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.