NCEA Level 1 Physics -Static Electricity

Static Electricity

Is caused by electron transfer

Results from an imbalance of charges

Can occur by induction, friction, and contact

(ALWAYS describe the direction of motion of charges during the charging process)

Charging by Friction

Rubbing 2 insulating materials together causes electron transfer

Lose electrons = positively charge

Gain electrons = negatively charged

Conductors (e.g. copper) don't normally get charged by friction

Charging by Contact

Happens when you touch an uncharged object to a charged object

Charging by Induction

Induction charging creates a charged object without actually touching the charged object to another object

Charges

Never created but are transferred from one surface to another

Charge is conserved -the total number of charges between the objects stays the same

Unlike charges attract

Only negative charges (electrons) are displaced

Charged objects attract (initially) uncharged objects

Earthing

The ground acts as if it is a reservoir of charges

Electrons can flow in and out of the earth without altering its charge

Electrical discharge in air (e.g. lightning and lightning conductors)

1. Ice particles fall to bottom of storm cloud (heavy) rub past water vapour, ice particles now negatively charge

2. Bottom cloud now negative

3. Repels electrons on lightning conductor down to ground, giving air and lightning conductor (charges accumulate at points) positive charge

4. Positive ions (from the air) repelled away from positively charged conductor and towards negatively charged bottom of thundercloud.

5. Thundercloud neutralised in that area, less likely to strike lightning

6. If lightning does occur, will go down conductor on building: point closest to clouds on building, charges accumulate at points, charge easily earthed

Conductors

Allow charges to move freely through them

Metals

Charged by by friction: allow charges to move freely through them (electrons repel each other and spread out). Any charge transferred/induced is quickly conducted away (e.g to earth), leaving the metal neutral

Insulators

Do not allow charges to move freely through them

e.g plastic, wood

Charged by friction: do not lose charge easily as charges cannot move freely through them

Uniform charge distribution

A charged metal sphere will have a uniform charge density all over its surface

Non-uniform charge distribution

Charges accumulate at points

Spark

A electrical discharge/A stream of electrons

Why does charged spray (coming out of a spray gun) stick to both sides of the leaves?

The positive spray droplets repel each other and this causes them spread out (even behind leaves) onto leaves

Why do clothes in a dryer sometimes stick to each other (static electricity)

Rubbing of clothes in the dryer causes electron transfer and the clothes acquire opposite charges due excess/deficit negative charge. Since opposite charges attract, they cling together

As clothes are insulators/are now dry, they do not quickly discharge

Like charges....

Repel

Unlike charges...

Attract

Properties of a lightning conductor

Sharp point, highest point, made of thick copper