Download R4 - Great Neck Public Schools

Electricity and Magnetism Key Points
• CHARGE “q” – created by excess or deficiency of electrons
• Grounding – a ground is anything that can accept or give large amounts of charge (such as the earth)
• Opposites attract / Likes repel Generally, neutral objects are attracted to all charged objects.
• Charge must be a multiple of 1e = 1.6e-19 C (Milikan’s Oil Drop finding)
• Charge is preferred to be in coulombs only. Convert everything to C when charge is needed.
• Insulators – charge moves poorly
Conductors – charge moves well
(rules: (1) when conductors are in equilibrium, there is 0 charge on the inside and therefore 0 field inside the
conductor (2) charge accumulates are sharp points)
•Creating charge  contact, induction – always end up with opposite charge of the nearby charged object,
•Polarization (insulators) – charge redistribution by atoms spinning in place to align
• Charge conservation – the total charge in a system is conserved (ex: +2C sphere and -6 C sphere are touched
and separated, each sphere has (+2 + -6)/2 = -2 C each
• Electric force between 2 charges is determined by Fe = k q q / r2. To see how changing factors affect one
another start with everything being “1” then make the requested change to see the new factor
•Electric Field – the force you would get if you put a charge in a certain location (force per charge). E = Fe / q
(With “q” being a charge that is placed in the field E). E = kQ / r2 (With “Q” being a charge that is making a field)
• Field lines
- lines are drawn based on the way they would effect a small + test charge, – charges move opposite field lines.
- lines come out from + (pushed away) and drawn in towards – (pulled towards).
- Field between two parallel plates is constant and uniform  the force acting on a charge between the plates is
constant
•Potential and Potential Difference – (Voltage).
- Potential is energy per charge (V = W / q). Potential is “like” the push that moves charges around.
• W = work or energy. Units of J (joules).
Can be converted to electronvolts = eV. (electronvolt is a unit of ENERGY) 1 eV= 1.6e-19 J
• I = Current = the rate and amount of charge flow = Q / t
units = Amps (coulombs/sec)
•R = Resistance = how hard it is for charge to flow through.
Units = Ohms Ω
Factors that effect resistance
R=ρL/A
ρ = resistivity
L =length of wire
A = cross sectional area of wire
Temperature – an object at a higher temperature has more resistance.
•OHMS law.
V = IR
Ohmic materials obey ohms law. (when you graph the V and I you get a straight line)
•Circuits
- A closed circuit is one where current flows through it. Current will only flow when a switch is closed
- Having just a single wire in a circuit branch with no resistors or other devices on it will divert all current
to that wire and other branches get nothing, this is called a short circuit
• Series = only one path for charge to move through
 with only one path. the current through every item in that circuit must be the same and voltages get shared
• Parallel = more than one path for charge to move through
 with multiple pathways, current splits so each differs. With independent connections, each item gets the
total voltage.
• Too many items can overload the circuit as it puts out more current for every item added (fuses are used to
limit this, and “blow” when a certain current is reached).
•The charge in and out of a junction must be conserved 5A in equal 3A + 2A out
• Meters- Ammeters for current, must be in series (next to things) and the meter itself has a low resistance
- Voltmeters for voltage, must be in parallel (around things) and the meter itself has a high resistance.
• Magnetism
- Created from spinning/moving charges.
- Magnets always have 2 poles
- A compass is a mini-magnet that is free to rotate, It has its own N and S pole.
- Field lines are always drawn in the direction that the north pole of a compass would point.
The compass follows the field
 so lines always come out of the N side of a magnet and loop around into the S side of a magnet
•Electromagnetic Induction
- when a wire is moved across magnetic field lines, there is an induced potential difference
NOTE: YOU MUST CUT ACROSS MAGNETIC FIELD LINES AND BE PULLING IT OUT OF THE LINES TO INDUCE
POTENTIAL AND CURRENTS. To get the most potential, you cut across at 90 degrees.
Physics Regents Review HW #4 – Electricity and Magnetism
Wells - THIS HW WILL COUNT AS A DOUBLE HW GRADE.
MULTIPLE
PROBLEMS
CHOICE
1.)
2.)
3.)
4.)
1.)
(a)
(b)
5.)
6.)
7.)
8.)
9.)
2.)
(a)
(b)
10)
11)
12)
3.) on sheet
13)
14)
15)
16)
17)
4.)
18)
19)
20)
21)
22)
23)
24)
5.) (a) on sheet
(b)
(c)
25)
26)
27)
6.) on sheet
28)
29)
30)
31)
32)
33)
34)
7.) (a)
(b)
Name: ___________________
MULTIPLE CHOICE
1.) If a small sphere possesses an excess of 5 electrons, the net charge on the sphere is
(1) –3.2 x 10–20 C
(2) –8.0 x 10–19 C
(3) –8.0 x 1019 C
(4) –3.2 x 1020 C
2.) An object with a net charge of 4.80 x 10–6 coulomb experiences an electrostatic force having
–2
a magnitude of 6.00 x
newton when placed near a negatively charged metal sphere. What is the electric
field strength at this location?
