17: The Electric fields Flashcards
An object with [ ] has an electric field around it. What does that mean?
Charge
A region where it can attract or repel other charges
Can charge be positive, negative, or both?
Both
Oppositely charge particles….
Attract each other
Like charges…
Repel
What will happen if a charged object is placed in an electric field?
It will experience a force
When can you model something as a point charge? What does that mean?
If the charge object is a sphere with evenly distributed charge (spherically symmetrical), it will act as if all of its charge is at its centre so you can model it as a point charge
What can electric fields be represented by?
Field lines
What do you use Coulomb’s law for?
To find the force of attraction or repulsion between two point charges
Is the force negative or positive if the charges are opposite?
If they are like charges?
Opposite = negative Like = positive force
Q and q are point charges. Describe the force on q given Q’s force.
The force on q is always equal and opposite to the force on Q
The further apart the charges are….
Which law is this?
The weaker the force between them
Inverse square law
What is k in Coulomb’s law equation?
Electric force constant, given to us. About 9 * 10⁹ Nm²C⁻²
What is electric field strength?
Defined as the force per unit positive charge - the force that a charge of +1C would experience if it was placed in an electric field
What is the unit for electric field strength?
N\C
What kind of field does a point charge have?
A radial field
In a radial field, what does the electric field strength depend on?
The distance, r from the point charge Q. Or from the centre of the source of the radial field
Describe the relationship between field strength and distance. What does this look like in a diagram?
Field strength decrease as you go further away from Q - on a diagram, the field lines get further apart
Inverse square law. Field strength is proportional to 1/r²
If Q is the positive point charge, and q the positive test charge. What is the direction of the field lines?
q would be repelled, so the field lines point away from Q
If Q is a negative point charge and q is a small positive charge. What is the direction of the field lines?
q would be attracted to Q, so the field lines point towards Q
What does a charge in an electric field have?
An electric potential energy
What is electric potential energy?
The work that would need to be done to move a small charge q from infinity to a distance r away from a point charge Q
What does the graph of electric potential energy against radius look like for a repulsive field?
In the 1st quadrant. A 1/r graph. So steep nearby axis then gradient getting smaller before flattening out along x axis
What does the graph of electric potential energy against radius look like for an attractive field?
In the fourth quadrant. - 1/r curve . So steep going to negative infinity at the y axis, and almost flat at the x axis going towards 0 (y) at positive infinity (x)
What is the gradient of an electric potential energy graph against r?
The electric force at that point
An infinite distance from Q, what potential energy does a charged particle q have?
Zero potential energy
Describe the relationship between potential energy and distance in a repulsive field? What sign are the charges of q and Q?
Q and q are both positive (or both -ve). You have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases
Describe the relationship between potential energy and distance in a attractive field? What sign are the charges of q and Q?
Q is negative and q is positive. The charge q gains potential energy as r increases (epe becomes less negative)
Explain the effect of moving a unit charge between two distances R (bigger) and r. Use force, graphs, and energy in your answer
You change the charge’s electric potential energy when you move it between two distances. In order to do this, you have to apply a force and do work.
The force applied is equal to the electric force. For a point charge q, you can plot the electric force against distance from a charge producing the electric field, Q
This is an inverse square law and the area under the curve between R and r gives the change in electric potential energy
What is electric potential? Velectric
Is the electrical potential energy per unit positive charge. The electrical potential at a point in an electric field is the work done to bring a unit positive charge from infinity to that point.
What is the electric potential at infinity, from the charge?
Zero
What sign is electric potential for repulsive and attractive forces?
Repulsive force = positive potential
Attractive force = negative potential
Describe the graph of potential against distance for a repulsive field
What is the gradient?
V is initially positive and tends to zero as r increase towards infinity. 1/r graph
The gradient of a tangent is the electric field strength
In the first quadrant
Describe the graph of potential against distance for an attractive field
In the fourth quadrant
V is initially negative and tends to zero as r increase towards infinity
1/r graph
What is the relationship between electric field strength and electric potential?
The electric field strength is equal to the negative of the rate of change of electric potential with distance
What is the area underneath an electric field strength against distance graph equal to?
Electric potential
What is the gradient of an electric potential graph against distance equal to?
Electric field strength
Why is there a negative sign in the relationship between electric field strength and the rate of change of electrical potential with distance?
To increase the charge’s potential, and potential energy, you have to do work against the force - they ‘act’ in the opposite direction
Describe field strength in a uniform field
Field strength is the same everywhere
How can you produce a uniform electric uniform field? Describe the equipotentials
By connecting two parallel plates to the opposite poles of a battery
The equipotential surfaces are parallel to the plates, and perpendicular to the field lines. They’re also evenly spaced and symmetric
What is the equation for electric field strength in a uniform field?
