Kerala Syllabus Class 8 Basic Science: Chapter 10 Static Electricity - Questions and Answers
Std 8: Physics: Chapter 10: Static Electricity: Questions and Answers
♦ Activity
Take a water-filled container with a hole at the bottom. Hold a plastic scale near the water stream.
Hold the scale after rubbing it well on dry hair. What do you observe?
Observation: After rubbing, when the scale is brought near the water stream, the water stream bends towards the scale.
Inference: Rubbed objects can attract other things.
♦ Now rub the pair of objects given below against each other. Check if they can attract small pieces of paper. Can you find new rubbing pairs? Add your findings to the table.
• When some pairs of objects are rubbed against each other, some of them gain the ability to attract other objects.
• This ability to attract when rubbed is due to the accumulation of electric charge.
♦ What are electric charges?
Electric charge is the fundamental factor that causes attraction or repulsion between objects. There are two types of charges: positive and negative.
♦ The main particles of an atom are illustrated below. Let’s analyse the figure.
• The central part of an atom is the nucleus. It contains positively charged protons and neutral neutrons.
• Negatively charged electrons revolve around the nucleus.
♦ What are the main particles in an atom?
Protons, neutrons, and electrons
♦ The central part of an atom is the nucleus. What are the main particles in the nucleus?
Protons and neutrons
♦ What is the charge of each of these particles?
• Proton → positive charge
• Neutron → neutral (no charge)
• Electron → negative charge
♦ Electrons revolve around the nucleus. What is the charge of electrons?
neutral (no charge)
♦ Find the charge of atoms in the table given below.
• What is the reason for an atom to be electrically neutral?
An atom is neutral because the number of positive charges and negative charges is equal.
• What is the charge obtained if the number of electrons in an atom is greater than the number of protons?
Negative charge
• What is the charge obtained if the number of protons in an atom is greater than the number of electrons?
Positive charge
When rubbed, substances get charged due to the transfer of electrons. This changes the number of electrons in the atom.
| When an atom loses electrons, it becomes positively charged and when it gains electrons, it becomes negatively charged. |
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♦ Complete the table given below based on the discussions so far.
Proton
♦ Which is the negatively charged particle in an atom?
Electron
♦ Which is the uncharged particle in an atom?
Neutron
♦ If the number of electrons and protons in an atom are equal, the atom will be electrically ..........
Neutral
♦ Hang two rubber balloons touching each other. Rub them together with a piece of wool in between [Figure 10.7(a)]. Remove the wool. Can you see the balloons repelling each other? [Figure 10.7(b)]
Negative charge
♦ What can be understood from the repulsion between them?
Both balloons get negetive charge, and they repel each other.
♦ What can be understood from the repulsion between them?
Like charges repel.
♦ Bring the rubbed part of the wool near the balloon. What do you observe?
When the rubbed part of the wool is brought near the balloon, the balloon get attractd towards the wool.
♦ What charge does the wool gain after rubbing?
Positive charge
♦ What can be understood from the attraction between them?
Unlike charges attract.
♦ Activity
Do charged objects attract uncharged (neutral) objects?
Inference: Charged objects have the ability to attract neutral objects.
♦ Is attraction or repulsion the most suitable indication to confirm the charge of an object?
The most suitable indicator to confirm the charge of two objects is not attraction, but repulsion.
♦ Explain the following terms.
• Electricity - It is a form of energy produced due to the presence or flow of charges.
• Charging - The process of making objects charged through electron transfer is called charging.
• Insulators are substances that do not conduct electricity.
• Static electricity - If electric charge remains in the same place without being able to flow, such electricity is called static electricity.
♦ Which type of objects get an electric charge when rubbed?
Insulators
♦ If the number of electrons is less than the number of protons in an atom, the charge of that atom will be ..........
Positive
♦ How do atoms acquire an electric charge?
Due to electron transfer while rubbing
♦ How does an atom become negatively charged?
When the atom gains electrons.
♦ When a glass rod is rubbed with silk,
i) Silk loses electrons.
ii) Glass rod loses electrons.
iii) Silk gains negative charge.
iv) Glass rod gains negative charge.
a) i and ii are correct
b) ii and iii are correct
c) iii and iv are correct
d) iv and 1 are correct.
Answer: b) ii and iii are correct
♦ If a balloon is rubbed on wool, what charge does the wool acquire?