(1) 1.25 x 104 N/C directed away from the sphere
(2) 1.25 x 104 N/C directed toward the sphere
(3) 2.88 x 10–8 N/C directed away from the sphere
(4) 2.88 x 10–8 N/C directed toward the sphere
3.) The magnitude of the electrostatic force between two point charges is F. If the distance
between the charges is doubled, the electrostatic force between the charges will become
(1) F/4 (3) F/2
(2) 2F (4) 4F
4.) The charge-to-mass ratio of an electron is
11
(1) 5.69 x 10–12 C/kg (3) 1.76 x
C/kg
–11
12
C/kg (4) 5.69 x
C/kg
5.) An object possessing an excess of 6.0 x 106 electrons has a net charge of
–13
(1) 2.7 x 10–26 C (3) 3.8 x
C
–24
–13
C (4) 9.6 x
C
6.) When a neutral metal sphere is charged by contact with a positively charged glass rod, the sphere
(1) loses electrons (3) loses protons
(2) gains electrons (4) gains protons
7.) The diagram below represents a source of potential difference connected to two large, parallel metal plates
–3
separated by a distance of 4.0 x
meter. Which statement best describes the electric field strength between
the plates?
(1) It is zero at point B.
(2) It is a maximum at point B.
(3) It is a maximum at point C.
(4) It is the same at points A, B, and C.
8.) A negatively charged plastic comb is brought close to, but does not touch, a small piece of paper. If the comb
and the paper are attracted to each other, the charge on the paper
(1) may be negative or neutral
(2) may be positive or neutral
(3) must be negative
(4) must be positive
9.) The diagram below shows two small metal spheres, A and B. Each sphere possesses a net charge of
–6
4.0 x
coulomb. The spheres are separated by a distance of 1.0 meter. Which combination of charged
spheres and separation distance produces an electrostatic force of the same magnitude as the electrostatic force
between spheres A and B?
10.) Which graph best represents the relationship between the magnitude of the electric field strength, E, around
a point charge and the distance, r, from the point charge?
11.) A 10-meter length of wire with a cross-sectional area of 3.0 x 10–6 square meter has a resistance of
–2
9.4 x
ohm at 20° Celsius. The wire is most likely made of
(1) silver (3) aluminum
(2) copper (4) tungsten
12.) The diagram below shows three neutral metal spheres, x, y, and z, in contact and on insulating stands.
Which diagram best represents the charge distribution on the spheres when a positively charged rod is brought
near sphere x, but does not touch it?
13.) A microwave oven operating at 120 volts is used to heat a hot dog. If the oven draws 12.5 amperes of
current for 45 seconds, what is the power dissipated by the oven?
(1) 33 W
(2) 1.5 x 103 W
(3) 5.4 x 103 W
(4) 6.8 x 104 W
14.) In a television set, an electron beam with a current of 5.0 x 10–6 ampere is directed at the screen.
Approximately how many electrons are transferred to the screen in 60 seconds?
(1) 1.2 x 107
(2) 5.3 x 1011
(3) 1.9 x 1015
(4) 6.3 x 1018
15.) If 4.8 x 10–17 joule of work is required to move an electron between two points in an electric
field, what is the electric potential difference between these points?
2
(1) 1.6 x 10–19 V (3) 3.0 x
V
–17
2
(2) 4.8 x 10 V (4) 4.8 x 10 V
16.) The potential difference applied to a circuit element remains constant as the resistance of the
element is varied. Which graph best represents the relationship between power (P) and resistance (R) of this
element?
17.) An operating electric heater draws a current of 10 amperes and has a resistance of 12 ohms. How much
energy does the heater use in 60 seconds?
(1) 120 J (3) 7200 J
(2) 1200 J (4) 72,000 J
18.) The table below lists various characteristics of two metallic wires, A and B.
If wire A has resistance R, then wire B has resistance
(1) R (3) R/2
(2) 2R (4) 4R
19.) In a flashlight, a battery provides a total of 3.0 volts to a bulb. If the flashlight bulb has an operating
resistance of 5.0 ohms, the current through the bulb is
(1) 0.30 A (3) 1.5 A
(2) 0.60 A (4) 1.7 A
20.) A complete circuit is left on for several minutes, causing the connecting copper wire to become
hot. As the temperature of the wire increases, the electrical resistance of the wire
(1) decreases
(2) increases
(3) remains the same
21.) Several pieces of copper wire, all having the same length but different diameters, are kept at room
temperature. Which graph best represents the resistance, R, of the wires as a function of their cross-sectional
areas, A?
