Unit 8 Fields
Honors Physics
May 2014
Barton
FIELDS - general
Answer the following question in the
space provided in your packet:
Using your own prior knowledge
and life experience, how do you
know gravitational, electric, and
magnetic fields exist? (give specific
examples if you can think of any)
Real World Reading
 Read the article at this site:
http://oceanservice.noaa.gov/education/kits/tides/tide
s06_variations.html
 Answer the following question in a 5-6 sentence
paragraph: (answer in the space provided in the packet)
 What is the role of gravitation in tidal
patterns?
Gravitational Fields
 Gravity is a non-contact force
 Gravity is always attractive
 Newton’s Law of Universal Gravitation:
𝑮𝒎𝟏 𝒎𝟐
𝑭𝒈 =
𝒅𝟐
 Fg – force of gravity (N)
 G – universal gravitation constant G=6.67*10-11 Nkg2/m2
 m1 & m2 – masses of objects exerting the gravitational force on
one another (kg)
 d – distance between the centers of the two masses (m)
 Sometimes an r is used instead of a d in this equation
 Additional information:
http://www.physicsclassroom.com/class/circles/u6l3c.cfm
Gravitational Field Drawings - Earth
Gravitational field
around the earth – the
length of the arrows
indicate the strength of the
field at that distance from
the earth
Gravitational Field Drawings – Earth &
Moon Interaction – 2 different examples
Example 1:
Moon
Earth
Example 2:
Moon
Earth
Answer to gravitational field question from
packet
3. In which diagram do the field lines best represent the
gravitational field around Earth?
Answers to gravitational potential energy
questions from packet
Answer the following questions about jumping into a pool from a high
diving board, a low diving board, and the side of the pool:
2.Which will cause you to hit the water with the greatest speed?
- the high dive
3. How does the speed relate to the kinetic energy?
- the greater the speed, the greater the kinetic energy  KE =½ mv2
4.What causes you to have that kinetic energy?
- the height from which you jump
5. How is this energy related to the height from which you jumped?
- higher the height, the more total energy you will have
6.Which will give you the greatest initial gravitational potential energy?
- the high dive; gravitational potential energy depends upon height relative to the
earth’s surface  Pegrav= mgh
8. As a ball falls freely toward the
ground, its total mechanical
energy
A. Decreases
B. Increases
C. Remains the same
7. A block slides along the frictionless path shown in the figure below. From greatest to least rank
the amount of gravitational potential energy the block has at the different points labeled on the
track.
A. A, B, C, D
B. C, D, B, A
C. D, C, B, A
D. A, B, D, C
Mechanical Energy – Gravitational Potential
Energy and Kinetic Energy
 Mechanical Energy – energy acquired/lost by object when
work is done




ME = ET = KE + PE = K + U
Kinetic Energy (KE or K) – energy of motion K = ½mv2
Gravitational Potential Energy (PE or U) – stored energy of
position relative to surface U = mgh
Law of Conservation of Energy – energy cannot be created or
destroyed
Energy can be…
 Transferred – given from one object to another
 Transformed – changed from one type of energy into
another
Answers to gravitational potential energy
questions from packet
9. A ball is dropped from the top of a
cliff. Which graph best represents the
relationship between the ball’s total
energy and elapsed time as the ball falls
to the ground? [Neglect friction.]
10. A child, starting from rest at the top of a
playground slide, reaches a speed of 7.0 meters
per second at the bottom of the slide. What is
the vertical height of the slide? (ignore friction)
[hint: remember, the total initial energy = total final
energy]
A. 0.71 m
B. 1.4 m
C. 2.5 m
D. 3.5 m
Magnets – Drawing Magnetic Fields
Magnets – Magnetic Fields
 Magnetic force can be attractive or repulsive
 Field lines are vectors
 Produce CLOSED LOOPS
 Point from NORTH to SOUTH outside the magnet
 Point from SOUTH to NORTH inside the magnet
 **Magnets are POLARIZED, which means they have TWO
DISTINCT, OPPOSITE ENDS**
 Magnetic fields are produced by
MAGNETS & MOVING
CHARGES
 MOVING CHARGES
experience magnetic fields
 like ELECTRIC FIELDS
Magnets – Magnetic Fields
Earth’s Magnetic Field
 The earth’s MAGNETIC SOUTH pole is very near
to the earth’s GEOGRAPHIC NORTH pole
 The earth’s MAGNETIC NORTH pole is very near
to the earth’s GEOGRAPHIC SOUTH pole
Magnets
 Natural Magnets:
 domains inside have ELECTRONS aligned in their
SPINS
 magnets cause other materials to become POLARIZED
 they become temporary magnets; ie: nail or paperclip
 Permanent Magnets:
 domains are “PERMANENTLY” ALIGNED
 Non-magnets:
 domains inside have ELECTRONS with UNALIGNED
SPINS
Not aligned
(non-magnetic)
Aligned
(magnetic)
Answers to magnetic field questions
from packet
The diagram below shows the magnetic field
lines between two magnetic poles, A and B.
Which statement describes the polarity of
magnetic poles A and B?
1.
a.
b.
c.
d.
2. In which diagram do the field lines best
represent the magnetic field around Earth?
A is a north pole and B is a south pole.
A is a south pole and B is a north pole.
Both A and B are north poles.
Both A and B are south poles
4. When two ring magnets are placed on a pencil, magnet A remains suspended above magnet B, as shown below. Which
statement describes the gravitational force and the magnetic force acting on magnet A due to magnet B?
