Unit D (1-3)

Question 1

What are Kepler’s 3 laws of motion?

Answer 1

1) each planet orbits the Sun along an elliptical path. The sun is located at a focus of the elliptical orbit.
2) The imaginary line joining the Sun and a planet sweeps equal areas of space in equal time intervals as the planet follows its orbit.
3) The square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet to the Sun. T^2 ∝ r^3

Question 2

What is Newton’s law of Gravitation?

Answer 2

Every object in the Universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The direction of the force is along the line joining the objects.
F = (Gm1m2)/r^2

Question 3

How can Kepler’s 3rd law be derived from Newton’s Law of Gravitation? Remember what shape
are we assuming for the orbit

Answer 3

F = ma
(GMm)/r^2 = ma
a = v^2/r
(GMm)/r^2 = m(v^2/r)
(GM)/r = v^2
v = (2pir)/T
(GM)/r = [(2pir)/T]^2
T^2 = (4pi^2*r^3)/GM

Question 4

What is the definition of gravitational field strength?

Answer 4

Gravitational field strength, g, at a point in a gravitational field is defined as the gravitational force per unit mass acting on a point mass, m.
g = F/m = GM/r^2

Question 5

Where is g = 0 between two masses?

Answer 5

When the forces between the two masses equal each other
Fm = FM
If the magnitudes of g1 and g2 are equal, there is no resultant gravitational field at that point.

Question 6

What do the gravitation field lines look like close to Earth? Far from a mass

Answer 6

Close to the earth, they are perpendicular.
Gravitational fields act at a distance. No contact is needed for a gravitational field to act on a mass.Gravitational fields have infinite range. Every gravitational field extends for an infinite distance. However, the field becomes very weak at large distances away from the mass that is the source of the field.

Question 7

What do we mean by extended mass Vs point mass?

Answer 7

We treat all masses as if they are point masses with all the mass at the center of gravity of that object

Question 8

What is charging by induction? Friction? Contact?

Answer 8

Induction:
There is no contact between the objects.
The charge of one object is redistributed.
If this object is connected to a grounding wire, negative charges are transferred to the Earth.
The two objects become charged with opposite charges.
Friction:
There is contact and relative motion between the objects.
Negative charge transfers from one object to the other.
The two objects become charged with opposite charges.
Contact:
There is contact between the objects.
Negative charge transfers to the more positively charged object.
The two objects become charged with the same amount of charge.

Question 9

What are the key differences between insulators and conductors?

Answer 9

A conductor allows current to flow easily through it. Insulators don’t allow current to flow through it.

Question 10

What is Coulomb’s law?

Answer 10

Coulomb’s law, which gives the magnitude of the electric force between two point charges. A point charge is a charge whose volume is so small that it can be considered as negligible.
F = (k*q1q2)/r^2

Question 11

Define electric field strength

Answer 11

We define the strength of the electric field as the force per unit charge that is experienced by a small positive test charge.
E = F/q = V/d

Question 12

How do we draw electric field lines

Answer 12

The field lines show the direction of the electric force on a positive charge. The field lines are directed radially outwards for a positive point charge and inwards for a negative point charge. The electric field is a radial field. A positive charge placed on a field line will move along the line in the direction of the arrow.

Question 13

Can E be 0 between charges of the same sign? Opposite sign?

Answer 13

For the electric field at point ‘P’ to be zero, the charges must be of the same sign (both positive or both negative) and at different distances from point ‘P’. If the charges are of opposite signs, the electric field at point ‘P’ cannot be zero.

Question 14

What formula’s apply to parallel plate capacitors

Answer 14

E = V/d
E = electric field strength
between the plates newtons per coulomb (N  C-1)
or volts per metre (V m-1)
V= electric potential difference
between the plates volts (V)
d = distance between the plates

Question 15

What is the charge and hence electric field inside a conductor?

Answer 15

Charge lives on the surface of a conductor inside a conductor inside a conductor E= 0 NC-1 as Q effectively = 0 C
the inside of the sphere does not contain excess charges. This tells us that the electric field strength inside the sphere is zero.

Question 16

Understand the Millikan Experiment - formula and reason for doing it

Answer 16

The aim was to measure the charge of an electron. He used a small cloud of oil drops sprayed on a chamber and they were ionized by friction. One drop is selected and allowed to fall. When no electric field si applied, drap and buoyancy forces balance out weight - no net force terminal velocity is found. Then an electric field is applied so that the drop is held stationary and Fe is calculated.

Question 17

Can we have single N and S poles?

Answer 17

NO a magnet will always have a north and south pole no matter how many times it is broken

Question 18

What do magnetic field line drawings look like for a single magnet and between two magnets

Answer 18

The lines go from the north end to the south end of the single magnet.
For two magnets, still go from north to south, but depending on which pole is facing the other will determine if the lines curve away from each other or curve towards each other.

Question 19

What is RHR #1 and when does it apply?

Answer 19

Point thumb in direction of current to get the direction of the field lines and curve fingers around (connects I and B) x = into the page o = out of the page

Question 20

What is RHR#2 and when does it apply?

Answer 20

thumb = force (F)
index finger = current (I)
middle finger = magnetic field (B)
**remember, for electrons the current is in the opposite direction

Question 21

If F = ILB sin0 and F/L = mu I1I2/(2pi r) what is the magnetic field strength B of a wire at distance
r?

Answer 21

B = (uI)/(2pir)

Question 22

What is the force between parallel wires when the current is in the same direction? Opposite
direction?

Answer 22

When current is in same direction, the force is attractive.
When current is in opposite direction, the force is repulsive.

Question 23

How can we turn F = BIL into F = qvB for a single particle?

Answer 23

I = q/t
F = B (q/t) L
L/t = v
F = Bqv = qvB

Question 24

Be able to apply F = ma = qE = qV/d and SUVAT to calculate acceleration/velocity problems

Answer 24

SUVAT equations are in formula booklet, don’t forget to look at the different equations in unit A and B.5 specifically for motion

Question 25

Be able to use E and M to find if a particle can travel in a straight line or not

Answer 25

to travel in a straight line, E has to equal M to balance out the forces so the particle can travel in a straight line

Question 26

Understand how the Bainbridge Mass Spectrometer works

Answer 26

The mass spectrometer is a device used for measuring atomic mass of the isotopes of any element.
1) ionization occurs (all particles have same charge)
2) acceleration applied (KE increase)
3) velocity selection, constant (electric and magnetic fields in between two plates, FE = FB)
4) isotopes are separated in a magnetic field and their radius will show the mass with the equation
r = mv/qB (uniform circular motion)

Question 27

How do you find the equation that shows a charged particle in a magnetic field undergoing circular motion?

Answer 27

F = ma and a = v^2/r
so
ma = m(v^2/r) = qvb
r = mv/qB