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PHYS 102: Study Guide
Combo of 2 electrons and 3 protons have net charge:
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+1.6 x 10^-19 Coul
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An electron is just above a fixed position. Direction of force on the proton is:
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UP
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An electron is just above a fixed position. Direction of force on the proton is up. Which one has greater magnitude force?
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both feel same
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Rubber rod rubbed w fur. Fur brought near bulb of electroscope. Sign on charge of leaves on electroscope is:
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positive b/c pur positively charged so leaves charged positively by polarization
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Stream of water deflected toward electrically charged object. Sign on charge on object is:
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Can't be determined. water deflected towards object regardless of + or -. water still neutral but polarized
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Balloon charged, clings to wall. Sign on balloon
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Can't be determined, wall neutral but polorized
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How does magnitude of the electric force between two point charges change as distance is increases? Force:
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decreases
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Compared with electric force, gravitation force between two protons is:
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very much smaller
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If the distance between two charged particles is tripled, what happens to magnitude of the electric force each exerts on the other:
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reduced to 1/9th bc 1/3 tripled
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How is the magnitude of the electric field due to a point charge reduced when the distance is tripled:
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reduced to 1/9th
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SI units for electric field is:
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N/C
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In space, electric force acts vertically upward of an electron. Direction of electric field at that point is:
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Down, b/c electron has negative charge
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If charge of electron were pos and proton neg, what is overall effect on universe?
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No effect, simple sign conversion
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An electrically neutral object can be given a net charge. Does this violate law of conservation?
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No, charges are simply moved from one object to another
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If neutral piece of metal becomes negative, does its mass increase or decrease?
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mass decreases b/c it loses electrons
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What if it becomes pos charges?
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increases b/c it gains electrons
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Why is it important for fuel trucks to have metal chains reaching from frame to ground?
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metal=conductor, allows any excess electrical charge that may have been induced on the truck (by friction of rubber on the road, for example) to leak off to ground instead of building up on the truck
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Is there an overall gain or loss of electrons by an object when it is electrically polarized?
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No, charges simply change location
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Explain steps to create an electroscope that is positively charged by induction
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Starting with an uncharged electroscope, touch the bulb with your finger. Then bring a negatively charged rod near the bulb without touching it. This repels electrons from the rod. Removing your finger while keeping the negatively charged rod close would leave the electroscope with a net positive charge. If you bring a charged object near the top and the leaves collapse, the object must be negatively charged in order to push electrons to the leaf area and at least partially cancel the positive charge there.
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Suppose neg charged balloon replaced with pos charged glass rod. Which way would water bend now?
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same way, the positive charges in the rod would attract the negative ends of the water molecules, causing the stream to bend toward the rod
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2 points charges initially exert an electric force of magnitude F on one another. Suppose the charge of one was doubled and the other tripled. What would new force be?
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(2q1)(3q2) = 6q1q2, which makes the force 6 times as strong, of 6F
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Safe to stay inside cat during electric storm?
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YES metal frame electric frame inside to 0
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electrotatic conditions, is it found the excess on a conductor in uniformly spread over its surface? what shape?
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surface must be spherical
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Tall buildings have lightning rods to protect. Why rods in shape and taller?
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charges accumulate at sharp points, electric field strongest there, it will hit rod first
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Explain why electrode A is labeled with plus when it has excess of electrons, which carry negative charge?
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It is less negative than electrode B
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When a battery with significant internal resistance us part of a complete circuit, the voltage across its terminals and its
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terminal volage
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As battery ages, assuming it is always connected into same complete circuit, its:
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terminal voltage decreases
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when 4 1.5V batteries connected, output age of combo is measured 1.5V, batteries connected in:
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parallel
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When someone whose car has dead battery, how should ur car battery be connected in relation to dead battery:
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in parallel
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eight
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eight
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To move 3.0C of charge from one electrode to another, a 12V battery must do how much work?
