Advanced Higher Physics Unit 2 Electric Fields

Size of charged particles Watch the last five minutes of the following video: Powers of ten video

Static Electricity What do these things have in common? Crackles when combing hair. Cling film sticking to your hands. Clothes clinging to each other in a dryer. Getting a shock when rubbing your feet on a carpet. Lightning. They are all caused by static electricity. Static electricity is due to electric charge that builds up on the surface of an insulator. The charge that has built up cannot easily flow away from the insulator, which is why it is called static electricity.

Where does static charge come from? electron (negative charge) All materials are made of atoms, which contain electric charges. proton (positive charge) Around the outside of an atom are electrons, which have a negative charge. The nucleus at the centre of an atom contains protons which have a positive charge. An atom has equal amounts of negative and positive charges which cancel each other out. This means an atom has no overall charge. Electrons do not always stay attached to atoms and can sometimes be removed by rubbing.

Where does static charge come from? Static charge can build up when two materials are rubbed together, such as a plastic comb moving through hair. When this happens electrons are transferred from one material to the other: One material ends up with more electrons, so it now has an overall negative charge. One material ends up with fewer electrons, so it now has an overall positive charge.

Creating static charge An insulating material can be charged by friction. For example, if an insulator is rubbed with a cloth, it can become charged in one of two ways: A. Electrons move from the cloth to the insulator. B. Electrons move from the insulator to the cloth.

Charging a polythene rod

Charging a polythene rod If an insulator made of polythene is rubbed with a cloth, electrons move from the cloth to the insulator. What charge does the cloth now have? The cloth is positively charged. What charge does the polythene insulator now have? The insulator is negatively charged.

Charging an acetate rod

Charging an acetate rod If an insulator made of acetate is rubbed with a cloth, electrons move from the insulator to the cloth. What charge does the cloth now have? The cloth is negatively charged. What charge does the polythene insulator now have? The insulator is positively charged.

Static charge

Identifying unknown charge If a rod has an unknown charge, how can the unknown charge be identified using a positively charged rod? ? If the unknown charge is brought near to a positively charged rod and it is attracted to this rod, then the unknown charge must be ________. negative If the unknown charge is brought near to a positively charged rod and it is repelled by this rod, then the unknown charge must be ________. positive

Identifying unknown charge If a rod has an unknown charge, how can the unknown charge be identified using a negatively charged rod? ? If the unknown charge is brought near to a negatively charged rod and it is attracted to this rod, then the unknown charge must be ________. positive If the unknown charge is brought near to a negatively charged rod and it is repelled by this rod, then the unknown charge must be ________. negative

Inducing a temporary charge If a negatively charged rod is brought near to a piece of paper, the paper sticks to the rod. The paper is uncharged (equal amounts of + and -), so why does it stick to the rod? + - + - + - As the negatively charged rod approaches the paper, the electrons in the paper are repelled away from the rod. This makes one side of the paper positive and one side negative. A charge has been induced on the paper and the positive side of the paper is attracted to the negative rod.

Inducing a temporary charge If a negatively charged rod is brought near to a piece of paper, the paper sticks to the rod. The paper is uncharged (equal amounts of + and -), so why does it stick to the rod? + - + - + - As the negatively charged rod approaches the paper, the electrons in the paper are repelled away from the rod. This makes one side of the paper positive and one side negative. A charge has been induced on the paper and the positive side of the paper is attracted to the negative rod.

Activity 1 Charged rod (acetate or polythene) Charged rod free to move Use the following apparatus to show that: Like charges repel each others Unlike charges attract each others

Uses of static electricity Static electricity can be dangerous but it can also be useful, as long as it is used carefully. Examples of uses of static electricity are: ___________________ Photocopiers Printers Spray painting Pollutant removers

How a photocopier works

Electrostatic paint spray Static electricity can be used to spray a car with paint: - + - - - - - - - - - - - - - - Paint gun nozzle has a positive charge. Car is negatively charged. electrostatic generator The nozzle of the paint gun is connected to one terminal of an electrostatic generator. The other terminal is connected to the metal panel, which is earthed.

Electrostatic paint spray The spray gun is designed to produce tiny charged droplets of paint. - + + - - - - - - - - - - - - - - - Paint gun nozzle has a positive charge. Car is negatively charged. electrostatic generator As a result the charged droplets are attracted to the car body panel. This gives a uniform coating of paint. Also, the droplets travel along the lines of force of the electrostatic field to reach hidden parts of the panel.

Danger of static electricity Filling fuel, rollers for paper and grain shoots are situations where charge can be a problem. Static can build up as the fuel flows along the pipe or paper rolls over rollers or grain shoots out of tubes. This can easily lead to a spark and then an explosion. To prevent this happening, the nozzles or rollers are made out of metal so any charge build up is conducted away. Large petrol tankers always have earthing straps between the tanker and the storage tank to prevent the risk of sparks.

The Gold Leaf Electroscope An electroscope can be used to detect the presence and type of charge on both metallic and non metallic objects.

Activity 2 Follow the instructions on the following slides to charge an Electroscope.

Charging an electroscope

Charging an electroscope 1. Bring up a negative charge close to electroscope Polythene rod

Charging an electroscope 2. Earth cap with finger. Polythene rod

Charging an electroscope 3. Remove finger. Polythene rod

Charging an electroscope 3. Remove rod. Polythene rod The electroscope has become positively charged.

Charging an electroscope 1. Bring up a positive charge close to electroscope Acetate rod

Charging an electroscope 2. Earth cap with finger Acetate rod

Charging an electroscope 3. Remove rod Acetate rod The electroscope has become negatively charged.

Activity 3 Using the laptop go onto: START ―› All Programs ―› Core Programs ―› Subject Area ―› Science ―› Physics ―› Virtual Physical Laboratory ―› Electrostatics ―› Electrostatic Induction Use animation and information from the next slides to charge two identical spheres with equal and opposite charges.

Charging two identical sphere with equal and opposite charges 1. Two initially uncharged metal spheres are touching.

Charging two identical sphere with equal and opposite charges 2. A positively charged rod is brought closed.

Charging two identical sphere with equal and opposite charges 3. Spheres are separated.

Charging two identical sphere with equal and opposite charges 4. Charged rod is removed. The two spheres have equal and opposite charges.

Activity 4 Using electroscope and charged rod try “testing for a charge” using information from next slides.

Testing the sign of a charge An electroscope is charged. For example positively.

Testing the sign of a charge A plastic object is rubbed and brought near electroscope.

Testing the sign of a charge If leaf rises further, the object is charged positively.

Testing the sign of a charge If leaf goes down, the object is charged negatively.

Faraday’s Ice-Pail Experiment A positively charged sphere is suspended in the can without touching the walls of the base. The leaf rises. Charged sphere touches the can. Sphere becomes neutral but the can stays positively charged.