Unit 4: Electric fields Flashcards
What is said about like charges and unlike charges?
Like charges repel and unlike charges attract
What is the makeup of an uncharged atom?
An uncharged atom contains equal numbers of protons and electrons
How can most plastic materials be charged easily?
Most plastic materials can be charged easily by rubbing a dry cloth on them as electrons are transferred
What is the makeup of electrical conductors such as metals?
They contain lots of free electrons - they move about inside the metal and are not attached to any one atom
How is any charge that is given to an object neutralised?
By electrons transferring between the conductor and the Earth, so in order to charge a metal it must first be isolated from the Earth
What is the makeup of insulators?
Insulating materials do not contain free electrons, some insulators are easy to charge because their surface atoms easily gain or lose electrons
What is an electric current?
A flow of charge
What is the relationship deduced from the shuttling ball experiment?
I = Qf = charge Q/time for 1 cycle
What is the formula for the number of electrons?
Q/e
How is an isolated conductor neutralised or discharged?
If the isolated conductor is charged positively then ‘earthed’ electrons transfer from the Earth to the conductor to neutralise it or discharge it
How will an earthed conductor become charged?
If a charged object is placed near it
How do any two charged objects affect each other?
Any two charged objects exert equal and opposite forces on each other without being directly in contact
What surrounds each charge?
An electric field
What do antistatic materials allow?
Charge to flow across the surface
What is field line?
The path of a free positive test charge
How can you tell what field lines look like?
Patterns produced by semolina grains sprinkled in oil, an electric field is set up across the surface of the oil by connecting two metal conductors in the oil to the output terminals of a high voltage supply unit; the grains line up along the field lines
Describe the field lines of oppositely charged points
The field lines become more concentrated at the points, a positive test charge released from an off-centre position would follow a curved path to the negative point charge
Describe the field lines of a point near a plate
The field lines are concentrated at the point object but are at right angles to the plate where they meet, the field is strongest where the lines are most concentrated
Describe the field lines of oppositely charged particles
The field lines are parallel and meet the plates at right angles, the field is uniform between the plates as the field lines are parallel to each other
What is a gold leaf electroscope used for?
To detect charge
What happens if a charged object is in contact with the metal cap of the electroscope?
Some of the charge on the object transfers to the electroscope; as a result, the gold leaf and the metal stem which is attached to the cap gain the same type of charge and the leaf rises because it is repelled by the stem
What happens if another object with the same type of charge is brought near the electroscope?
The leaf rises further because the object forces some charge on the cap to transfer to the leaf and stem
What happens to a charged object in an electric field?
It experiences a force due to the field
When may an object be used as a ‘test’ charge to measure the strength of an electric field at any position in the field?
Provided the object’s size and charge are both sufficiently small
What is the electric field strength, E, at a point defined as?
The force per unit charge on a positive test charge placed at that point
What are the units of E?
Newton per coulomb or volts per metre
What is the formula for E in terms of F?
E = F/Q
On what condition is air an insulator?
Provided it is not subjected to an electric field that is too strong - such a field ionises the air molecules by putting electrons out of the molecules
When does a lightning strike to the ground occur?
When a cloud becomes more and more charged and the electric field in the air becomes stronger and stronger, the insulating property of air suddenly breaks down and a massive discharge of electric charge occurs between the cloud and the ground
What is a lightning conductor and how does it make a thunderstorm lighting strike less likely?
It is a metal rod at the top of a tall building, the rod is connected to the ground by means of a thick metal conductor, when a charged cloud is overhead, it creates a very strong electric field near the tip of the conductor, air molecules near the tip are ionised and they discharge the thundercloud
What type of quantity is the electric field strength?
A vector in the same direction as the force on a positive test charge
In which direction is the electric field strength?
In the direction of the field line
What is the relationship between the direction of the force and the direction of the electric field?
The force on a small charge in an electric field is in the same direction as the electric field if the charge is positive and in the opposite direction to the electric field if the charge is negative
Why does the charge on a ‘test’ charge need to be very small?
A large amount of charge would affect the charges that cause the field thus altering the electric field strength
What is the field pattern for two oppositely charged flat plates?
The field lines are parallel to each other, at right angles to the plates and from the positive plate to the negative plate, the field is uniform
Describe a uniform field
The field strength has the same magnitude and direction everywhere
What is the formula for E in terms of V?
E = V/d
What is the formula for F in terms of V?
F = QV/d
How does a greater charge affect the electric field?
The greater the charge on a body, the stronger the electric field is
How is the charge arranged on a charged metal conductor?
The charge on it is spread across its surface, the more concentrated the charge is on the surface, the greater the strength of the electric field is above the surface
What is the electric field strength between plates proportional to?
The charge per unit area on the facing surfaces
In which direction should the field lines point from?
From positive to negative
What is the formula for Eε0?
Eε0 = Q/A
What is the farad?
The unit of capacitance, it is equivalent to 1 coulomb per volt
What is ε0?
It is referred to as the permittivity of free space and it represents the charge per unit area on a surface in a vacuum that produces an electric field of strength 1 volt per metre between the plates
What happens when a charged object X moves towards another object Y that has the same type of charge?
