4.2 Electrical quantities

4.2.1 Electric charge

1. Objects can be given one of two types of electric charge:

• Positive charges: the type of electric charge carried in the nucleus of an atom
• Negative charges: the type of electric charge carried by electrons

2. Unlike charges attract and like charges repel

3. Experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges

Experiment 01:

Producing and Observing Electrostatic Charges by Friction

Objective:
To demonstrate how friction produces electrostatic charges and to observe attraction and repulsion.

Materials:

• 2 Plastic rods
• Cloth
• Cradle or support to suspend the rod

Procedure:

a. Charge the Rod:

• Rub the plastic rod with a cloth. Both the rod and the cloth become charged.

b. Test Attraction:

• Hang the rod in a cradle so it can move freely. Bring the charged cloth close to the rod. Observe that the rod moves towards the cloth.

c. Test Repulsion:

• Rub a second plastic rod with the same cloth. Bring it near the suspended rod. Observe that the two rods repel each other.

Observations:

• The rod and cloth attract each other (opposite charges).
• The two rods repel each other (same charges).

Explanation:

• Rubbing transfers negative charges (electrons) to the rod, making it negatively charged. The cloth becomes positively charged due to a loss of electrons.
• Similar charges repel, and opposite charges attract.

Conclusion:

• Like charges repel (negative-negative or positive-positive).
• Unlike charges attract (negative-positive).

4. Explain that charging of solids by friction involves only a transfer of negative charge (electrons)

Friction and Charging

1. What Happens During Charging?
   • Friction transfers electrons (negative charges) between two materials.
   • Example: When a polythene rod is rubbed with a cloth, electrons move from the cloth to the rod.

2. How Objects Get Charged:
   • Gaining electrons → becomes negatively charged.
   • Losing electrons → becomes positively charged.
   • Only electrons are transferred during charging. Positive charge(Proton) is not transferred.

Key Points:

• Electrons are negatively charged and loosely held on the outside of atoms.
• The atom’s nucleus is positively charged.
• An atom is neutral when it has an equal number of positive (protons) and negative charges (electrons).

Explaining Charging with Atomic Structure

1. Neutral Atom:
   • An atom consists of:
     • Protons: Positively charged particles in the nucleus.
     • Electrons: Negatively charged particles orbiting the nucleus.
     • Neutrons: Neutral particles in the nucleus.
   • In a neutral atom:
     • The number of protons = The number of electrons.
     • Overall charge = Neutral (0).

2. Losing an Electron (Positive Ion Formation):
   • When an atom loses an electron, the balance is disrupted:
     • Protons > Electrons.
     • Overall charge = Positive (+).
   • Example:
     • A neutral sodium (Na) atom has 11 protons (+11) and 11 electrons (-11).
     • If it loses 1 electron, it becomes Na⁺ with 11 protons (+11) and 10 electrons (-10).

3. Gaining an Electron (Negative Ion Formation):
   • When an atom gains an electron, the balance is disrupted in the opposite way
     • Electrons > Protons.
     • Overall charge = Negative (-).
   • Example:
     • A neutral chlorine (Cl) atom has 17 protons (+17) and 17 electrons (-17).
     • If it gains 1 electron, it becomes Cl⁻ with 17 protons (+17) and 18 electrons (-18).

Key Examples of static electricity

1. Plastic Comb and Hair:
   • When you comb your hair, the comb becomes negatively charged (gains electrons), and your hair becomes positively charged (loses electrons).
   • Observation: The comb can attract small pieces of paper because of the static charge.

2. Balloon and Wool Cloth:
   • Rubbing a balloon on a wool cloth transfers electrons from the cloth to the balloon.
   • The balloon becomes negatively charged, while the wool cloth becomes positively charged.
   • Observation: The balloon sticks to walls or attracts your hair.

3. Walking on a Carpet (Static Shock):
   • Friction between your shoes and the carpet transfers electrons to you.
   • Result: Your body becomes negatively charged.
   • When you touch a metal doorknob, the electrons discharge suddenly, causing a static shock.

5. Describe an experiment to distinguish between electrical conductors and insulators

Experiment 2:

Distinguishing Between Electrical Conductors and Insulators

Objective:
To test materials and classify them as conductors or insulators.

Materials:

• Battery (cell)
• Lamp
• Connecting wires with crocodile clips
• Various materials to test (e.g., metal, plastic, wood, glass)

Steps:

1. Set up the circuit:
   • Connect the battery and lamp using wires to form a circuit. Ensure the lamp lights.
2. Introduce a gap:
   • Remove one wire to create a gap in the circuit. Observe that the lamp no longer lights.
3. Test materials:
   • Attach a material in the gap using crocodile clips.
   • Observe if the lamp lights:
     • If it lights: The material is a conductor.
     • If it does not light: The material is an insulator.
4. Record results:
   • Create a table listing the materials tested and whether they are conductors or insulators.

Conclusion:

• Conductors allow electricity to pass through them (e.g., metals like copper and aluminum).
• Insulators block electricity (e.g., plastic, rubber, glass).

6. Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples

Conductors and Insulators: Using a Simple Electron Model

1. What Are Conductors?
   • Metals are good conductors because they have free electrons that can move easily between atoms.
   • Examples: Copper, gold, aluminum.
   • When a metal is charged, the electrons move through the metal and into the Earth (unless insulated).
2. What Are Insulators?
   • Non-metals are insulators because their electrons are tightly bound to their atoms.
   • Examples: Plastic, rubber, glass.
   • In insulators, charges stay localized because electrons cannot move freely.
3. Key Differences:
   • Conductors: Allow the flow of electrons (electric current).
   • Insulators: Prevent the flow of electrons.

7. State that charge is measured in coulombs

• Electric charge is a property of matter, measured in coulombs (C).
• The coulomb is named after Charles-Augustin de Coulomb, who discovered the relationship between the force, size of charges, and their distance apart.

Key points:

• Charge of one electron = -1.6 x 10⁻¹⁹ C
• Charge of one proton = +1.6 x 10⁻¹⁹ C
• It takes over 6 million million million electrons to make 1 coulomb of charge.

Key point:

Coulomb (C): SI unit of electric charge.

8. Describe an electric field as a region in which an electric charge experiences a force

• An electric field is a region where an electric charge feels a force, even without contact.
• A charged object can affect nearby charged or uncharged objects via its electric field.

9. State that the direction of an electric field at a point is the direction of the force on a positive charge at that point

• The direction of an electric field is defined by the force experienced by a positive charge placed in the field.

Key Points:

• Positive charges: Field lines point away from the charge.
• Negative charges: Field lines point toward the charge.

10. Describe simple electric field patterns, including the direction of the field:

a. Around a point charge

• Positive charge: Field lines radiate outwards.
• Negative charge: Field lines radiate inwards.

b. Around a charged conducting sphere

• Similar to a point charge:
  • Positive sphere: Field lines radiate outward.
  • Negative sphere: Field lines radiate inward.
• Field lines are always perpendicular (at right angles)to the surface of a conducting sphere