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24 Cards in this Set

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  • Back
If you charge a pocket comb by rubbing it with a silk scarf, how can you determine if thecomb is positively or negatively charged?
Place it close to a charged object that you know the charge of. If repels it is the same charge, is attracts it is the opposite charge.
Contrast the net charge on a conductor to the “free charges” in the conductor
Net Charge: the amount of extra electrons over a proton

Free Charges: charges that can move throughout an object

When an electroscope is charged, its two leaves repel each other and remain at an angle. Whatbalances the electrivc force of repulsion so that the leaves don’t separate further?
gravity stops leaves from continuing to separate
Why does a shirt or blouse taken from a clothes dryer sometimes cling to your body?
The friction produces a charge on the clothes. This charge then induces a charge on the body causing our body and the clothes to attract
A positively charged rod is brought close to a neutral piece of paper, which it attracts. Draw adiagram showing the separation of charge and explain why attraction occurs.
(++++++) (- +)



When the rod is brought close to the object the opposite charges in the uncharges object are attracted to the charged object. This causes the uncharged object to become polarized and attracted to the charged object

Why does a plastic ruler that has been rubbed with a cloth have the ability to pick up smallpieces of paper? Whyt is this difficult to do on a humid day?
The plastic ruler is charged and when it gets closer to the paper the paper becomes polarized which allows the paper to be attracted to the ruler

Humidity stops the charge from the ruler from reaches the paper so the paper so not become polarized

The form of Coulomb’s law is very similar to that for Newton’s law of universal gravitation.What the the differences between these two laws? Compare also graviational mass and electriccharge.
Gravity is much weaker than electrostatic forces.

Gravity is always attractive forces


Electrostatic can be both attractive and repulsive forces

We are not normally aware of the gravitational or electric force between two ordinaryobjects. What is the reason in each case? Give an example where we are aware of each one andwhy.
Gravity: this force is extremely weak so we do not normally notice it. We do however account for it in weight.

Electric: we normally do not notice this because most objects are neutral and do not have a charge. It takes both objects to have a charge for something to happen. This force is noticed with an electrically charged comb is able to pick up paper

. Is the electric force a conservative force? Why or why not?
Yes it is a conservative force because the amount of force at the beginning is also the amount of force at the end
Explain why the test charges we use when measuring electric fields must be small
We use small charges because it is easier to measure the effects of the charge. With larger charges measuring the effect can change the effect
When determining an electric field, must we use a positive test charge, or would a negativeone do as well? Explain
No we do not need to use a positive charge, a negative one will work just as good. The magnitude will also be the same either way because it is force/mag. of charge
Why can electric field lines never cross?
The electric field direction is the direction of the tangent to the electrical field lines. If two directions could cross then there would be two field lines which cannot happen
Sketch the electric field lines for a uniform line of charge which is infinitely long.
^^^^^^^^^^^^ horizontal view

============ - view from end shows


vvvvvvvvvvvvvv 5 different arrows pointing out

If the electric flux through a closed surface is zero, is the electric field necessarily zero at allpoints on the surface? Explain
No because the field lines pass through the surface of the area but after then reach the middle they get cancelled
A point charge is surrounded by a spherical Gaussian surface of radius r. If the sphere isreplaced by a cube of side r, will ΦE be larger, smaller, or the same? Explain.
the electric flux will be the same for the cube because the flux has to do with the charge enclosed not the shape of the surface
If two points are at the same potential, does this mean that no work is done in moving a testcharge from one point to the other?
no Net work is being done but there might be work being done just the work is both positive and negative so they cancel each other out
If a negative charge is initially at rest in an electric field, will it move toward a region ofhigher potential or lower potential?
move toward a region of higher potential
Is there a point along the line joining two equal positive charges where the electric field iszero? Where the electric potential is zero? Explain.
no there is not because for two fields to add to make zero they have to be opposite charges. If both were positive then they would add instead
When a battery is connected to a capacitor, why do the two plates acquire charges of thesame magnitude? Will this be true if the two conductors are different sizes or shapes?
batteries are not a source of charge, but a source of electrical energy. This means that when a battery is connected it is not losing any charge. This also does not change with shape or size
We have seen that the capacitance C depends on the size, shape, and position of the twoconductors, as well as on the dielectric constant K. What then did we mean when we said that Cis a constant in Eq. 21-7 (Q = CV)?
means capacitance is independent from from voltage. This means that the capacitance does not change unless the capacitor is changed
When a flashlight is operated, what is being used up: battery current, battery voltage, batteryenergy, battery power, or battery resistance? Explain.
battery energy. batteries change chemical energy into electrical energy. Light bulbs turn electrical energy into light and heat
The equation P = V2/R indicates that the power dissipated in a resistor decreases if theresistance is increased whereas the equation P = I 2R implies the opposite. Is there acontradiction here? Explain.
no because they are used at different times

P=V2/R is used when voltage is constant


P=I2R is used when current is constant

Which draws more current, a 100 W lightbulb or a 75 W bulb? Which has the higherresistance?
the 100 W bulb draws more current than the 75 W bulb

the 75 W bulb has the higher resistance

Is current used up in a resistor? Explain
no, current is flow of electric charge and charge is conserved. The current is the same on both sides of the resistor