Physics - Electricity Flashcards
How to create charge between two objects?
Two insulators moving relative to each other - friction makes them both either positive/negative,t charged
Plastic with duster/hair being combed
Can conductors become charged?
Only retain the charge if they are insulated from their surroundings ; if not insulated then charge will leak away
Induction
A neutral object becomes magnetised when placed near a magnet
Electrostatic induction ; charged object induces charge on a neutral object (charge is polarised to opposite sides)
How to keep the charge permanent?
If one end of an object charged by induction is momentarily earthed then that charge will leak away and become permanently charged (with the other charge)
Total charge?
Charge is always conserved - for every object that becomes positively charged, another one must become negatively charged
Which way the electrons move?
Determined by whichever object has nuclei that attract the electrons less strongly (loses electrons)
If an object is made out of insulating material (balloon)
Charge will not flow away
Two factors which affect the electrostatic force
Larger the charges = larger the force
Larger distance = smaller the force
Sparking
Air between two objected becomes ionised by a large voltage and starts conducting
Two charged objects that have air between them can discharge by a spark between them (when charge is large enough/distance is small enough)
How can risk of sparking be eliminated
By earthing ; or if they are connected together by a wire then electrostatic charging cannot take place
Photocopying and printing
Charge being placed on the paper ; exposed to toner powder which sticks to the paper at those locations as a result of electrostatic induction
Aircraft refuelling
Large volumes of fuel flow through the pipe - large amounts of friction - pipe is electrostatically charged - thus pipe is always earthed to prevent build up of charge
Battery
Group of cells (two cells)
Series connection
Connected in a line
Parallel connection
Branches
Direct current
Always in the same direction
Alternating current
Repeatedly changes direction ; usually very rapidly
Sources of dc
Cells or batteries
Power supply from mains electricity
Converted from ac to dc using diodes as a rectifier ; diode only allows current in one direction (direction of the arrow)
Source of ac
Generators in power stations produce ac
Ac current?
Change is repeated regularly to produce a waveform
Frequency of mains supply in U.K.
50Hz (current changes 100 times each second to produce 50 complete to and fro cycles )
What is ac wave in the form of?
Sin wave
Positive in one direction
Negative in the other
Voltage on ac supply
Switches from positive to negative and back