In general work must be done to move charge X towards charge Y, their electric potential energy increases as X moves towards Y
How does the electric potential energy of X change when it is moved from infinity towards Y?
The potential energy of X increases from zero if its moved from infinity towards Y
What must be overcome for X to move closer to Y?
The electric field of Y causes a force of repulsion to act on X and this force must be overcome to move X closer to Y
What is the electric potential at a certain position in any electric field defined as?
The work done per unit positive charge on a ‘positive test charge’ (i.e. a small positively charged object) when it is moved from infinity to that position OR the potential energy per unit positive charge of a ‘positive test charge’ at that position
What is the position of zero potential energy?
Infinity
What is the unit of electric potential?
The volt which is equal to 1J(C^-1)
What is the formula for electric potential, V, in terms of electric potential energy, Ep?
V = Ep/Q
What is the formula for the change in work done in terms of V?
ΔW = Q(V2 - V1)
What are equipotentials?
Lines of constant potential
What can be said about a test charge moving along an equipotential?
It has constant potential energy and no work is done by the electric field on the test charge because the force due to the field is at right angles to the equipotential
What is the relationship between the lines of force and the equipotentials?
The lines of force of the electric field cross the equipotential lines at right angles
What is the potential gradient at any position in an electric field?
The change of potential per unit change of distance in a given direction
What do closer equipotentials mean?
The closer the equipotentials are, the greater the potential gradient is at right angles to the equipotentials
What type of quantity is electric potential?
A scalar
Describe the equipotentials between plates
They are equally spaced lines parallel to the plates - the potential gradient is constant, the potential increases in the opposite direction to the electric field and is equal to V/d
What is the electric field strength equal to?
The negative of the potential gradient
What can a Van de Graaff generator do?
Can produce sparks in air
How does a Van de Graaff generator work?
Charge created when the rubber belt rubs against a pad is carried by the belt up to the metal dome of the generator, as charge gathers on the dome, the potential difference between the dome and the Earth increases until sparking occurs
What does the spark from the dome do?
A spark suddenly transfers energy from the dome, work must be done to charge the dome because a force is needed to move the charge on the belt up to the dome, the electric potential energy of the dome increases as it charges up, some or all of this energy is transferred from the dome when the spark is created
What is the force between two charges proportional to?
1/r^2 and Q
What does the force between two charged objects depend on?
How close they are to each other
What is the formula for F?
Q1Q2/4πε0r^2
What type of law is Coulomb’s law of force between point charges?
An inverse square law
When does Coulomb’s law apply?
Strictly to a vacuum
How does doubling the separation of two point charges affect the force?
It reduces the force to a quarter
What is the formula for E in terms of r^2?
E = Q/4πε0r^2
What is a point charge?
A convenient expression for a charged object in a situation where distances under consideration are much greater than the size of the object
What is a ‘test’ charge in an electric field?
It is a point charge that does not alter the electric field in which it is placed
What does a negative value of E correspond to?
It means that the field lines will point inwards to the main charge (i.e. not the test charge)
What does the resultant force per unit charge on the test charge give?
The resultant electric field strength at the position of the test charge
What is the resultant electric field strength when one charge is negative and the other is positive?
You add the electric field strengths
What is the resultant electric field strength when both charges are positive?
You subtract the electric field strengths
What is the resultant electric field strength when the forces are at right angles to each other?
E^2 = E1^2 + E2^2
What is the resultant electric field strength?
The vector sum of the individual electric field strengths
What can be said about the electric field lines of a force surrounding a point charge?
They are radial, the equipotentials are therefore concentric circles centred on Q
What is the electric field strength inversely proportional to?
The square of the distance r
What is the formula for the electric potential, V?
V = Q/4πε0r
What is the electric potential inversely proportional to?
The distance r therefore it is not an inverse square law
What does a negative E and negative V mean?
A negative E means a field that acts towards a negative charge whereas a negative V means a value less than 0
Compare how E varies with distance to how V varies with distance
E varies with distance more sharply than V does
What happens to the charge of a spherical metal conductor insulated from the ground?
The charge spreads out across the surface with the greatest concentration where the surface is most curved
Can electric fields be attractive or repulsive?
Yes
What is electric charge measured in?
Coulombs
What sign will F be if the charges repel?
Positive
What sign will F be if the charges attract?
Negative
What is the effect on F if the point charges aren’t in a vacuum?
The size of the force will depend on the permittivity of the material between them
Does field strength depend on where you are in the field?
Yes
How can a uniform field be produced?
By connecting two parallel plates to the opposite poles of a battery
What is the magnitude of electric potential as distance increases?
The absolute magnitude of V is greatest on the surface of the charge and decreases as the distance from the charge increases - V will be zero at an infinite distance from the charge
What sign is V for a repulsive force and for an attractive force?
For a repulsive force, V is positive and for an attractive force, V is negative
What does the amount of energy you need to move a charge across a potential difference depend on?
The size of the charge you’re moving and the size of the potential difference you want to movie it across
