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