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W = qΔV = (3.0 C)(12 V) = 36 J
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In circuit diagram, battery represented by:
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two unequal length lines
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More charge flows past given point on wire when wire has a current of
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all same currents because q = It.
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Least current
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1.5 C passing a given point in 1.5 min because I = q/t.
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Convention current in what direction in dentist X-ray
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West bc Electrons have negative charges but conventional current is defined for positive charges.
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In a current carrying metal wire, the drift velocity of the electrons is in the order of:
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a few millimeters per seconds.. The electrical signal (the energy) through the wires is transmitted near the speed of light, but the actual motion of the charges is quite slow, of the order of a few mm/s.
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When light switch goes on, electric energy gets to bulb at speed on order of:
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speed of light
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Ohm is another term for the
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volt per amp
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If you double the voltage across a resistor while the same time cutting resistance to 1/3 OG value, what happens?
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increases by 6 times
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Both length and diameter of cylinder resistor are doubled? what happened to resistance?
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diameter, increase by a factor of 4.. length, doubles the resistance
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Electric power unit watt is equivalent to:
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A2xohm, J/s, V2/ohm
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If voltage across resistor doubled, the expended in resistor is:
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increased by a factor of 4 Since P = V2/R, doubling the voltage will increase the power by a factor of 4.
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Current halved, power expend by resistor is:
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decreases by factor of 4 Since P = I2R, halving the current will reduce the power by a factor of ( )2 = .
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The drift of speed of electrons in a complete circuit is a few mm per sec. Yet lamp is 3.0m away from light switch and turns on when u flip switch. Why?
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electric field in the wire propagate at close to the speed of light
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Filaments in bulbs fail after bulb turned on rather than when they've been on for awhile. Why?
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When the light is first turned on, the filament is cool so its resistance is low and the current is high. Once the lamp is hot, its resistance increases and current decreases, so there is less chance of burning out since the rate of heating is now lower.
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Why voltage drops in parallel
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When internal resistance increases the terminal voltage drops.
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What would happen to power outlet when you plugged a 240V europe outlet designed for 120V from united states?
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power output would quadruple and it would overheat and burn out.
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Which one gives off more power when heating across 12 V battery, 5 ohm or 10 ohm resistor?
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the smaller (5.0 Ω) resistor dissipates more power than the larger one.
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Which of allowing must be same for resistors in series?
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current
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Which must be same in parallel?
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voltage
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2 resistors connected to serries 12V battery, RA has 9V across it, which one has least resistance?
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B
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Correctly measure voltage, volt meter connected in:
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in parallel with it b/c volts must be same
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Are the voltage drops across resistors in series generally the same? If not, under what circumstances could they be the same?
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No but if all resistors equal, the voltages across are the same
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Are the joule heating rates for resistors in series the same?
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No, they must have the same resistance if they want to be
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Are current in resistors same in parallel?
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No, if resistors are all equal, currents are the same
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Joule heating in resistors same in parallel?
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No, hey must have the same resistance if they want to be
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If a large resistor and a small resistor are connected in series, will value of effective resistance be closer to that of the large resistance or that of the small? What about parallel?
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series, effective resistance closed to value of larger... parallel, closer to smaller
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60W bulb and 40W bulb in 120V, which brighter?
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40W bulb glows brightest, 60W has smaller resistance
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What would happen is ammeter connect in parallel with current carrying circuit element?
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ampmeter has low resistance so it would burn out
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What would happen is voltmeter connect in parallel with current carrying circuit element?
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voltmeter has high resistance so it circuit drop would be close to 0
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Explain why it is safe for bird to perch with both feet on same high volt wire, even if insulation was worn through?
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A conductor has very low resistance. The resistance of the wire between the feet is very small; so the voltage (or potential difference) between the feet is also very small. This means that the current through the bird is also small.
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Current generates:
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magnetic field
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Moving magnetic field generates:
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current
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