A. The gravitational force is attractive and the magnetic force is repulsive.
B. The gravitational force is repulsive and the magnetic force is attractive.
C. Both the gravitational force and the magnetic force are attractive.
D. Both the gravitational force and the magnetic force are repulsive
Electric Fields
 charges change the space around them, this
change is the electric field produced by
that charge
 charges interact w/each other and exert
forces on each other through their electric
fields
Electric Fields Created by Charges &
Charged Plates
Properties of electric fields
 E-fields are produced by charges and we represent them
with arrows (vectors)
 the lines NEVER cross but they do bend
 the closer the lines, the stronger the E-field
(magnitude)
 E-field lines point in the direction that a positive
charge feels a force (direction)
 E-field is zero inside a conductor
 E-field lines are perpendicular to metallic surfaces
 Supplemental electric field information:
http://physics.bu.edu/~duffy/py106/Electricfield.html
Equations
Electric Force
units: N
𝑭
𝑬=
𝒒
Electrostatic constant
= 8.99*109 Nm2/C2
𝒌𝒒
𝑬= 𝟐
𝒓
Charge in the field
units: C
Electric Field strength
units: N/C or V/m
Electric Force
units: N
Charge creating
the field
units: C
Distance
units: m
Electrostatic constant
= 8.99*109 Nm2/C2
Charges
units: C
𝒌𝒒𝟏 𝒒𝟐
𝑭𝒆 =
𝒓𝟐
Distance
units: m
Answers to electric field questions in packet
15. The center of a -0.0035 C charge is 0.44 m to the left of the center of an 0.008 C charge. Determine the magnitude and
direction of the electrostatic force on the -0.0035 C charge.
F = (kq1q2)/r2 = [(8.99*109)(0.0035)(0.008)]/(0.442) = 1.3*106 N to the right
16. A positive charge is inside an electric field and is experiencing an electric force to the right. Use words and a diagram to show
the direction in which the field points.
Field points to the right (see slide 19)
17. A negative charge is inside a leftward pointing electric field. Use words and a diagram to show the direction in which the charge
feels a force.
Negative charge feels a force to the right (see slide 19)
18. An electric field is measured using a positive charge of 3.0*10-6 C. This charge experiences a force of 0.12 N to the right. What
are the magnitude and direction of the electric field strength at the location of the charge?
E = F/q = (0.12N)/(3*10-6C) = 4*104 N/C to the right
19. A negative charge of 4.5*10-6 C produces an electric field. What is the electric field strength and direction at a distance of 25 cm?
25 cm = 0.25 m
E = (kq)/r2 = [(8.99*109)(4.5*10-6C)]/(.25m)2 = 6.5*105V/m
20. The distance between an electron and a proton is varied. Which pair of graphs best represents the relationship between
gravitational force, Fg, and distance, r, and the relationship between electrostatic force, Fe, and distance, r, for these particles?
Conductors & Insulators
 Conductors – materials that allow electrons to move freely from
atom to atom
 Insulators – impede the flow of electrons from atom to atom
Charging by Conduction
 a neutral object is touched by a charged object
 Neutral object either gains or loses electrons to become
charged (protons are NOT transferred)
Neutral object becomes
negative  gains e-
Neutral object becomes
positive  loses e-
Charging by Induction
 Charged object is brought near a neutral object causing
polarization
 Polarization – separating opposite charges within an object
Answers to charging question from packet
21. Two metal spheres that are initially uncharged are mounted on
insulating stands, as shown below. A negatively charged rubber
rod is brought close to, but does not make contact with, sphere X.
Sphere Y is then brought close to X on the side opposite to the
rubber rod. Y is allowed to touch X and then is removed some
distance away. The rubber rod is then moved far away from X and
Y. What are the final charges on the spheres?
Sphere X Sphere Y
A. Zero
Zero
B. Negative Negative
C. Negative Positive
D. Positive Negative
E. Positive Positive
MAGNETIC FIELD FORCES on moving charges
 magnetic fields apply a force to moving charges & current-carrying wires
 Force on a charged, moving particle in a magnetic field:
F = qvB
 q = particle’s charge (units: coulombs, C)
 v = particle’s velocity (units: m/s)
 B = magnetic field strength(units: T, Tesla)
 **How do we determine the direction of the force?**
 Right Hand Rule: Relationship between force, magnetic field, and velocity of a POSITIVE
PARTICLE
 fingers: DIRECTION OF MAGNETIC FIELD (B)
 thumb: DIRECTION OF VELOCITY (v)
 palm: DIRECTION OF FORCE (F)
***For negative particle, opposite of right hand rule***
*** a particle must be moving PERPENDICULAR to the magnetic field ***
*** a particle moving PARALLEL to mag. field will feel NO FORCE (F =0)***
MAGNETIC FIELD FORCES – moving charges
Determine the direction of the unknown variable for a
proton moving in the field using the given information:
F is out of
the page
F is down
F is left
MAGNETIC FIELD FORCES – current-carrying
wire
 Force on a current-carrying wire in a magnetic field:
F = ILB
 F = FORCE (units: N)
 I = CURRENT (units: A)
 L = WIRE LENGTH (units: m)
 B = MAGNETIC FIELD STRENGTH (units: T, teslas)
 **How do we determine direction?** APPLY RIGHT HAND RULE
 THUMB IS CURRENT (I) (instead of velocity)