5&10 Fields (Electric, Magnetic, Gravitational)

Question 1

Explain how a charged plastic rod can attract a small uncharged piece of paper

Answer 1

when the charged rod is placed near the paper, some electrons are attracted to the surface of the paper near the rod

Question 2

A plastic rod and a copper rod both have their ends rubbed against a cloth. Explain why the end of the plastic rod becomes electrically charged but the end of the copper rod stays electrically neutral.

Answer 2

Friction causes transfer of electrons from cloth to the rods

Plastic = insulator = electron charge stay at the end

Copper = conductor = electrons easily redistribute

Question 3

Describe how electric current flows through a conductor

Answer 3

1. potential difference is applied between two points in the conductor → electric field is created → electric field exerts force on electrons → electrons accelerate and drift along the conductor → electrical PE converted to KE →collide with and transfer energy to the atoms

Question 4

mean drift velocity (v)

Answer 4

average velocity that a particle moves through a conductor due to an electric field

Question 5

electric potential difference (V)

Answer 5

energy transfered per unit charge

Question 6

electric potential energy

Answer 6

work done to bring a small positive test charge from infinity to its current position

Question 7

potential difference across a component =

Answer 7

the difference in the electric potential energy of electrons before they enter the component and after they leave it

Question 8

electronvolt, eV

Answer 8

the work done to move 1 electron through a potential difference of 1V

Question 9

electric current (I)

Answer 9

the rate of flow of charge
past a given cross-section

Question 10

an electric field is created when

Answer 10

a potential difference is applied between two points in a conductor

Question 11

electromotive force (emf),
ξ [volt, V]”

Answer 11

power supplied per unit current (V=P/I)
energy supplied per unit charge (V=E/Q)

Question 12

resistance, R [ohm, Ω]

Answer 12

opposition to current flow in an electrical circuit

Question 13

secondary energy source

Answer 13

generated from primary energy source

Question 14

renewable energy source

Answer 14

can naturally replenish; doesn’t run out

Question 15

pros/cons of solar energy

Answer 15

• pros: doesn’t pollute atmosphere, renewable
• cons: expensive to set up, inefficiency (light energy wasted as heat, gets reflected…)

Question 16

electric field

Answer 16

the reigion where a charge experiences a electric force

Question 17

electric field strength

Answer 17

E = F/q
electric force per unit charge experienced by a positive test charge placed at a point in the field

Question 18

uniform field

Answer 18

same field strength throughout;
represented by parallel and equally spaced field lines

Question 19

Coulomb’s Law

Answer 19

F = k(q1q2)/r^2
The attractive/repulsive electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation

Question 20

An alpha particle is situated 2.0 mm away from a gold nucleus in a vacuum. Calculate the magnitude of the electrostatic force acting on each charges.

Atomic number of gold = 79

Atomic number of helium = 2

Answer 20

F = k(q1q2)/r^2
* r = 2 x 10^-3
* q1= 2e (two protons) = 2x1.6x10^-19
* q2 = 79e (79 protons) = 79x1.6x10^-19
* k = 8.99 x 10^9

Question 21

Ohm’s law

Answer 21

current is proportional to potential difference (V=IR);
if temperature stays constant

Question 22

rules for electric field lines

Answer 22

• cannot cross
• meet surface at 90˚

Question 23

electric field lines reprsesent…

Answer 23

direction + strength of electric field

Question 24

The conventional direction of current is

Answer 24

from + to - (opposite as the flow of electrons)

Question 25

DC vs. AC

Answer 25

• Direct Current (DC) flows in one direction only
• Alternating Current (AC) is used in high voltage devices; flows one direction around the circuit and reverses

Question 26

When electrons flow through a component in the circuit (e.g. a resistor)…

Answer 26

1. electrons transfer electric potential energy to the component
2. component transforms electric potential energy into other forms of energy (eg. thermal, light)

Question 27

Magnitude of the force on a current carrying conductor depends on the angle of the conductor to the external magnetic field.
- The max force occurs when…
- The min force occurs when …

Answer 27

F = BIL sinø
- max F occurs when ø=90, when the conductor (I) is perpendicular to B
- min F occurs when ø=0, when the conductor (I) is parallel to B

Question 28

equation for force on a current-carrying conductor:

Answer 28

F = (N) BIL sinø

Question 29

equation for force on a moving charge in a magnetic field:

Answer 29

F = Bev sinø

Question 30

The force experienced a charge in a magnetic field is always perpendicular to its motion. Hence, the force is a centripedal force and the charge travels in circular motion. Based on this info, how can we find the radius of a moving charge’s path in a magnetic field?

