Topic  5

 

What is static electricity, how can it be used, and what is the connection between static electricity and electric currents?

 

In the exam you need to know how to:

  • to explain why static electricity is dangerous in some situations and how precautions can be taken to ensure that the electrostatic charge is discharged safely

    •

  • to explain how static electricity can be useful.

You need to know about:

  • When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material onto the other.

  • The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

  • When two electrically charged bodies are brought together they exert a force on each other.

  • Two bodies that carry the same type of charge repel. Two bodies that carry different types of charge attract.

  • Electrical charges can move easily through some substances, e.g. metals.

  • The rate of flow of electrical charge is called the current.

  • A charged body can be discharged by connecting it to earth with a conductor. Charge then flows through the conductor.

  • The greater the charge on an isolated body the greater the potential difference between the body and earth. If the potential difference becomes high enough a spark may jump across the gap between the body and any earthed conductor which is brought near it.

  • Electrostatic charges can be useful, for example in photocopiers and smoke precipitators and the basic operation of these devices.

 

Key Words

Charge

Positive

Negative

Electron

Insulator

Conductor

 

Static Electricity can be described as "electricity at rest".  You will have discovered static electricity for yourself when taking off your school jumper on a very cold, dry day.  When I was at school (more years ago than I care to remember), there was a nylon carpet in one of the study rooms, and you could get quite a charge off it by shuffling up and down it.

 

Static electricity occurs in insulating materials (materials that do not conduct electricity), and arises due to the separation of charge.

 

Current electricity flows in conducting materials (e.g. metals).

 

 

How is static electricity made?

You will remember how in atoms there is a positively charged nucleus with protons and neutrons in it.  Surrounding the nucleus there are electrons that are negatively charged.  If an electron is removed from the atom, the atom is positively charged.  If an electron is added to the atom it gets negatively charged.

 

Question 1  What happens to the protons?  ANSWER

 

Let's suppose we rub a polythene rod with a cloth:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Electrons from the cloth are rubbed onto the polythene rod.

 

Question 2  What charge does this give the rod?  ANSWER

 

 

Now suppose we charge an acetate rod with the cloth:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This time we rub electrons off the rod onto the cloth.  The rod becomes positively charged, and the cloth becomes negatively charged.

 

If we hang the rods so that they can swing freely, we observe the following:

We can conclude from this simple experiment that:

 

like charges repel; unlike charges attract.

 

Investigating Charged Objects

The Gold Leaf Electroscope is a common instrument to investigate statically charged objects.  Here is a picture of one:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Let's think what happens if we bring a charged rod up to the cap.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The negative charge on the polythene repels the electrons from the cap.  The electrons go down the rod some to the bottom and some to the gold leaf.  They repel and make the leaf rise.

 

It is important to understand that only the electrons move.  The protons are in the nucleus and NEVER move.

 

Question 3  A positively charged rod is brought up to the cap of the gold leaf electroscope.  What happens and why?  ANSWER

 

 

Charge and Potential Difference

If we crowd more electrons onto a surface, we increase the charge and we also increase the potential difference (p.d.).  The potential difference is also called the voltage, and is measured in volts (V).  In static electricity, we have high voltages.  It is easy to charge up an object to 30 000 V.  If you touch that, you will only get a small shock, because the current is so tiny.

 

The most spectacular static electricity machine you will come across is the Van der Graaf generator.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The machine pumps electrons upwards to the top sphere.  The electrons crowd onto the top sphere and make the potential difference very large indeed.

 

If the breakdown voltage of air, 3000 V/mm is exceeded, a spark will jump to the discharge sphere.  The charge will be conducted back to the base through the wire.

 

When a big charge is being made on the top sphere, you can hear the motor slowing down as it has to work harder.

 

Question 4  A spark is 10 cm long.  What voltage is this?  ANSWER

 

Physics students love the Van der Graaf.  Big sparks fly about the place, and students love to give each other shocks.  A student (or teacher) can stand on the bench and their hair stands on end as in the picture below.  It works better if you stand in a plastic tray.

 

 

 

Once when I was doing this a spark jumped from my foot, across the plastic tray and the bench, to the base of the machine.  The distance must have been about 40 cm, indicating a voltage of 1.2 million volts.  I certainly felt it, and my class remembered it for a long time!

 

Question 5  Why would a shock of this sort of voltage not kill you?  ANSWER

 

 

Static Electricity can be Dangerous

Aeroplanes are expensive machines to run.  They use huge amounts of fuel.  A Jumbo Jet needs about 80 tonnes of fuel.  They only earn money when they are flying, so it's important to get the fuel on board as quickly as possible.  However rapid pumping of liquid fuel can result in a massive static charge building up that will lead to a spark.  It doesn't take a genius to see how dangerous that could be.

 

Therefore a bonding line is placed between the aeroplane and the pump to conduct the charge to earth.  In the picture below you can see the wire connecting the plane to the tanker passing by the men's feet.

 

 

The same danger can occur when powders are pumped at high speed.

 

Lightning is the ultimate display of static electricity.  The heat of the ground causes moist air to rise.  As it gets colder the higher up you go, ice forms, and as ice particles bump into each other, a separation of charge occurs as shown in the picture below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The charges separate.  The positive charges go to the top of the cloud.  The negative charges are at the bottom of the cloud.  They induce positive charges in the ground by repelling electrons into the ground, leaving the positives where they are.

 

If the potential difference gets big enough, a spark will jump.  The voltage is about 20 billion volts and the current about 20 000 amps.  Fortunately the spark only lasts for about 1/1000th of a second.

 

The energy can set fire to things, boil the sap in trees to make them explode, or kill a person who is struck (many people do survive though).  The intense heating of the air results in a loud bang, the echoes of which give the distinctive rumbling sound of thunder.

 

 

Static Electricity can be Useful

Your teacher will give you photocopied sheets.  The photocopier on which they were produced uses static electricity.

 

If you want to know how the photocopier works, click on the link below.

 

http://electronics.howstuffworks.com/photocopier.htm

 

Another use is the electrostatic dust precipitator.  The idea is shown in the picture below:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

When the dust particle passes between the plates, electrons on the dust particle are repelled leaving the positives where they are.  This is a process called induction.  The left side of the particle is more negative, so the particle is attracted to the positive plate.  It sticks there. 

 

This device can remove over 95 % of dust particles.

 

Coal fired power stations produce huge amounts of ash, much of which goes up the chimney.  Dust precipitators remove the vast majority of this.  The plates are periodically struck by heavy mechanical hammers and the dust falls off into bins.

 

Now answer Question 6

 

Summary

  • Static electricity occurs in non-conducting materials.

  • Static electricity arises due to separation of charge.

  • Like charges repel; unlike charges attract.

  • The greater the charge, the greater the potential difference.

  • Static voltages are high, but discharge currents are tiny.

  • Static electricity can be discharged by conducting it away with a wire.

  • Static electricity can be dangerous, but can also be useful

 

 

Now try the Topic Quiz

 

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