O lvl Phy: Static Electricity

Anderson Sec Sch 2007 Sec 4 CA1 Q14

The figure below shows a person walking on a carpet and approaching a metal door which is earthed.

(a) It is found that the carpet on which the person has walked on carries positive charges. Explain briefly how these charges are formed.

(b) What kind of electric charges can be found on the body of the person?

(c) When the person is close to the metal door, sketch the distribution of electric charges on the person and the metal door in the following figure.

(d) What happens when the person touches the metal door? Explain briefly.

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Answer:

(a) When the person's shoes rubbed against the carpet, there is friction. Electrons are transferred from the carpet to the person. The person has an excess of electrons and is negative.

(b) Negative.

(c)

(d) The negative charges on the person's body will be discharge through the door, which is earthed. The person will feel a slight and short electric shock.

O lvl Phy: Static Electricity

The diagram shows a large conducting sphere X mounted on an insulating pillar. The sphere X is given a positive charge. The small, uncharged metal sphere S, suspended on an insulating thread, is brought near X.
(a) Show on the diagram the charges induced on S.

(b) What will happen to sphere S? Explain your answer.

(c) S is moved horizontally so that it touches X. Explain what happens to S.

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Answer:

(a)

(b) S will be attracted by X. This is because negative charges are induced nearer to sphere X whereas the positive charges induced are further away from X. Thus, there is a net attractive force between the sphere X and S.

(c) When S touches X, some electrons flow from S to X and S is now positively charged (Note: S was originally neutral). Since X also possess positive charges, repulsion occurs and S moves to the right, away from X.

O lvl Phy: Static Electricity

(a) Name two methods of electrostatic charging.


A polythene strip may be charged by rubbing with a soft cloth. Before rubbing the polythene strip with a soft cloth, both the strip and the cloth are neutral. After rubbing, the polythene strip is charged negatively.

(b) Does a neutral body possess any charges? Explain.

(c) Is the soft cloth charged after rubbing? If it is, is it charged positively or negatively?

(d) What happens when the polythene strip is rubbed with the soft cloth?

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Answer:

(a) Friction and induction

(b) Yes. There are both positive and negative charges, but it is neutral because the total number of positive charges is equal to the total number of negative charges.

(c) Yes. It is positively charged.

(d) Friction between the 2 materials pull electrons from the cloth to the polythene strip. The polythene strip will possess extra negative charges and becomes negatively charged, whereas the soft cloth loses negative charges and therefore becomes positively charged.

O lvl Phy: Static Electricity

Dec 1989 P2 Q5

In the figure below, L is a large conducting sphere connected through a switch to the positive terminal of a supply of high voltage. A light conducting sphere S hangs from an insulating thread as shown. Both spheres are initially uncharged.

(a) Describe the movement of electrons which takes place when the switch is closed and sphere L becomes charged.

(b) Describe and explain the effect produced by the charge on sphere L on the conducting sphere S.

(c) Sphere S is now connected to earth. Describe the effect this has on (i) the charge on S, (ii) the position of S.

(d) Describe an experiment you could carry out to identify the sign of any charge remaining on S after the earth connection has been removed.

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Answer:

(a) Electrons from the sphere L moves to the positive of the high voltage supply and the sphere becomes positively charged.

(b) Some electrons from the farther end of S moves to the nearer end, causing negative charge at the nearer end. The sphere S is attracted towards L and comes to the position shown above.

(c)
(i) The charge on S is negative as some electrons from the earth drifts to S.
(ii) S is at the nearest position possible from L.

(d) One way is to use a negatively charged gold leaf electroscope. The negatively charged gold leaf electroscope is brought near the sphere S. The divergence of the gold leaf will increase, hence showing that the remaining charge on S is negative.

O lvl Phy: Static Electricity

Anglican High Prelims 2005 P2 Q5

The figure below shows a paint spray that uses principles in electrostatics.

(a) The pain droplets become positively charged by friction. State where and how the droplets get charged.
(b) This method is sed to apply paint to a grounded metal gate. Explain why the droplets are attracted to the gate.
(c) State and explain another advantage of this method of painting.

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Answer:

(a) Nozzle.
The nozzle is charged positively. When the paint is sprayed, the positive charges are 'brushed' off onto the droplets.

(b) The gate will be induced to a negative charge (as the positively charged spray droplets near).
Since unlike charges attract, the positively charged paint droplets will be attracted to the gate.

(c) The paint will be more evenly distributed because positively charged paint droplets will repel each other.

O lvl Phy: Static Electricity

Mayflower Sec Sem 1 2005 P2 Q5

When an aircraft is being refuelled, the fuel can become negatively charged as it flows along the metal pipe to the fuel tanks.

(a) Explain how the fuel becomes negatively charged.
(b) What type of charge does the metal pipe have?
(c) Explain why a build-up of charge on the wings of the aircraft could be dangerous.
(d) During refuelling, the metal airframe (pipe) is connected to the ground. Explain how this makes the process safer.

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Answer:

(a) The friction between the fuel and the metal pipe causes the metal pipe to lose electrons to the fuel.

(b) Positive charge

(c) The build-up of charges might produce sparks. Since fuel is highly flammable, the wings might burst into flames or an explosion might occur.

(d) Excess charges flow to/from the ground and the pipe is neutralised. This prevents build-up of charges and thus, prevents a fire from occurring due to sparks from the static charges.

O lvl Phy: Static Electricity

RGS Prelims 1991 P2 Q6

Two metal spheres X and Y on insulating stands are placed in contact and a positively charged sphere S on an insulating stand is brought near X and Y as shown in the figure below.

(a) Draw on the figure the charges, if any, on each of the spheres X and Y.
(Note: sketch it out, not draw on your monitor screen)

(b) If X is earthed without removing S, state the kind of charge, if any, on X and Y.

(c) If X is earthed in the presence of S, then having removed earth connection followed by the withdrawal of S, what will be the charge on X and Y if they are separated?

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Answer:

(a)

(b) If X is earthed,
X will be neutral; there's no charges on X. The electrons will flow down the path to earth.

Y will have positive charges.


(c) After removing the earth connection,

X and Y will be as shown. With the withdrawal of S, the positive charges in Y will distribute itself evenly between X and Y since they are in contact.

Thus, the charges on both X and Y will be positive if they are separated.

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