5.2 Charge and Field Flashcards
Any object with charge has what around it?
An electric field.
What is an electric field?
A region around a charged object that can attract or repel other charges.
What is electric charge measured in?
Coulombs, C.
Oppositely charged particles…
Attract each other.
Like charges…
Repel.
What happens to a changed object if it is placed in an electric field?
It will experience a force.
What is Coulomb’s law?
F = (k Q1 Q2) / r^2
A negative F from Coulombs law means the force is…
Attractive.
Compare the force on Q1 with the force on Q2.
Equal but opposite.
What is electric field strength defined as?
Force per unit charge.
What is electric field strength, E?
The force that a charge of +1C would experience if placed in the electric field. E is a vector pointing in the direction that the charge would move.
What are the units of field strength?
NC^-1
What kind of field does a point charge have?
A radial field.
Give the equation for field strength in terms of force, F on a test charge, q.
E = F / q
In a radial field, what is E inversely proportional to?
r^2
Give the equation for E in a radial field, distance r away from the point charge Q.
E = kQ/r^2
Where would the field lines point in a field caused by a negative Q?
Towards Q.
What is the definition of electric potential energy, EPE?
The worst hat would need to be done to move a small charge, q, from infinity to a distance, r, away from a point charge.
Give the equation for EPE in a radial field.
EPE = kQq/r
What would the sign of the EPE be for a test charge in a repulsive field be?
Positive.
What does the gradient of an EPE vs r graph tell you?
The electric force at that point.
What is electric potential defined as?
Potential energy per unit charge.
What is the symbol for electric potential?
V
Give the equation for V in terms of EPE and q.
V = EPE / q
What is electric potential, V, measured in?
Volts
What is the equation for electric potential in a radial field?
V = kQ/r
What does the gradient of tangent to a V against r graph tell you?
The field strength at that point.
Describe the field strength in a uniform electric field.
The same everywhere in the field.
How can you produce a uniform field?
Connect two parallel plates to the opposite poles of a battery.
Give the equation for field strength in a uniform field.
E = V/d where V is the p.d. between the plates and d is the distance between the plates.
What are the units of E?
Vm^-1
Describe the field lines in a uniform field between two plates.
Parallel to each other, going from the more positive plate to the other plate.
Describe the equipotential surfaces in a uniform field.
Parallel to the plates and perpendicular to the field lines.
What happens to an object moving in a fluid?
It experienced a viscous drag force.
In which direction does viscous drag force act?
The opposite direction to velocity.
Give the equation for Stoke’s law.
F = 6πηrv
What do η, r and v represent in Stoke’s law?
η is the viscosity of the fluid. r and v are the radius and velocity of the object respectively.
What did the atomiser do in Millikan’s Oil-Drop experiment?
Created a fine mist of oil drops, charged by friction.
What was the purpose of the charged plates in Millikan’s Oil-Drop experiment?
They produced a field that exerted a force on the charged particles.
Give two forces acting on an oil drop before the electric field is switched on.
Weight of the drop and viscous force from the air.
Give the equation for weight of the oil drop.
W = 4/3 π r^3 ρ g
Rearrange Stoke’s law and the weight of the oil drop to make r^2 the subject of the equation.
r^2 = 9ηv / 2ρg
Give two forces acting on a stationary oil drop after the electric field had been turned on.
Weight of the drop and force due to the uniform electric field.
Give two equations for electric field strength.
E = F/q = V/d
Combine the two equations for electric field strength.
F = QV/d where Q is the charge on the oil drop.
What was Millikan’s Oil-Drop experiment evidence for?
Charge is quantised since the charge on any oil drop was a multiple of -1.6 x 10^-19
Give three equations used to derive the equation for force on a singly charge particle moving through a magnetic field.
F = BIl, I = q/t, t = l/v
Give the equation for force on a singly charged particle moving in a magnetic field.
F = qvB
In which direction is current when an electron travels in a magnetic field?
The opposite direction to the movement of the electron.
Which direction does the force act on a charged particle moving through a magnetic field?
Perpendicular to motion.
Describe the motion of a charged particle moving through a perpendicular magnetic field.
Circular.
Rearrange two equations and make r the subject to find the radius of the circular motion of a particle travelling in a magnetic field.
r = mv/Bq
