Section 2: Electricity Flashcards
(42 cards)
Unit of charge
Coulomb (C)
+ve and -ve charges - cancel out?
+ve and -ve charges DON’T cancel out
The effects of the +ve and -ve charge can cancel out
Rubbing/contact between two objects causes…
Charges from one object to transfer to the other object (either +ve or -ve charge can transfer)
Conservation
Electrical charge is conserved
Grounding / earthing
Connecting an electrically charged object to the ground with a wire (or a water film) will result in the object losing its electric charge
Which charge can flow through a metal wire
Electrons (-ve charge)
Coulomb’s law in a vacuum
The size of the electric force between 2 point charges in a vacuum is given by Coulomb’s law
An inverse square law; strength of force depends on 1/r^2
Superposition principle
If there’s more than 2 charges, the 3rd charge doesn’t affect the force acting between the 2 charges (i.e. not affecting by ‘extra’ charges)
Field
A quantity that varies in space
Electric fields are…
Vector fields
Electric fields - units
N/C (newton/coulomb)
Direction of electric fields
The direction a positive charge will move
Negative charges go in opposite direction to field
Electric field lines
The closer the field lines, the stronger the field
Electric shielding
An object can be ‘shielded’ from an external electric field by enclosing it in a metal can
If metal can is exposed to an external electric field, the e- in the metal can will move and arrange themselves so there is no electric field in the can
What is voltage (V)
The electrical potential energy per coulomb of charge
Always measured with respect to something
What does it mean if 1C of charge changes 6V from point B to A
1C of charge at A has 6J more electrical PE than at B
When a charge is moved against an electric force…
Work is done on the charge –> causes charge to gain potential energy
Millikan’s oil drop experiment
Weight force and electrostatic force on charge (oil drop) act in opposite directions
If V is adjusted until the drop is stationary, these 2 forces are balanced
Equipotential surface
A surface perpendicular to the field lines
Electrical PE is constant
Strength of fields and equipotential surfaces
In a strong field, the field lines are close tgt and so are the equipotential surfaces
Capacitance (C) - units
Farad (F)
One farad = one C/V
What is a capacitor
A device that stores electric charge
Made of a pair of conductors separated by an insulator
What is capacitance (C)
The property of a capacitor to store charge
Increased C –> more charge the capacitor can store for a given applied V
Ways to increase capacitance
Increase SA of plates - can store more charge on plates
Decrease distance between plates - stronger attraction of e- to +ve plate and cancels out more -ve charge repulsion
Put insulator between plates (dielectric)
