15 — Static electricity

Question 1

Electric field

Answer 1

Electric field is a region in which an electric charge experiences an electric force

Question 2

Direction of electric field

Answer 2

Direction of electric field is the direction of the force on the small positive charge

Question 3

Insulator

Answer 3

An insulator is a material where electrons r not free to move about within them

Question 4

Conductor

Answer 4

A conductor is a material where electrons r free to move about within them

Question 5

Applications of charging by friction

Answer 5

Transfer of fluids and dry bulk
Air flow across surface of planes and vehicles
Spray painting
Electrostatic precipitator

Question 6

Suggest a method to charge 2 conductors

Answer 6

1. Place a +ve/-ve charged rod near conductor
2. Separate the 2 conductors in presence of charged rod
3. Charged rod is removed and both conductors acquire OPP. Charges

Question 7

Suggest a method to charge insulators

Answer 7

Friction.
Rubbing 2 insulators causes electrons w weaker bonds from x material to lose electrons to y material, thus becoming positively charged.

Question 8

Suggest a method to charge a single conductor

Answer 8

induction
1. Bring a negatively charged rod near the conductor but not touching it. Electrons r repelled by negatively charged rod. Attach the earth wire to connect the conductor w the earth thru earthing. Positive charges r induced to side nearer to negatively charged rod.
2. Negative charges r repelled and travels down to Earth via earthing. Positive charges r induced towards the side w the charged rod.
3. Remove earthing then remove charged rod. The conductor becomes positively charged.

Question 9

Suggest methods to neutralise charges from charged insulators

Answer 9

A. Place charged insulator in damp surroundings
-> Electrons in neutral water molecules lose electrons that neutralise some positive charges in rod

B. Ionisation (heating air molecules)
-> air molecules lose electron due to ionisation, resulting in a net positive charge. Electrons lost is attracted to positively charged rod to neutralise the positive rod.

Question 10

Discharging

Answer 10

Neutralising a charged body to remove excess charges from it

Question 11

Suggest a way to neutralise charged conductors

Answer 11

Earthing -> flow of electrons via earthing to neutralise positive charges

Question 12

How to prevent an electric shock

Answer 12

1. Use a humidifier to increase humidity of room
2. Use anti-static products to remove charges

Question 13

Spray painting

Answer 13

Paint droplets from an aerosol usually become charged by rubbing against nozzle of spray, becoming charged the same net charge as nozzle. Since paint particles r all positive, they spread evenly due to electrostatic repulsion and are attracted to negative charged body.

Question 14

Electrostatic precipitator

Answer 14

A strong electric field exists betw +ve and -ve plates, ionising surrounding air molecules. The free electrons get attached to dust and ash particles, making them negatively charged. -ve charged dust and ash particles attract to +ve plates, mechanically shaken to get rid of dust and ash.

Question 15

Earthing

Answer 15

A grounding wire that electrons can travel
-> single conductor charged -ve, e- travel down wire to earth to neutralise excess -ve charge
-> single conductor charged +ve, e- travel up wire from earth to neutralise excess +ve charge

Question 16

Answ tech of neutralising charges

Answer 16

_ charges of sphere Q r induced to side nearer to P. They attract/repel due to unlike charges attract/like charges repel.

Question 17

Hazards

Answer 17

1. Petrol tankers and aeroplanes
   -> friction betw lorry/aeroplanes and air build up posiitve charges on surface. When a sudden discharge occurs, a spark from this electric charge can ignite the petrol vapour and cause it to explode.
   -> metal chains or special conducting tyres r used for earthing of charges
2. Operating theatres
   -> gases used to anaesthetize patients r easily flammable -> static charges produced due to friction betw fabrics can cause sparks. Anaesthetic machines, trolleys, n patients r linked to ground by metal chains. Medical staffs wear anti-statics conducting shoes.

Question 18

Explain how the fuel in hose is charged

Answer 18

The fuel becomes charged by friction betw the fuel and the surface of the hose when the fuel is passing thru the hose.

Question 19

State another example where electrostatic charging is a potential hazard.

Answer 19

Lightning is an atmospheric discharge of electrostatic charges and can carry large currents. Thus, it is dangerous.

Question 20

How earthing prevents the aircraft from the risk of explosion

Answer 20

Connecting a wire from the aircraft to earth provides an earthing path for excess positive charges, thus preventing build-up of positive charges on the aircraft.

Question 21

How distance affects force

Answer 21

+ve charges on bottom of metal strip closer to sphere than -ve charges on top of the strip -> forces betw +ve charges n sphere is larger than the force betw -ve charges n the sphere.

Question 22

The tv screen is charged positively. Explain why dust particles r attracted to the screen.

Answer 22

The dust particles may be made of conductive material n become charged by induction, with the side closer to screen having more e-, becoming -ve and the side further from screen having less e-, becoming +ve. Since unlike charges attract, the dust particles r attracted to the glass screen.

Question 23

Explain why the reading on the balance increases when the positive plastic rod is brought close to the positive glass rod.

Answer 23

The plastic rod is +ve charged. When the plastic rod is brought near glass rod, both rods repel due to like charges repel, causing the glass rod to exert a larger force on the balance due to its weight and the repulsive electrostatic force, causing reading to increase.

Question 24

State how the experiment (2 insulator rod on a balance ) shows that there is an electric field around the plastic rod

Answer 24

When there is an electric field around the plastic rod, a charged obj (PR) will experience a force when it is in the electric field, and thus affects the force measured by the balance.

Question 25

Explain why the +ve charged rod is attracted to the uncharged metal sphere.

