroscope but do not touch it.
•Since opposite charges attract (law of charges), negative electrons will flow from
the ground into the electroscope.
•Disconnect the ground wire.
•Remove the positively charged object.
•The electroscope is now negatively charged.
end
Electric Field:
•Is the direction a small, __________
positive
test charge will
travel when placed in the field.
Drawing Electric Fields:
-
+
end
Drawing Electric Fields around charged objects:
end
end
end
?
+
end
Drawing Electric Fields between charged plates:
-
+
Electric field is
constant between
the plates so long
as the distance
between them
doesn’t change.
zero
Electric Fields inside a spherical conductor is _______________
end
Charges flow along the outside of a conductor.
− − −
−
−
− − −
If there’s an imperfection along the surface of the sphere, that
more
location will have ______________
charge.
−−
−
−−
−
−
−
− − −
end
Electric Field Formulas:
E
•Electric Field has this notation: _______________
•Energy has this notation: _______________
E
•Do not use +/- notation in formulas. Use the direction of a small
____________
test charge to find direction.
positive
Electric field a distance from a charged object.
Units of N/C
Electric field is used to find the force a charged
object experiences when placed in the electric field.
Units of N/C
Electric field between electric plates.
Units of N/C or V/m
end
Examples:
Calculate the electric field 2.55 x 10-3 m away from a 3.82 mC object.
(8.99x109)(3.82x10-6C)
=
= 5.28x109 N/C
-3
2
(2.55x10 )
If a 1.12 mC charged sphere is brought to a distance of 2.55 x 10-3 m
away from the object in the last question, what electrical force does
it experience?
= (5.28x109)(1.12x10-6C) = 5.92x103 N
If the 1.12 mC charged sphere has a mass of 3.00 g, what
acceleration will it experience in the electric field?
=
5.92x103 N
3.00x10-3 kg
= 1.97x106 m/s2
end
Examples:
Calculate the electric field between the plates below. Draw the
direction of the electric field as well.
+
9 V separated
by 2.50 cm
If an electron is placed in between the plates, what electrical force
does it experience? What is the direction of this force?
The direction is up towards the positive plate.
end
Examples:
Calculate the electric field midpoint between two point charges of
4.00 mC and 3.25 mC that are 6.84 x 10-1 m apart.
4.0
3.25
0.342 m
0.342 m
The electric fields point in opposite directions so subtract.
to the right
end
Examples:
Calculate the electric field at point “p”
+1.66 mC
3.00 cm
-2.82 mC
p
4.00 cm
end
+1.66 mC
3.00 cm
-2.82 mC
p
4.00 cm
Combine the electric fields using pythagorean theorem.
end
E of S
end
Electrical Energy:
•Electric charges have electrical ____________
potential
energy when placed
objects or between __________
charged plates.
near other charged _________
•Electric potential is also referred to as __________.
voltage
•Unlike electric field, the ___________
voltage
experienced by a charge
does change as it moves between electric plates.
end
Potential Difference:
There is 100 V between the plates. What is the potential difference
experienced by the positive particle in locations 1, 2, 3 and 4?
+ plate
4 100 V
3 75 V
2 50 V
+ 1 0V
- plate
end
Potential Difference:
There is 100 V between the plates. What is the potential difference
experienced by the negative particle in locations 1, 2, and 3?
+ plate
3 0V
2 25 V
- 1 100 V
- plate
end
Examples:
A proton is moved through two parallel plates that are 5.00 cm
apart and have a potential difference of 90.0 V between them. How
much work is done against the electric field when the proton moves
3.00 cm parallel to the plates?
+
The proton is not moved through a
potential difference.
+
-
No work is done against the
electric field.
What other force could be present to ‘make’ the
electron follow a straight path between the plates?
A magnetic force. What direction would
the field need to be oriented in?
end
Examples:
A proton is moved through two parallel plates that are 5.00 cm
apart and have a potential difference of 90.0 V between them. How
much work is done against the field when the proton moves 3.00
cm towards the positive plate?
+
3.00 cm
+
-
Method 1
The proton is moved through 3/5 of the
potential difference.
V=(3/5) (90.0 V) = 54 V
end
+
3.00 cm
+
Method 2
The electric field is constant between the
plates.
-
end
1.50x103 V
+
3.50x103 V
Calculate the electric potential
difference gained by the moving
charge.
Though the electric field
remains constant, the charged
object does experience a
change in voltage.
end
Calculate the speed of the 3.00x10-12 C and
4.00x10-15 kg charge as it reaches the
opposite plate if it started from rest..
3.50x103 V
end
Examples:
Calculate the vertical displacement of the electron as it travels
between the plates if the plates are 9.00 cm long.
+
7.00 X 106 m/s
e-
9.00 cm
90 V separated
by 6.50 cm
-
end
+
7.00 X 106 m/s
e-
9.00 cm
Horizontally
Vertically
0
end
Examples:
How many electrons travel through a wire in 0.100 s if the current is
3.00 A ?
end
Coulomb’s Law:
Coulomb used a __________
torsion
balance to measure the electric forces
acting on between charged spheres so he could derive the following
formula:
His apparatus looked like:
end
Cavendish:
Cavendish used a __________
torsion
balance to measure the gravitational
forces acting on between charged spheres so he could derive the
following formula (this was done before Coulomb’s exp):
His apparatus looked like:
end
Examples:
What is the electric force between two charges of 5.00 mC and
3.00 mC if they are separated by 2.00 cm?
end
Examples:
How far apart are two point charges of 2.00 𝜇C and 4.00 𝜇C if they
produce an electric force of 5.00 x 10-1 N on each other?
=
(8.99𝑥109 )(2𝑥10−6 )(4𝑥10−6 )
0.5
= 3.79 m
end
Examples:
Two point charges produce an electric force of 6.00 x 10-2 N. What
will be the force between them if their charges are tripled and the
distance between them halved?
end
Examples:
Find the net electric force on the middle sphere.
-2.00 mC
3.00 mC
4.00 cm
4.00 cm
(8.99𝑥109 )(2𝑥10−6 )(3𝑥10−6 )
=
3.00 mC
(0.04)2
= 33.7 𝑁
(8.99𝑥109 )(2𝑥10−6 )(3𝑥10−6 )
=
(0.04)2
50.6
84.3 𝑁
end
Examples:
Two equally charged, identical spheres are 0.100 m apart and have
an electric force of 4.00 x 10-2 N between them. What is the
magnitude of the charge on each sphere?
end
Examples:
Find the net electric force on the middle sphere.
-2.00 mC
3.00 mC
(8.99𝑥109 )(3𝑥10−6 )(3𝑥10−6 )
=
(0.04)2
4.00 cm
4.00 cm
50.6 𝑁
3.00 mC
(8.99𝑥109 )(2𝑥10−6 )(3𝑥10−6 )
=
(0.04)2
33.7 𝑁
end
Use pythagorean theorem to combine the forces.
33.7 𝑁
50.6
𝑡𝑎𝑛
−1
33.7125
50.56875
33.7
= (33.7)2 +(50.6)2
60.8 𝑁
60.8 𝑁 𝑎𝑡 33.7° 𝑊 𝑜𝑓 𝑁
end