Answer 30

Bev = mv^2/r
→ r = mv/Be

Question 31

Explain whether the speed of an electron is the same when entering and exiting the magnetic field.

Answer 31

F perpendicular to v
→ no work done
→ no change in KE
→ no change in speed

Question 32

Give two reasons why soft iron is used for the core of electromagnet.

Answer 32

1. easily magnetized and demagnetized
2. produces strong magnetic field (high permeability)

Question 33

Explain the motor effect

Answer 33

When a current flows across a magnetic field, the field due to the current and the external field interact to produce a force on the current.

Question 34

Compare the deflections of alpha particles, beta-minus particles and gamma rays as they pass through the same magnetic field.

Answer 34

gamma: no mass no deflection
beta: deflected
alpha: + so deflected in the opposite direction from beta, greater mass so deflected less

Question 35

Explain whether it is possible for an electron beam to pass through 1. a magnetic field and 2. an electric field without having its velocity changed

Answer 35

if the electrons move parallel to the magnetic field, it will not experience any force; but any charge will experience force in electric field

Question 36

test mass/charge means

Answer 36

has no effect on the field it is placed in

Question 37

zero gravitational potential energy

Answer 37

a system of masses that are INFINITELY APART that there are zero gravitational forces between them

Question 38

gravitational potential energy (Ep or GPE)

Answer 38

WORK DONE to bring the masses from infinity to a point

Question 39

gravitational potential (Vp)

Answer 39

WORK DONE PER UNIT MASS;
to bring a small test mass;
from infinity to a point

Question 40

electric potential (Ve)

Answer 40

WORK DONE PER UNIT CHARGE;
to bring a small positive test charge;
from infinity to a point

Question 41

gravitational potential difference (∆Vg)

Answer 41

WORK DONE PER UNIT MASS;
to bring a small test mass;
between two points

Question 42

equipotential

Answer 42

always perpendicular to field lines

Question 43

Explain why the equation ∆Ep = mg∆h cannot be used to determine the increase in GPE when a satellite is put into orbit around the Earth.

Answer 43

not uniform field, g varies between Earth surface and the height of the orbit

Question 44

Explain why a satellite in orbit around the Earth has negative gravitational potential energy

Answer 44

GPE is calculated with reference to infinity, where GPE is considered zero.

Since a huge amount of energy need to be supplied to move the satellite from Earth surface to infinity, it must have negative energy near the Earth surface.

Question 45

If a satellite moves to a lower orbit, does its GPE increase or decrease?

Answer 45

decrease (becomes more negative)

Question 46

escape speed

Answer 46

speed of an object at a planet’s surface;
so that it will escape from the gravitational field and travel to infinity

Question 47

Newton’s Universal Law of Gravitation

Answer 47

F = GMm/r^2

the force of gravity between two objects is inversely proportional the square of the separation of their centres and directly proportional to the product of their masses

Question 48

equipotential lines are…

Answer 48

always perpendicular to field lines!

Question 49

Two positive charges are separated by a small distance. Is the electric potential energy stored in this situation positive or negative?

Answer 49

positive

Question 50

Two positive charges are separated by a small distance. Explain the energy changes that will occur if the charges are able to move freely.

Answer 50

they would be repelled from each other → gain kinetic energy, lose electric potential energy

Question 51

How much energy is transferred when a charge of −26nC moves between charged sphere places where the potentials are +1.0kV and +1.5kV?

Why may the answer be positive or negative?

Answer 51

(-26 x10^-9)(±0.5x10^3)

if it moves from +1.0kV to +1.5kV, ∆Vp is positive; vice versa

Question 52

A graph shows the variation of gravitational potential V with distance r from the surface of planet A to planet B. Explain why the gradient of the graph is zero at a point.

Answer 52

gradient = ∆V/∆r = g

this is where the force of attraction of A on B and B on A are equal and opposite → resultant force = 0 → acceleration (gravitational field strength, g) = 0

Question 53

A graph shows the variation of gravitational potential V with distance r from the surface of planet A to planet B. Explain why the graph between A and B are negative.

Answer 53

gravitational potential V is defined as 0 at infinity;
gravitational field is attractive so energy is required to move AWAY from A.