5&10 Fields (Electric, Magnetic, Gravitational) Flashcards
Explain how a charged plastic rod can attract a small uncharged piece of paper
when the charged rod is placed near the paper, some electrons are attracted to the surface of the paper near the rod
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.
Friction causes transfer of electrons from cloth to the rods
Plastic = insulator = electron charge stay at the end
Copper = conductor = electrons easily redistribute
Describe how electric current flows through a conductor
- 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
mean drift velocity (v)
average velocity that a particle moves through a conductor due to an electric field
electric potential difference (V)
energy transfered per unit charge
electric potential energy
work done to bring a small positive test charge from infinity to its current position
potential difference across a component =
the difference in the electric potential energy of electrons before they enter the component and after they leave it
electronvolt, eV
the work done to move 1 electron through a potential difference of 1V
electric current (I)
the rate of flow of charge
past a given cross-section
an electric field is created when
a potential difference is applied between two points in a conductor
electromotive force (emf),
ξ [volt, V]”
power supplied per unit current (V=P/I)
energy supplied per unit charge (V=E/Q)
resistance, R [ohm, Ω]
opposition to current flow in an electrical circuit
secondary energy source
generated from primary energy source
renewable energy source
can naturally replenish; doesn’t run out
pros/cons of solar energy
- pros: doesn’t pollute atmosphere, renewable
- cons: expensive to set up, inefficiency (light energy wasted as heat, gets reflected…)
electric field
the reigion where a charge experiences a electric force
electric field strength
E = F/q
electric force per unit charge experienced by a positive test charge placed at a point in the field
uniform field
same field strength throughout;
represented by parallel and equally spaced field lines
Coulomb’s Law
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
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
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
Ohm’s law
current is proportional to potential difference (V=IR);
if temperature stays constant
rules for electric field lines
- cannot cross
- meet surface at 90˚
electric field lines reprsesent…
direction + strength of electric field
The conventional direction of current is
from + to - (opposite as the flow of electrons)
DC vs. AC
- 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
When electrons flow through a component in the circuit (e.g. a resistor)…
- electrons transfer electric potential energy to the component
- component transforms electric potential energy into other forms of energy (eg. thermal, light)
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 …
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
equation for force on a current-carrying conductor:
F = (N) BIL sinø
equation for force on a moving charge in a magnetic field:
F = Bev sinø
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?
Bev = mv^2/r
→ r = mv/Be
Explain whether the speed of an electron is the same when entering and exiting the magnetic field.
F perpendicular to v
→ no work done
→ no change in KE
→ no change in speed
Give two reasons why soft iron is used for the core of electromagnet.
- easily magnetized and demagnetized
- produces strong magnetic field (high permeability)
Explain the motor effect
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.
Compare the deflections of alpha particles, beta-minus particles and gamma rays as they pass through the same magnetic field.
gamma: no mass no deflection
beta: deflected
alpha: + so deflected in the opposite direction from beta, greater mass so deflected less
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
if the electrons move parallel to the magnetic field, it will not experience any force; but any charge will experience force in electric field
test mass/charge means
has no effect on the field it is placed in
zero gravitational potential energy
a system of masses that are INFINITELY APART that there are zero gravitational forces between them
gravitational potential energy (Ep or GPE)
WORK DONE to bring the masses from infinity to a point
gravitational potential (Vp)
WORK DONE PER UNIT MASS;
to bring a small test mass;
from infinity to a point
electric potential (Ve)
WORK DONE PER UNIT CHARGE;
to bring a small positive test charge;
from infinity to a point
gravitational potential difference (∆Vg)
WORK DONE PER UNIT MASS;
to bring a small test mass;
between two points
equipotential
always perpendicular to field lines
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.
not uniform field, g varies between Earth surface and the height of the orbit
Explain why a satellite in orbit around the Earth has negative gravitational potential energy
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.
If a satellite moves to a lower orbit, does its GPE increase or decrease?
decrease (becomes more negative)
escape speed
speed of an object at a planet’s surface;
so that it will escape from the gravitational field and travel to infinity
Newton’s Universal Law of Gravitation
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
equipotential lines are…
always perpendicular to field lines!
Two positive charges are separated by a small distance. Is the electric potential energy stored in this situation positive or negative?
positive
Two positive charges are separated by a small distance. Explain the energy changes that will occur if the charges are able to move freely.
they would be repelled from each other → gain kinetic energy, lose electric potential energy
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?
(-26 x10^-9)(±0.5x10^3)
if it moves from +1.0kV to +1.5kV, ∆Vp is positive; vice versa
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.
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
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.
gravitational potential V is defined as 0 at infinity;
gravitational field is attractive so energy is required to move AWAY from A.
