Trimester 2 General Physics Final

1.  What happens to a charged particle in an electric field?

Positive particles are accelerated with the field.

Negative particles are accelerated against the field.

2.  What happens to a charged particle in a magnetic field?

If the charged particle is moving and is moving at an angle to the field, it will be pushed at a right angle to the direction of the field.  Positve particles follow the right hand rule.  Negative particles move in the opposite direction that a positive particle would.

3.  What would happen to an electron if you dropped it while standing at the equator?

The electron would begin to fall down, but then, it would get pushed to the west due to the earth’s north pointing magnetic field.  (Thumb toward ground, fingers toward north, palm faces east, because the particle is negative it will go west.)

4.  How does the right hand rule for a moving particle in a magnetic field work?

The right hand rule for a moving charged particle in a magnetic field states that if the particle is moving at an angle to the field, it will be deflected at a right angle to the field's direction. Positive particles follow the right hand rule, while negative particles move in the opposite direction.

5.  How does the right hand rule for a long, straight, current carrying wire work?

You put your thumb in the direction of the current, and wrap your fingers around the wire.  The magnetic field will form concentric circles around the wire with a direction the same as the direction of your fingers.

6.  How does the right hand rule for finding the direction of force on a current carrying wire work?

Put your thumb in the direction of the current, fingers in the direction of the field, and palm faces the direction of the force.

7.  What does the magnetic field around a long straight wire look like?

Concentric circles around the wire.

8.  What’s the deal with the geographic and magnetic north poles of the earth?

Geographic north is actually the magnetic south pole of the earth.  Imagine a big bar magnet lined up with the earth axis.  Because field lines point from the north pole to the south pole of a magnet, and we know that our field lines go north, they must be heading to the south pole of the earth’s magnet.

9.  What is the difference between soft and hard magnetic materials?

Soft magnetic materials like iron magnetize easily when placed in an external magnetic field, but lose it when the field is removed.  Hard magnetic materials like Al, Ni, and Co, are difficult to get magnetized, but once they are, they maintain their permanent magnetic field.

10.  What are magnetic domains and what significance do they have in regards to magnets?

Magnetic domains are tiny regions within a metal that have a magnetic field pointing in a direction.  Oftentimes the fields of these domains are all pointing in random directions so the material is not magnetic.  But when they all point in the same general direction (due to an external magnetic field), the whole material becomes a magnet.

12.  What are the rules of magnetic field lines?

Magnetic field lines start on north and end at south, they form closed loops, they cannot cross.

13.  Where is the magnetic field around a magnet the greatest?

Near the poles.

14.  How do the poles of magnets interact with each other?

Opposites attract/Like poles repel

15.  How does the equivalent resistance of resistors wired in parallel compare to the resistance of any one of the resistors?

The equivalent resistance is less than the value of the smallest resistor.

16.  How does the equivalent resistance of resistors wired in series compare to the resistance of any one of the resistors?

The equivalent resistance is greater than any of the resistors in the circuit.

17.  If a circuit only has one element in it (say a light bulb), how does the voltage drop across the device compare to the voltage from the source.

All of the voltage is used in the device.  For example, if the voltage from the battery is 12V, the voltage across the device would also be 12V.

18.  What does the wattage rating in a light bulb indicate?

It indicates the rate at which heat and light are dissipated in the bulb.

19.  What factors determine the resistance of a wire?

Length (longer = more resistance)

Diameter (thinner = more resistance)

Temperature (hotter = more resistance)

Material (materials like copper, gold, silver, have lower resistances)

20.  What is drift speed, and how does it compare to the speed of electrons between collisions?

Drift speed is the average velocity it takes an electron to move along the length of the wire.  It is very slow because of the random motion of the particle as it runs into other particle.  In between collisions, the speed of the electrons is very fast.

21.  Why do lights turn on right away if the drift speed of electrons is so slow?

The turning on of the lights does not depend on an electron getting from one point in the wire to get to another point.  ALL the particles in the wire begin moving essentially at the same time because the electric field is created at almost the speed of light.  So electrons in the light bulb filament begin moving when you flip the switch.  That movement creates the light.

22.  What is current?

Current is the flow of electrons.

23.  What happens to the electric potential energy if a charged particle moves in an electric field?

If the particle does what it is supposed to do (positive with the field, negative against), then it will lose potential energy.  If work is done on the particle to make it do the opposite, then potential energy will increase.

24.  What forces are involved when two charges interact?  What do the forces depend on?  What happens if their masses change?  What happens if the distance between them changes?

