Section 2: Electricity Flashcards
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)
Dielectric strength
The max electric field a dielectric can withstand before breaking down and conducting
Dielectric constant
When a dielectric is placed in an electric field, the dielectric polarises
The electric field created by the dielectric polarising (E_polarised) creates an opposite electric field to the applied electric field –> net electric field is smaller than applied electric field
What is κ (kappa)
Dielectric constant - how many times the electric field is weaker in the dielectric compared to what it would be if no dielectric is present (i.e. vacuum)
What does the dielectric constant (κ) depend on
The material between the capacitor plates
Cell
A single chemical unit that makes electricity
Battery
Several cells connected in series, so their voltages are added tgt
Electric current
The rate of flow of charge
Amount of charge going through a given X-section of a conductor in one second
Direction of current
Direction of positive charge flow
Ampere-hours
Measures the quantity of charge that a fully charged battery can supply
e.g. a 100Ah battery can supply 1A for 100 hours, or 10A for 10 hours
Resistance
The opposition to current
Resistance symbols
Box or jaggered line
Amp (A) is equivalent to
C/s
Resistance depends on…
Proportional to length
Inversely proportional to area
Proportional to type of material
Resistors - series and parallel
Resistors in series add, so R total is greater than any one of the resistors
Resistors in parallel: R total is less than individual resistances
Electrical power
How rapidly an electrical device can transfer electrical energy into other forms
Measured in W = J/s
R = R₀ [1 + α(T - T₀)]
R = resistance at temp T R₀ = resistance at temp T₀ α = temp coefficient of resistivity - can be +ve or -ve
EMF of battery
Voltage measured when no current is being pulled out of battery
Voltmeter, ammeter and resistance
A voltmeter should have high resistance so almost no current flows through it
An ammeter should have low resistance so there’s almost no voltage drop across it