Answer 25

When +ve rod is brought near metal sphere, e- of sphere r induced towards the side near rod since e- r -ve charged n unlike charges attract and +ve charges r induced to side further away from +ve rod. Thus, the sphere will be attracted to +ve rod as attraction due to unlike charges attract since -ve charges in sphere is closer to the +ve rod than +ve charges in sphere. Thus, attraction forces will be stronger than repulsion due to like charges and the sphere n rod r attracted.

Question 26

Describe how to charge the sphere uniformly over its surface by induction w a +ve charged rod.

Answer 26

When the +ve rod is held near, the sphere is earthed by conducting wire. E- will move up the conducting wire from earth to due to unlike poles attract, causing metal sphere to have a net negative charge.
The conducting wire is then removed and +ve rod can be removed. Excess e- will redistribute evenly since like charges repel and the metal sphere will be charge -ve uniformly over its surface.

Question 27

Explain why the sphere is attracted more strongly to rod when charged than when an uncharged.

Answer 27

After the e- r induced to the sphere, the sphere has a net negative charge. The attraction to +ve rod will be stronger by unlike poles attract since there is no repulsion from the +ve side of the sphere.

Question 28

A neutral ball is displaced to touch the negative plate and then released. The ball is then repelled by the negative plate.e x plain what causes the repulsion betw the sphere and the -ve plate.

Answer 28

Negative charges from the negative plate is induced to the conducting sphere, causing the sphere to be negatively charged. Since like charges repel, the ball is repelled by the negative plate.

Question 29

The sphere is repelled and attracted by 2 oppositely charged plates, causing it to swing between and touches the 2 plates. Why does the swinging sphere eventually stop.

Answer 29

When all excess electrons have been transferred from the negative plate to the positive plate, both plates become neutral and the neutral sphere is not attracted to any plate.

Question 30

Suggest why gold leaf electroscope angle of deflection increases.

Answer 30

As the number of rubs increases, the glass rod loses more and more electrons and becomes more and more positively charged.

Question 31

Suggest one possible source of error when using electroscope.

Answer 31

The glass rod to be brought near the metal plate of electroscope has to be discharged betw rubs to prevent charge accumulation. It can be discharged by running under a tap.

Question 32

Explain why the threads fan out at the ends of a metal ball connected to a dc supply.

Answer 32

There is an accumulation of positive charges at the end of each thread and these like charges repel one another.

Question 33

After the dc supply was removed, the threads fanning out on the metal ball remained in the same position. Explain.

Answer 33

The positive charges remained on the ball and the thread even after the dc supply was removed.

Question 34

Explain why the galvanometer deflects momentarily.

Answer 34

+ve sphere induces e- in metal plate to side near sphere by unlike charges attract, and +ve charges to surface opp. To sphere by like charges repel. This creates a positive potential difference betw the plate and earth, causing e- to flow from earth towards metal plate. The flow of e- causes the galvanometer to deflect. The deflection stops when +ve charges r neutralised, causing the galvanometer to return to zero position.

Question 35

Explain why an electrostatic discharge rod is necessary.

Answer 35

Electrostatic charges can dmg aviation electronics or interfere aviation electronic communication devices. Electrostatic discharge rods help to remove static charges from aircraft in flight.

Question 36

Explain the danger posed when an aircraft is positively charged.

Answer 36

As fuel flows through the hose, friction against the hose and fuel causes electrostatic charging of fuel and hose. A small discharge may cause a spark, since fuel is highly combustible, a fire or an explosion can occur.

Question 37

How to prevent aircraft from accumulating positive charges.

Answer 37

Aircraft and hose outlet can be connected w a ground wire to neutralise any charges that r produced by the fuelling process.

Question 38

Man becomes charged after walking on carpet.

Answer 38

Friction between the man and the carpet causes positive charges to accumulate in the man as more electrons r lost from body.

Question 39

State how the threads show that there is an electric field around the ball

Answer 39

The threads r repelled from one another. Thus there is a electrostatic force of repulsion and hence an electric field must be present around the charged body.

Question 40

Suggest why dry air allows electrostatic charges to be accumulated more easily

Answer 40

Dry air has less ions that can neutralise the charges on the door knob.

Question 41

What will happen to the stream of water from a water tap when a charged rod is brought near the water

Answer 41

Water nearer to the rod will be induced w positive charges, which is then attracted to negatively charged rods by unlike charges attract. Stream of water bend towards the rod.

Question 42

Explain why there is more positive charge on the RHS of x.

Answer 42

Conc of negative charges on the LHS of Y will repel the electrons in X, making the RHS more +ve charged.

Question 43

Explain why S is repelled aft touching C

Answer 43

Sphere S has a net +ve charge. Upon touching the surface of C, electrons in S will be induced to C, neutralising some of the +ve charges in C. However, C remains +ve charged and since S lose e-, S becomes positively charged as well. Since like charges repel, S is repelled aft touching surface of C.

Question 44

Explain why it is difficult to charge the glass rod under damp conditions

Answer 44

Any charges formed will be discharged easily under wet env.

Question 45

Suggest one way to determine if the glass rod is charged.

Answer 45

The rod is repelled by another similarly charged rod.

Question 46

Explain why the aluminium foil is attracted to the earthed iron plate.

Answer 46

The earthed iron plate is an electrical conductor with mobile electrons. When positively charged aluminium foil is brought near iron plate, electrons become attracted to the iron plate’s surface closest to and facing the foil by unlike charges attract. These electrons and the positive charges set up an electric field betw them and create an electric force on the foil towards the plate which is greater than the foil’s weight, causing the foil to be attracted to the plate.