Module 4: Electricity Flashcards
Electric force
The force responsible for binding nuclei and electrons to form atoms, binding atoms to form molecules, and binding molecules to form bulk materials
What does electrical force act upon?
Charged objects
Unit for charge
C coulombs
Smallest nonzero charge an object can have
qe
Value of qe
1.6 x 10^-19 C
Describe charging by friction
Electrons are transferred from one insulator to another when they are rubbed together
Conductor
A material where charge can move freely
Insulator
A material where charge cannot move freely
Two examples of conductors
Metals
Salty water
Four examples of insulators
Plastics
Oil
Glass
Undissolved salt
Do conductors have a high or low resistivities?
Low
Do insulators have high or low resistance?
High
Polarisation
The separation of charges in a neutral object
In which materials does polarisation occur to a larger extent?
Conductors
|q|
Absolute value of charge
In which direction does an electric field go?
Positive to negative
Function of a battery
Kees constant potential difference across terminals
How can we increase the capacitance? (3)
Decrease distance between plates
Increase size of plates
Add a capacitor in parallel
Relative permittivity
Range between 1-100
Can find actually permittivity by multiplying this with the permittivity of a vaccum
What circuit component does a cell membrane (and fluid around it) act like?
Capacitor
Membrane is insulator between two ‘plates’ of salty water
How does capacitance relate to our reaction times?
The capacitance of nerve cell membranes determines how fast you can think
The smaller the capacitance, the faster the reaction time
Which has a larger capacitance: capacitors in series or parallel?
Parallel
Which has a larger resistance: resistors in series or in parallel?
Series
Current can split through two resistors in parallel
Conventional current
The current in the direction of positive charge flow
Opposite direction to conduction (movement of electrons)
What property determines a specific objects resistance?
It’s geometry (additional to the materials resistance)
Kirchoffs law #1
The sum of potential charges around a loop is zero
Due to conservation of energy
Kircchoffs law #2
The current flowing into a junction is the same as the current flowing out
Due to conservation of charge
What happens to electrical energy as it passes through a resistor?
They lose energy- bashing into the metal atoms of the resistor
This energy is converted into heat
Why don’t electrons slow down as they travel ‘through’ the battery, like they do in a resistor?
The battery provides chemical energy- which is converted into electrical energy in the electrons
If potential is decreased in the direction of current flow, what happens to the power?
Dissipates
How do we increase the potential in the direction of current flow?
Power input
If two lights are connected in series, will they be as bright as one?
No- resistance is doubled (and voltage stays the same) so the current is halved
Power is halved
What happens to the brightness if we add two lightbulbs in parallel?
Same brightness- resistance is halved, so current is doubled
Power is doubled through two lights- so each is the same as before
Four factors determining how dangerous an electric shock can be
Amount of current (not voltage)
Path taken by current (through body)
Duration of shock
Frequency (DC more dangerous than AC)
Why is most electrical interference (e.g. defibrillator) done through the skin?
Because skin provides a large resistance that prevents current through the body from being too large
RC circuit
Resistor and capacitor in series
When does the charging of a capacitor finish?
When the voltage across the capacitor = voltage across the resistor
What happens to the rate of charge in a capacitor when discharging? What is this called?
Decreases as current approaches zero
Exponential decay
RC
Time constant/ RC constant
Time taken for charge to drop to 37% of previous value
For a circuit that quickly reaches equilibrium, will the time constant be small or large?
Small- small resistance and capacitance