Okay, so there are really two forces at work here.  Gravity and electric forces.  If you consider only gravity, the force is dependent on the mass of the particles and inversely proportional to the square of the distance between them.  More mass = more gravity, more distance = less gravity.  Because the distance is squared, distance affects the gravitational force more than mass.  Doubling the mass of one particle doubles the gravity.  Doubling the distance reduces the gravity to ¼ of what it was.

Of course none of this force even matters if the particles are charged, because the electric force is a kajillion times stronger than gravity (gravity needs a bunch of mass to be noticed).  The electric force is also distance dependent in the same way that gravity is, BUT it depends on the charges, not the mass.

F_g = Gm₁m₂/r²                             F_E = kq₁q₂/r²

25.  What is an electric field?

The electric field is a region of space that is influenced by a charge or group of charges.  Charged particles that are in an electric field experience a force.  It is similar to our gravitational force here on earth.  A mass in our field experiences a downward force due to the gravitational field.

26.  What are the rules for electric field lines?

Electric field lines go out from positive.  

Electric field lines go in to negative.

Electric field lines go from positive to negative.

Electric field lines cannot cross.

Electric field lines exist in numbers that are proportional to the charges involved.

27.  What happens to electric field strength as you move away from its source?

It decreases proportionally to the square of the distance.

28.  What does the electric field depend on?

E depends on charge and distance from the charge.

29.   If a conductor is charged, where does the excess charge reside?

On the outside of the conductor.

30.  What is the electric field inside a conductor?

0

31.  What are the similarities between electric and gravitational forces?

Both can attract, both are inverse square laws, both depend on fundamental property of nature (mass, charge)

Differences - electric is stronger, can also repel.

32.  What is charging by polarization?

When an insulator is charged by polarization, it means that the electrons don’t leave the atoms, but the nucleus within an atom is no longer centrally located in the atom leaving one end of the atom with a partial positive charge and the other side with a partial negative charge.

33.  What is charging by conduction?

When a charged object touches a conductor and the charges flow from one object to the other.  (Contact required)

34.  What is charging by induction?

When a charged object is brought near to a conductor, the conductor will have a charge separation.  Grounding the conductor by touching it will either remove or add charges.  Removing the finger and the charged object from the vicinity of the conductor will leave the conductor with excess charges or fewer charges, and it will then be charged. 

35.  What two methods will charge a conductor?

Conduction and Induction.

36.  What to methods will charge an insulator?

Conduction and polarization

37.  How can you charge something by induction?

Bring a charged object close to a conductive material without touching it. This causes the charges within the conductor to separate. If you ground the conductor during this process, it will lose or gain electrons. When you remove the ground and the charged object, the conductor is left with a net charge.

38.  Why do conductors conduct?

They have free electrons that move around.

39.  Why does rubbing a glass rod with silk make it positively charged?

The silk removes electrons from the glass.

40.  Why does rubbing a rubber rod with fur make it negatively charged?

The rubber rod takes electrons away from the fur.

41.  How do charges interact with one another?

Likes repel, opposites attract.

42.  How do changes in mass or distance affect the gravitational force?

Changes in mass or distance significantly affect gravitational force. The gravitational force between two objects depends on their masses and the distance between them. Increasing the mass of either object increases the gravitational force, while increasing the distance decreases the gravitational force, following the inverse square law. 

43.  Where do you consider the mass of an object to be located?

At its center.

44.  What is the difference between mass and weight?

Mass is how much stuff there is in an object.  Mass is the same no matter where you are.  Weight is the force of gravity on an object.  If gravity is weaker, the same mass will weigh less than where gravity is stronger.

45.  What is a centripetal force?

Centripetal force is the force directed toward the center of curvature in an object in circular motion.

46.  Why do you feel like you are being pushed to the right when your car turns left?

Inertia.  Your body wants to keep going in a straight line (Newton’s 1st Law).  The car is actually turning into you.  The car pushes into you so you are pushing into the car (Newton’s 3rd Law).

47.  What force causes your car to turn?

Friction between the tires and the road.  Friction is the centripetal force in this case.

48.  What is centripetal acceleration?  What does the word centripetal mean?

Centripetal acceleration is the “center seeking” (that’s what centripetal means) acceleration.  It’s caused by the centripetal force.

49.  What is tangential speed of an object moving in a circle?

The tangential speed of an object moving in a circle is the velocity the object has if it were going in a straight line at the point in space that it occupies.