Transcript Physics 4.
Physics 4.4
Charge
What is charge?
Where do you see charge around you?
Describe the atom in terms of charge?
Coulombs and elementary charges
1 coulomb (C) is equal to 6.25 E18
elementary charges
1 elementary charge is equal to the charge
of one proton (+), one electron (-), or
1.60 E-19 C
Electrostatic forces between
objects near each other
What does a positively charged object do
the a:
Positive object
repel
Negative object
attract
Neutral object
alignment
Law of Conservation of Charge
Charge cannot be created or destroyed-the total
charge in the system remains constant.
Ex/ think about when you rub a balloon on your
hair. Your hair loses electrons and the balloon
gains negative charge!
When charged objects are in contact, to find
individual charges, add up the total charge and
divide by the number of objects. They all have
the same charge.
Coulomb’s Law
Electrostatic force (Fe) is directly
proportional to the product of the charges
(q1 x q2) and inversely proportional to the
square of the distance between the
charges (r²).
Coulomb’s Law
???
Charge on sphere A is +5.0 E-4 C.
Charge on sphere B is +3.0 E-5 C.
Distance separated is 3.0 meters.
Find: electrical force Fe.
Fe = kq1xq2 / r²
Electric Field
Any place where there is an electrostatic
force (attractive or repulsive) between the
two charged particles, we say the there is
an electric field between them.
E = Fe / q
Electric field strength tells how strong an
electric field is.
Units are N/C
???
At point P in an electric field, the
magnitude of the electrostatic force on a
proton is 4.0 E-10 newton. What is the
magnitude of the electric field intensity at
point P?
E = Fe / q
Electric fields around point charges
or spheres
An electric field (electric field lines or lines
of force) goes away from positive and
goes toward the negative. Electric field
lines never intersect (never cross) each
other.
Draw electric field lines around:
A positive point charge
A negative point charge
???
An object with a net charge of 4.80 E-6
coulomb experiences an electrostatic force
having a magnitude of 6.00 E-2 newton
when placed near a negatively charged
metal sphere. What is the electric field
strength at this location?
E = Fe / q
Electric field lines between two
parallel plates
Electric field lines go away from the
positive plate and go toward the negative
plate.
Electric field strength is the same
everywhere between the plates.
Where would an electron accelerate
towards? A proton? A neutron? Which
would accelerate faster? Why?
Potential Differences
Potential difference (or potential drop) is
the work or energy required to move a
charged particle towards a like charged
particle divided by charge in coulombs (C).
V=W/q
Units = volts (V) or joules per coulomb.
???
The energy required to move one
elementary charge through a potential
difference of 5.0 volts is:
V=W/q
Electric current
Number of charges that reach a point in a
wire in one second.
I = ∆q / t
Units = C / s or amperes (A).
???
If 10. coulombs of charge are transferred
through an electric circuit in 5.0 seconds,
then the current in the circuit is:
I = ∆q / t
Resistance
Resistance hinders the flow of charges,
which means the flow of current (I).
Increased temperature means bigger
resistance.
R = V / I is called Ohm’s Law.
Units are volts per ampere or ohm (Ω).
???
In a simple electric circuit, a 110 volt
electric heater draws 2.0 amperes of
current. The resistance of the heater is:
R=V/I
Resistivity
Characteristic of a material at a specific
temperature.
Resistance is affected by temperature plus
several factors which are displayed in this
equation:
R = ρ (resistivity) x L (length of wire)
A (cross-sectional area)
???
A 12.0 meter length of copper wire has a
resistance of 1.50 ohms. How long must
an aluminum wire with the same crosssectional area be to have the same
resistance?
R = ρL / A
Series Circuits
I = I1 = I2 = I3 =…
V = V1 + V2 + V3 +…
Req = R1 + R2 + R3 +…
Parallel Circuits
I = I1 + I2 + I3 +…
V = V1 = V2 = V3 =…
1
1
1
1
Req = R1 + R2 + R3 +…
Ammeters and Voltmeters
An ammeter is used to measure current
and is connected in series with the circuit
element.
A voltmeter is used to measure the voltage
and is connected in parallel.
Conservation of charge in a circuit
Power
Electric power is the product of voltage
and current.
P = IV = I²R = V²/R
Magnetic Field Strength
Magnetic field strength is strongest where
the field lines are closest.
Electromagnetic Induction
When a conductor of wire cuts across
magnetic field lines, one end of the wire
becomes more negative and one end
more positive, producing a potential
difference.
This induction is greatest when the wire
moves perpendicular to the magnetic field
lines at a greater speed.