SP11 - Static Electricity Flashcards
How can a material become charged?
If two insulating materials are rubbed together, one gains a positive charge and the other gains a negative charge, as electrons are transferred between the two. They attract objects with opposite charges, so may stick to each other.
Static electricity example - balloon rubbing on hair.
When you rub a balloon on hair (both insulating materials), electrons are transferred, causing them to become charged. The materials are insulating, so they cannot conduct this charge (the only way to get rid of the charge is to earth the object). Since the hairs all have the same charge, they repel each other and the hair stands out on end.
How does charging by induction work?
A charged object can affect the distribution of charges on an uncharged object. For example, a negatively charged balloon is placed on a wall, causing the electrons in the wall to be repelled (as they are both negative). The positive charges left behind (the induced charge) attracts the negatively charged electrons and the balloon sticks to the wall.
Why does lightening happen?
Static electricity builds up in clouds due to friction between particles of ice or water moved by air currents. When the charge is large enough, charged particles travel through the air between the clouds and the ground (the lighting induces a charge in the ground). This causes lightning and thunder.
How can sparking be avoided when refuelling cars and planes?
When fuelling vehicles there is a build up of static electricity due to the friction between the fuel and the pipe. If this charge caused a spark, it would be very dangerous as the fuel could catch on fire and cause an explosion. This is avoided using a bonding line (copper wire connecting the fuel tank to the earth). This conductor earths the plane by carrying the charge away from the plane, removing the risk of any sparks.
How is static electricity used in insecticide sprayers?
The insecticide is given an electrostatic charge as it leaves the sprayer. The droplets of insecticide repel each other as they have the same charge, insuring a fine mist of insecticide which covers a large area. It also induces a charge in the earth, causing it to fall more quickly, making it less likely to be blown away.
How is static electricity used in spray painting?
The paint droplets are charged as they leave the sprayer. The object being painted is also given a charge (using electrodes). They are charged with the opposite charge to the paint droplets, which ensures an even coverage and minimal wastage.
How is static electricity used in photocopiers?
- an image of the document is projected onto a positively charged copying plate
- the plate loses its charge in the light areas and keeps the positive charge in the dark areas (the text)
- a negatively charged black toner powder is applied to the plate and sticks to the part where there is a positive charge
- the toner is transferred onto a new blank sheet of white paper
- the paper is heated to make sure the powder sticks
- the photocopy of the document is made.
- inkjet printers work in a similar way, but instead of black toner powder, a small jet of coloured ink is negatively charged and attracted to the correct place on the page.
What is an electric field?
The force field around a charged object (the volume of space around it in which it can affect other objects).
How should field lines be drawn?
Field lines should:
- never cross
- shows where the field is strongest (which is where the lines are closest together)
- shows the direction of the force on a charge in the field
- start on a positively charged object, e.g. the point charge, and end on a negatively charged object (if there is only one object the field lines keep going, becoming more widely spaced.
What are the electric fields like between two parallel plates?
It is uniform (the same in all places), with the direction always going from positive to negative.