22.) Which physical quantity is correctly paired with its unit?
(1) power and watt•seconds
(2) energy and newton•seconds
(3) electric current and amperes/coulomb
(4) electric potential difference and joules/coulomb
23.) The graph below represents the relationship
between the potential difference (V) across a
resistor and the current (I) through the resistor.
Through which entire interval does the resistor obey Ohm’s law?
(1) AB (3) CD
(2) BC (4) AD
24.) How much electrical energy is required to move
a 4.00-microcoulomb charge through a potential
difference of 36.0 volts?
(1) 9.00 × 106 J (3) 1.44 × 10–4 J
(2) 144 J (4) 1.11 × 10–7 J
25.) Two identical resistors connected in parallel have an equivalent resistance of 40 ohms. What
is the resistance of each resistor?
(1) 20 Ω
(2) 40 Ω
(3) 80 Ω
(4) 160 Ω
26.) Which circuit diagram below correctly shows the connection of ammeter A and voltmeter V to measure the
current through and potential difference across resistor R?
27.) In which circuit would ammeter A show the greatest current?
28.) In the circuit diagram shown below, ammeter A1 reads 10. amperes.
What is the reading of ammeter A2?
(1) 6.0 A (3) 20. A
(2) 10. A (4) 4.0 A
29.) The diagram below represents currents in part of an electric circuit. What is the reading of ammeter A?
(1) 1 A (3) 3 A
(2) 2 A (4) 4 A
Base your answers to 30 through 32 on the information and diagram below. A 20.-ohm resistor and a
30.-ohm resistor are connected in parallel to a 12-volt battery as shown. An ammeter is connected as shown.
30.) What is the equivalent resistance of the circuit?
31.) What is the current reading of the ammeter?
(1) 1.0 A (3) 0.40 A
(2) 0.60 A (4) 0.20 A
32.) What is the power of the 30.-ohm resistor?
(1) 4.8 W (3) 30. W
(2) 12 W (4) 75 W
33.) The diagram below shows a wire moving to the right at speed v
through a uniform magnetic field that is directed into the page. As the
speed of the wire is increased, the induced potential difference will
(1) decrease
(2) increase
(3) remain the same
34.) The diagram below shows a bar magnet. Which arrow best represents the
direction of the needle of a compass placed at point A?
(1) ←
(3) ↑
(2) →
(4) ↓
PROBLEMS
1.) A 1.00-meter length of nichrome wire with a cross-sectional area of 7.85 x
to a 1.50-volt battery.
–7
meter2 is connected
(a) Calculate the resistance of the wire. [Show all work, including the equation and substitution
with units.] [2]
(b) Determine the current in the wire. [1]
2.) A proton starts from rest and gains 8.35 x 10–14 joule of kinetic energy as it accelerates between points A and
B in an electric field.
(a) What is the final speed of the proton?
(b) Calculate the potential difference between points A and B in the electric field. [Show all work, including the
equation and substitution with units.] [2]
3.) On the diagram of a bar magnet, draw a minimum of four field lines to show the magnitude and direction of
the magnetic field in the region surrounding the bar magnet. [2]
4.) The diagram below represents a wire conductor, RS, positioned perpendicular to a uniform magnetic field
directed into the page. Describe the direction in which the wire could be moved to produce the maximum
potential difference across its ends, R and S. [1]
5.) You are given a 12-volt battery, ammeter A, voltmeter V, resistor R1, and resistor R2. Resistor R2 has a value
of 3.0 ohms.
(a) Using appropriate symbols from the Reference Tables for Physical Setting/Physics, draw and label a complete
circuit showing:
• resistors R1 and R2 connected in parallel with the battery [1]
• the ammeter connected to measure the current through resistor R1, only [1]
• the voltmeter connected to measure the potential drop across resistor R1 [1]
(b) If the total current in the circuit is 6.0 amperes, determine the equivalent resistance of the circuit. [1]
(c) If the total current in the circuit is 6.0 amperes, determine the resistance of resistor R1. [Show all
calculations, including the equation and substitution with units.] [2]
6.) Your school’s physics laboratory has the following equipment available for conducting experiments:
accelerometers
lasers
stopwatches
ammeters
light bulbs
thermometers
bar magnets
meter sticks
voltmeters
batteries
power supplies
wires
electromagnets
spark timers
Explain how you would find the resistance of an unknown resistor in the laboratory. Your explanation must
include:
Measurements required [1]
Complete circuit diagram [2]
Equipment needed [1]
Any equation(s) needed to calculate the resistance [1]
7.)
(a) If switch S1 is open, what is the reading of ammeter A
(b) If switch S1 is closed, what is the equivalent resistance of the circuit