Physics 30: Forces & Fields
Electrostatics
● The study of stationary electric charges.
● Electrical ground: Source of charge.
● Semiconductors can be good conductors or insulators(depending on the situation).
● Only electrons flow in solid conductors. Charge flow may involve positive charges in
solutions or in a vacuum.
Conductors
● Conductors allow charge to flow.
● In conductors, electrons in the outermost regions of the atom are free to move.
● Metals are good conductors; their electrons are free to move. Conductors are used to
make wires. Copper, aluminium, and mercury are good conductors.
Insulators
● Insulators are substances that do not allow charge to flow well because their electrons
cannot move freely. Electrons are tightly held in atoms of the insulator.
● In an insulator, electrons are tightly bound to the nucleus and are not free to move within
the substance.
● The insulating material in a lamp cord stops charges from leaving the wire and protects
people from electric shock.
● Good insulators: Plastic, rubber, glass, wood, and air.
Grounding
● The process of connecting a charged object to the Earth.
● Grounding transfers charge to and from Earth.
● This allows excess charge to leave an object. Grounding will neutralize a charged object.
Superconductors
● Superconductors have a negligible resistance at very low temperatures.
● Lower electrical resistance = higher conductivity.
● Decrease in temperature of a material = decrease of electrical resistance.
Charge Separation
● A charge separation can be induced when a charged object is brought close to a neutral
object. Electrons flow from one object to the other due to separation of charge.
● Since unlike charges attract, a charge separation is induced when the electrons in the
neutral sphere move towards the positively charged ruler.
● The attractive forces are larger than the repulsive force because of the difference in
distance. A net attractive force results.
, 3 Ways to Transfer Charge:
● Friction
● Conduction
● Induction
● Only electrons move when charge is transferred.
Friction as a Way to Transfer Charge
● Different substances have different affinity for electrons. By rubbing two different
objects together, the substance with the greater affinity for electrons will take electrons
away from the other. One object becomes negative; the other gets an equal but opposite
charge.
Conduction as a Way to Transfer Charge
● When two objects with different charges are brought into contact, electrons will flow
from the more negative/charged object to the uncharged object by touching the objects
together until a charge balance is obtained. Both objects then have the same charge.
Induction as a Way to Transfer Charge
● Charging by induction happens when charges in an uncharged object are rearranged
without direct contact with a charged object.
● Bringing a charged object near a neutral one causes a separation of charge. Electrons
flow from one object to the other due to separation of charge.
Charging by induction involves the following steps:
1. Bring a charged object close(but not touching) to a neutral object that you wish to
charge.
2. Ground the neutral object.
3. Remove the ground.
4. Remove the charged object.
To Charge An Electroscope Positively by Induction
1. Bring the negatively charged rod near the knob of the electroscope.
2. Ground the electroscope.
3. Remove the ground.
4. Remove the negatively charged rod.
● Originally, the electroscope is neutral, and so its leaves hang vertically(no repulsion).
When a positively-charged rod touches a neutral scope, the system pursues balanced
charges.
● Electrons transfer from the electroscope to the rod until they are both positively charged.
● Since both leaves are now positively-charged, they repel and the leaves move apart.
Electrostatics
● The study of stationary electric charges.
● Electrical ground: Source of charge.
● Semiconductors can be good conductors or insulators(depending on the situation).
● Only electrons flow in solid conductors. Charge flow may involve positive charges in
solutions or in a vacuum.
Conductors
● Conductors allow charge to flow.
● In conductors, electrons in the outermost regions of the atom are free to move.
● Metals are good conductors; their electrons are free to move. Conductors are used to
make wires. Copper, aluminium, and mercury are good conductors.
Insulators
● Insulators are substances that do not allow charge to flow well because their electrons
cannot move freely. Electrons are tightly held in atoms of the insulator.
● In an insulator, electrons are tightly bound to the nucleus and are not free to move within
the substance.
● The insulating material in a lamp cord stops charges from leaving the wire and protects
people from electric shock.
● Good insulators: Plastic, rubber, glass, wood, and air.
Grounding
● The process of connecting a charged object to the Earth.
● Grounding transfers charge to and from Earth.
● This allows excess charge to leave an object. Grounding will neutralize a charged object.
Superconductors
● Superconductors have a negligible resistance at very low temperatures.
● Lower electrical resistance = higher conductivity.
● Decrease in temperature of a material = decrease of electrical resistance.
Charge Separation
● A charge separation can be induced when a charged object is brought close to a neutral
object. Electrons flow from one object to the other due to separation of charge.
● Since unlike charges attract, a charge separation is induced when the electrons in the
neutral sphere move towards the positively charged ruler.
● The attractive forces are larger than the repulsive force because of the difference in
distance. A net attractive force results.
, 3 Ways to Transfer Charge:
● Friction
● Conduction
● Induction
● Only electrons move when charge is transferred.
Friction as a Way to Transfer Charge
● Different substances have different affinity for electrons. By rubbing two different
objects together, the substance with the greater affinity for electrons will take electrons
away from the other. One object becomes negative; the other gets an equal but opposite
charge.
Conduction as a Way to Transfer Charge
● When two objects with different charges are brought into contact, electrons will flow
from the more negative/charged object to the uncharged object by touching the objects
together until a charge balance is obtained. Both objects then have the same charge.
Induction as a Way to Transfer Charge
● Charging by induction happens when charges in an uncharged object are rearranged
without direct contact with a charged object.
● Bringing a charged object near a neutral one causes a separation of charge. Electrons
flow from one object to the other due to separation of charge.
Charging by induction involves the following steps:
1. Bring a charged object close(but not touching) to a neutral object that you wish to
charge.
2. Ground the neutral object.
3. Remove the ground.
4. Remove the charged object.
To Charge An Electroscope Positively by Induction
1. Bring the negatively charged rod near the knob of the electroscope.
2. Ground the electroscope.
3. Remove the ground.
4. Remove the negatively charged rod.
● Originally, the electroscope is neutral, and so its leaves hang vertically(no repulsion).
When a positively-charged rod touches a neutral scope, the system pursues balanced
charges.
● Electrons transfer from the electroscope to the rod until they are both positively charged.
● Since both leaves are now positively-charged, they repel and the leaves move apart.