Positive
♦ If a negatively charged silk is brought near a balloon that was charged by rubbing on wool, they will ......... each other.
Repel
♦ If a charged glass rod is brought near charged wool, they will ......... each other.
Repel
♦ A charged object attracts another object. Then the second object must be .........
(oppositely charged, neutral, oppositely charged or neutral, having same charge).
Answer: oppositely charged or neutral
♦ The suitable indication to confirm the presence of an electric charge is .........
(Attraction, Repulsion, Neutralisation, None of these).
Answer: Repulsion
♦ Write the names of four pairs of objects that can be electrified by rubbing.
Glass rod and silk
Ebonite and wool,
Rubber and wool,
Plastic comb and hair.
♦ Rub a polyester cloth with a PVC pipe and a steel spoon. What difference will you observe?
Charge will form on the PVC pipe, but not on the steel spoon.
♦ What are the main constituents of an atom?
Proton, Electron, Neutron.
♦ How do suitable objects acquire an electric charge when rubbed?
Due to electron transfer during rubbing.
♦ Why is repulsion the most suitable indication to know the presence of charge, and not attraction?
A charged object will attract an uncharged (neutral) object. But repulsion occurs only between charged objects (with like charges).
♦ Hang two pith balls by a thread in such a way that they touch each other. Look at Figure 10.11 (a). Rub a glass rod with a silk cloth and touch it to the pith balls. What do you observe? Look at Figure 10.11 (b). Again, bring the glass rod near the pith ball as in the figure 10.11 (c).
• When two pith balls are touched with rubbed glass rod, they get repelled.
• In fig (c) the pith ball get repelled.
| After touching the glass rod | Observations |
|---|---|
| Between the pith balls (Figure 10.11b) | Attraction / Repulsion |
| Between the pith balls and glass rod (Figure 10.11c) | Attraction / Repulsion |
♦ What charge does a glass rod acquire when rubbed with a silk cloth?
When rubbed glass rod gets positive charge
♦ According to the table, what is the charge of the pith balls?
When the glass rod touches the pith balls, they get positive charge.
Both balls get positive charge.
They repel because the same charges repel.
♦ Were the pith balls charged here by contact?
Here, the pith balls get charged by contact.
| Charging by conduction is the method of charging an object by direct physical contact with a charged object. Here, the two objects that come into contact will have similar charges. |
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♦ The method of charging a metal sphere using a negatively charged glass rod is shown in the picture. Observe the picture and write down the different steps.
Step 2 - Remove the rod. Then the electrons spread over the metal sphere.
♦ Observe the picture of electrostatic spray painting of vehicles in workshops.
Charge of the paint droplets is negative.
• What is the charge of the part to be painted?
Body of the vehicle is positively charged.
• Why do paint droplets stick to the charged surface?
Because of the attraction of positive charges and negative charges, paint stick on the surface.
♦ The functioning of a photocopier machine is shown in the image. While printing, a distribution of positive charges can be seen on the surface of the drum in the shape of the characters to be photocopied (e.g., the letter A).
Toner particles are given negative charge.
• What causes the toner particles to stick to the drum as it rotates?
A positive charge is maintained on the surface of the drum. So the toner particles stick to the drum.
• What is the charge given to the paper?
The paper is given positive charge
• Why do the toner particles stick to the paper?
Because of the attraction of unlike charges.
♦ The image shows small sparks forming on the fingers while touching a doorknob. Discuss the reasons for this.
♦ The image shows charges flowing when a person who has accumulated a charge touches an electronic device. Could this potentially damage the electronic components in the circuit?
♦ A charged plastic pen or scale will attract pieces of paper. Now, touch this pen or scale on the ground. Does it still attract?
A charged plastic pen or scale will attract pieces of paper. After touching the pen on the ground, it doesn't attract pieces of paper.
♦ Earthing
Bringing an object in contact with the earth in this manner is called earthing.
♦ Through earthing, an object loses its charge. What is the reason for this?
The Earth is an electron bank. Due to its size and conductivity, the Earth can donate and receive electrons. Therefore, charged objects are neutralised when earthed.
♦ How are positively and negatively charged objects neutralised when earthed? Explain based on the images provided below.
• When a negatively charged object is earthed, electrons flow from the object to the earth, and thus the object gets neutralised.
♦ Observe a child sliding down a ride in the park. Isn’t the child’s hair unusually raised?
Through friction
• Can you suggest a method to neutralise the charge?
