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Fields and Forces
• 6.1.1 State Newton’s Law of gravitation
• 6.1.2 Define gravitational field strength
• 6.1.3 Determine the gravitational field due
to one or more point masses.
• 6.2.4 State Coulomb’s Law.
• 6.2.5 Define electric field strength
• 6.2.6 Determine the electric field strength
due to one or more point charges.
Forces and Fields
• Newton’s universal law of gravitation
– G=6.667 x 10 -11 N m2 kg-2
– Gravitational field strength, g
• Unit mass, m
• Vector nature, always attractive force
• Coulomb’s law
– k= 8.99 x 109 N m2 C-2
– Electric field, E
• Unit charge, q (positive)
• Vector nature, attractive or repulsive
Forces and Fields
• 6.1.5 Solve problems involving gravitational
forces and fields.
• 6.2.8 Solve problems involving electric charges,
forces and fields.
• A star explodes and loses half its mass. Its
radius becomes half as large. Find the new
gravitational field strength on the surface of the
star in terms of the original one.
Forces and Fields
• Point P is halfway between the centers of
two equal spherical masses (m= 3x1022
kg) that are separated by a distance of
2x109 m. Point Q is located 109m, directly
south of Point P. What is the gravitational
field strength at Points P and Q?
Forces and Fields
• For an object in centripetal motion orbiting
a larger mass, use the universal
gravitation law to derive the expression
T2=(4P2R3)/GM
• Two equal charges q are suspended from
strings as drawn on the board. Show that
tan q = (kq2)/mgr2 .
(Remember sinq/cosq= tanq)
Forces and Fields
• 6.1.4 Derive an expression for
gravitational field strength at the surface of
a planet, assuming that all its mass is
concentrated at its center.
– Neptune
– Jupiter
Forces and Fields
• 6.2.7 Draw the electric field patterns for
different charge configurations.
• 6.2.1 State that there are two types of
electric charge.
• 6.2.2 State and apply the law of
conservation of charge.
• 6.2.3 Describe and explain the difference
in the electrical properties of conductors
and insulators.
Forces and Fields
• Draw the field lines for the following
charges
– One positive charge
– One negative charge
– Two equal positive charges
– One negative charge and a positive charge
with twice the magnitude
Forces and Fields
• Electric charging
– Friction
• Which particles are transferred?
– Induction
• A negatively charged rod is brought near two touching neutral
conductors. What is the charge on each conductor when
they’re separated?
– Conduction
• Two separated identical conducting spheres with charges of
4mC and -12mC are allowed to touch and then separated.
What’s the charge on each sphere?
Forces and Fields
• 6.3.1 State that moving charges give rise to magnetic
fields.
• 6.3.2 Draw magnetic field patterns due to currents.
• 6.3.3Determine the direction of the force on a currentcarrying conductor in a magnetic field.
• 6.3.4 Determine the direction of the force on a charge
moving in a magnetic field.
• 6.3.5 Define the magnitude and direction of a magnetic
field.
• 6.3.6 Solve problems involving magnetic forces, fields,
and currents.
Forces and Fields
• Permanent and Temporary magnets
• Right hand rule for magnetic fields
– Straight wire
– Flat circular coil
– Solenoid
• Right hand rule for Force on charges in
magnetic field
Forces and Fields
• Calculating Force
– F = BILsinq
– F = qvBsinq
• Derive expression for the centripetal motion of
charged particles.
• Magnetic Field, B
– Direction determined by a “North test pole”
– Vector
– Unit of measurement: Tesla
Forces and Fields
• A current I=15A is directed along the positive x axis and
perpendicularly to a magnetic field. The conductor
experiences a magnetic force per unit length of 0.12 N/m
in the negative y direction. Calculate the magnitude and
direction of the magnetic field in the region through
which the current passes.
• A wire carries a steady current of 2.40A. A straight
section of the wire is 0.750m long and lies along the x
axis within a uniform magnetic field of magnitude 1.60 T
in the positive z direction. If the current is in the +x
direction, what is the magnitude of the force on the
section of wire?
Forces and Fields
• A proton is moving in a circular orbit of radius 14
cm in a uniform magnetic field of magnitude 0.35
T, directed perpendicularly to the velocity of the
proton. Find the orbital speed of the proton.
• A particle of +2.0mC charge and a kinetic energy
of 0.090 J is fired into a uniform magnetic field of
magnitude 0.10 T. IF the particle moves in a
circular path of radius 3.0m, determine the
mass.