E = V/d
Where V is the potential difference between the plates and d is the distance between them
This formula comes from the fact that the rate of change of potential is constant for a uniform field
What is the equivalent of gravitational field strength in electric fields? What are the meanings of both?
Gravitational field strength = force per unit mass
Electrical field strength = force per unit positive charge
What is the equivalent of Newton’s law of gravitation for the force between two point masses in electrical fields? What is this law?
Coulomb’s law for the electric force between two point charges
Inverse square law
Are field lines drawn in the same way for gravitational fields and for electrical fields? Describe what they look like
Yes for a point mass (or a spherically symmetric mass) and for a negative point charge (or a spherically symmetric charge)
The negative point charge/heavier mass is at the centre with radial field lines pointing towards the centre, and the positive point charge, or lighter mass somewhere in the field
What are the 3 main difference between gravitational fields and electric fields?
1) gravitational forces are always attractive. Electric forces can be either attractive or repulsive
2) objects can be shielded from electric fields, but not from gravitational fields
3) the size of an electric force depends on the medium between the charges. For gravitational forces, this makes no difference.
In terms of forces, what happens when you drop an object into a fluid (like air)?
It experiences a viscous drag force. This force acts in the opposite direction to the velocity of the object, and is due to the viscosity of the fluid
What is Stoke’s law?
F =6πηrv
What is the setup of Millikan’s experiment?
A top plate and a bottom plate with a microscope looking between, at one end. At the other end it is connected to a circuit, with a voltmeter in parallel, and a variable p.d battery also in parallel
The top plate has a hole in with an atomiser in it.
Describe Millikan’s experiment
1) the atomiser creates a fine mist of oil drops that are charged by friction as they leave the atomiser (positively if they lose electrons, negatively if they gain electrons)
2) some of the drops fall through a hole in the top plate and can be viewed through the microscope (the eyepiece carries a scale to measure distances and velocities accurately)
3) Millikan could apply a potential difference between the two players, producing a field that exerted an upwards force on the charged drops. By adjusting the p.d he could vary the strength of the field
What are the forces on the oil drop when the electric field is turned off?
1) the weight of the drop, acting downwards
2) the viscous force from the wire, acting upwards
When will the oil drop reach terminal velocity?
Stop accelerating
Weight = viscous force mg = 6πηrv
What happens to the forces when Millikan turned on the electric field?
The field introduce an electric force on each drop
Once the electric field was turned on, what did Millikan do with the p.d?
He adjusted the applied p.d until each drop was stationary. Since the viscous force is proportional to the velocity of the object, once each drop stopped moving, the viscous force disappeared
When Millikan made the drops stationary, after turning on the electric field, what happened to the force?
The remaining forces were the weight (downwards) and force due to the uniform electric field (upwards)
Before the electric field was switched on, what could Millikan find in the first experiment?
He calculated the value of r, the radius of the oil drop
What could Millikan find in the second half of his experiment?
The value for q, the charge on the drop
What was the result of the second half of Millikan’s experiment?
The charge on any drop was always a whole number multiple of 1.60 *10⁻¹⁹ C
Millikan concluded that charge can never exist in smaller quantities than 1.60 *10⁻¹⁹ C. He assumed that this was the size of change carried by an electron
What was found in Millikan’s experiment that was later confirmed in other experiments?
Charge is “quantised”
It exist in discrete “packets” of size 1.60 *10⁻¹⁹ C - the fundamental unit of charge, e
This is the size of the charge carried by one electron
What is electric current in a wire caused by?
The flow of negatively charged electrons
Why does a current carrying wire experience a force in a magnetic field?
The negatively charge electrons are affected by magnetic fields
Which equation would you use to find the force acting on a single charged particle moving through, and perpendicular to, a magnetic field?
F=qvB
Charged particles in a magnetic field are deflected in a [ ] path
Circular
Describe the motion of charged particles in a magnetic field
By Fleming’s left hand rule the force on a moving charge in a magnetic field is always perpendicular to its direction of travel
Mathematically, that is the condition for circular motion
Describe when in real life the effect of charged particles in magnetic field travelling in circular motion is found. Briefly describe it
Found in particle accelerators such as cyclotrons and synchrotrons, which use electric and magnetic fields to accelerate particles to very high energies along circular paths
What can the radius of curvature of the path of a charged particle moving through a magnetic field give you information about? What can this be used for?
About the particle’s mass and charge. This means you can identify different particle ps by studying how they’re deflected
Which force are equivalent for a charged particle travelling along a circular path? What is the resulting equation?
The centripetal force and the electromagnetic force are equivalent
r=mv/qB
Different charged particles will have paths with different [ ] - the higher the ratio of m go q, the [ ] the radius of the path
Radii
Larger