To neutralise the charge accumulated on the hair, touch the ground.
♦ Charging by induction
Charging by induction is the method of charging an object without direct physical contact of a charged object.
♦ Electroscope
An instrument used to detect and measure the electric charge of objects is called an electroscope.
♦ Let’s make an electroscope.
Make a small hole in the centre of the cork. Insert a part of a straw through the hole and secure it. Pass the copper wire through it. Make a ‘V’ shape at the end of the copper wire and attach two small pieces of aluminium foil to it. These are the ‘leaves’ of the electroscope. At the other end of the copper wire, stick a ball of aluminium foil. Close the bottle with the cork.
♦ Bring a charged glass rod near the metal sphere. What do you observe?
The metal leaves diverge.
♦ What is the reason?
It is because the metal ball gets a positive charge due to induction, and the leaves get negative charges. Due to the repulsion of the same charges, the leaves repel.
♦ Bring a neutral object nearby. Do the leaves diverge now?
When a neutral (chargeless) object is brought near the metal sphere the leaves will not diverge. Thus, we can detect the charged objects.
| When an electrically charged object is brought near the metal sphere of an electroscope, the divergence between its leaves changes. However, neutral objects cause no movement in the leaves. |
|---|
♦ Capacitor
A capacitor is a device for storing electric charge.
♦ Capacitance
The ability of a capacitor to store charge is called capacitance. Its SI unit is Farad (F).
♦ Dielectrics
The insulators used between the plates of capacitors are called dielectrics. Materials like paper, air, and polyester are commonly used as dielectrics.
♦ Observe the picture. How are charges distributed on a metal surface?
• In electrical conductors, the charge is distributed only on the surface.
• The amount of charge will be very high at the sharp edges.
♦ How does lightning occur?
Various air currents in the atmosphere cause positive or negative charges to accumulate in clouds. Electrostatic induction causes opposite charges to appear on objects on Earth. The electric charge thus stored is sufficient to make the air an electrical conductor. This causes electricity to flow through the air in flashes and produce light. This is lightning.
♦ Did you notice the charged clouds shown in the picture? Observe the picture and answer the questions given below.
Air currents in the atmosphere cause electric charges to develop and accumulate in clouds. The charges may be positive or negative.
• Why are the objects on Earth oppositely charged?
Electrostatic induction causes opposite charges to appear on objects on Earth.
♦ What all can we do to protect ourselves from lightning?
• Do not seek shelter under tall or isolated trees.
• Do not attempt to operate electrical appliances.
• Stay away from walls
• Stay inside the room
• Avoid contact with water
• If you are out of room, seek lower ground
• Stay away from doors and windows
• Do not lie on the concrete floor
• Avoid tall and isolated areas if you are outdoors
♦ Lightning Conductor
It is used to protect buildings from lightning. Its parts are a metal rod with a number of pointed tips, and it is earthed with a copper plate.
♦ Lightning Arrester
A lightning arrester is a device used to protect electrical systems from lightning.
♦ What first aid should we provide if a person is struck by lightning?
• The person struck by lightning should be laid flat in a well-ventilated area.
• Massage the entire body to warm it up.
• Provide artificial respiration.
• Apply intermittent pressure on the chest.
• Treat injuries and burns, fractures and bleeding.
• Give emergency medical help.
Let’s Assess
1. Assume you are given an object with a positive charge. Write down a suitable method to charge a metal sphere using this.
| Object | Charge to be gained | Suitable charging method |
|---|---|---|
| Metal sphere | Positive | Conduction |
| Metal sphere | Negative | Induction |
2. At petrol pumps, the nozzle used for filling petrol is earthed. Why?
The flow of petrol can generate static electricity. It may cause a spark and ignite the highly flammable petrol vapours. To prevent this, nozzles are earthed.
3. A plastic pen can be easily charged by rubbing it on hair. However, this is not possible when a steel spoon is used instead of this. What is the reason?
A steel spoon is a good conductor. When rubbing, electrons are transferred, and steel's free electrons can move easily throughout the metal.
4. If a positively charged metal object comes into contact with another negatively charged metal object of equal magnitude, determine the charge on both objects.
Both objects become neutral
5. Poorly earthed lightning conductors are more harmful than beneficial. What is the reason?
A poorly earthed lightning conductor is very dangerous because it cannot safely conduct current into the earth. This leads to high risks of fires, damage to buildings, electric shock, damage to equipments etc.
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