Chapter 16 - charge and field Flashcards
What is an electric field?
A region around a charged object in which other charges can be attracted or repelled
What is the symbol for electric charge?
Q
What are the units of electric charge?
Coulombs, C
What is Coulomb’s law?
F = KQ1Q2/r^2
What is the equation used to find the force if attraction or repulsion between two charged particles?
Coulomb’s law
F = KQ1Q2/r^2
What is K?
1/4πε0
What is ε?
The permittivity of the material between two charges?
What is ε0?
The permittivity of free space (used for air between two charges)
8.85x10^-12
What are the units of the permittivity of free space?
C^2N-1m^-2
What is the electric field strength?
The force a positive test charge of 1C would experience at that point in the field
Are attractive forces positive or negative?
negative
Are repulsive forces positive or negative?
positive
What is the symbol for electric field strength?
E
Is electric field strength a vector or a scalar quantity?
vector
What are the units of electric field strength?
NC^-1
What kind of field does a point charge(or body that behaves as if charge is concentrated at the centre) have?
Radial field
What is the formula for electric field strength?
E = F/Q
In a radial electric field, what is the inverse square law for electric field strength?
E ∝ 1/r^2
What is the formula for electric field strength around a point charge?
E = KQ/r^2
In what directions do field lines/lines of force point for an electric field?
The direction in which a positive test charge would experience a force if it were in the field
How do field lines/lines of force show a stronger electric field?
Lines are closer together where the filed is stronger
How do field lines/lines of force show a uniform electric field?
They are equally spaced and parallel to each other
How can electric potential, V, be found graphically?
The area under an E/r graph
What kind of energy does a charge in an electric field have?
Electrical potential energy
What is the electrical potential energy?
The work which would need to be done to move a positive test charge to infinity (ie.outside the field completely)
At an infinite distance from Q, what electrical potential energy would test charge q have?
0J
What is the formula for electrical potential energy?
E = KQ1Q2/r
What is electrical potential?
Potential energy per unit charge
What are the units of electrical potential?
Volts, V
What is the symbol for electrical potential?
V
What is the formula which relates energy, potential difference and charge?
E = QV
E is energy not electric field strength
What is the formula which relates force, charge and energy?
F = QE (E = electric field strength not energy)
What is the formula for electric potential?
V = KQ/r
How can a uniform field be created?
Connecting two parallel plates to opposite poles of a battery
What is the formula for electric field strength between two parallel plates?
E = V/d
What are the alternative units for electric field strength used for between two parallel plates?
Vm^-1
Where are equipotential surfaces in an electric field diagram?
Perpendicular to the field lines
What is the charge on an electron, e?
-1.6 x10^-19 C
What happens when an object is dropped into a fluid like air?
It experiences a viscous drag force in opposition to the motion due to the viscosity of the fluid
What is Stoke’s law used for?
Finding the viscous force on a spherical object as it falls through a viscous fluid (like air)
What is Stoke’s law?
F = 6πŋrv ŋ = viscosity of fluid v = velocity r = radius of sphere
Describe the basic set up of Milikan’s oil drop experiment?
An atomiser creates a fine mist of oil drops charged by friction
They are sprayed between parallel plates connected to a variable potential difference
The drops can be viewed with a microscope with an eyepiece scale
The field between the plates exerts a force on the charged drops
In Milikan’s oil drop experiment, before the potential difference is applied, what are the forces acting on the charged oil drops?
When will they be equal?
The weight and the viscous force from the air
Equal when the drop reaches terminal velocity and stops accelerating
In Milikan’s oil drop experiment, how is the radius of the drop calculated?
r^2 = 9ŋv/2ρg
ŋ and ρ are measured in separate experiments
What is the symbol for density?
ρ
In Milikan’s oil drop experiment, what happens to the force when the electric field is switched on?
There is an electric force on the drop, acting upwards
In Milikan’s oil drop experiment, what happens to the forces on the drop when the potential difference is adjusted and the drop is stationary?
The viscous force is proportional to the velocity, so will be zero when the drop is stationary
The only forces acting are the weight of the drop and the force due to the electric field
These forces will be equal
What is the equation for the force from an electric field between parallel plates used in Milikan’s oil drop experiment?
F = QV/d (V = pd between plates)
What is the equation used in Milikan’s oil drop experiment which allows charge to be calculated?
QV/d = 4/3 πr^3ρg
4/3 πr^3ρg = the weight of the drop, volume x density = mass, x g
What did Milikan’s oil drop experiment find?
The charge on an oil drop was always a whole number multiple of -1.6x10^-19
This means charge is QUANTISED, the fundamental unit of charge is e, 1.6x10^-19 C
Why does a current carrying wire experience a force in a magnetic field?
The current is a flow of negatively charged electrons which will be deflected by a magnetic field, causing a force on the wire
What is the equation for force exerted on a current carrying wire?
F = BIL
for a magnetic field perpendicular to the current
What is the formula relating current, charge and time?
I = Q/t
What is the equation giving the current in terms of charge flowing through the wire?
I = Qv/l (v= velocity of charged particle, l = length of wire)
What is the equation used to find the force on a single charged particle moving through a magnetic field?
F = BQv (v = velocity of particle)
In what path are charged particles in a magnetic field deflected through?
A circular path
Why do charged particles in a magnetic field always get deflected in a circular path?
Fleming’s left hand rule
The direction of force is always perpendicular to the direction of travel, the condition for circular motion
When a charged particle is deflected in a circular path by a magnetic field, what is acting as the centripetal force?
The electromagnetic force from the magnetic field
What is the formula for finding the radius of the path a charged particle will be deflected through in a magnetic field?
r = mv/BQ
What is the symbol for magnetic field strength?
B
What are the units of magnetic field strength?
Teslas, T
