Unit 7 Electricity - Past Paper PDF

Unit 7: Electricity Contents: Definition of Electricity Electric Charge Properties of electric charge The Electric Force : Coulomb's Law Mathematical Formulation of Coulomb's Law 8 Questions on Coulombs Law Electric...

Unit 7: Electricity Contents: Definition of Electricity Electric Charge Properties of electric charge The Electric Force : Coulomb's Law Mathematical Formulation of Coulomb's Law 8 Questions on Coulombs Law Electric Field Electric Field due to a point charge Electric field lines 6 Questions on Electric Filed 12/6/2024 Dr. Khaled Ilaiwi 1  Definition of Electricity ❖ Electricity is phenomenon associated with stationary or moving electric charges. ❖ Many devices depend on the physics of electromagnetism (combination of electric and magnetic phenomena). This physics is the root of all technological devices presents today such as computers, television, medical instruments, telecommunications, house lighting. ❖ Phenomena of electric force discovered in ancient Greek times (700 BC). ❖ Presence of electric force is due to the Electric Charge 12/6/2024 Dr. Khaled Ilaiwi 2 Electric Charge ❖ Electric charge come from the particles making up the basic unit of matter (the atom) ❖ Atom is made of: The atom +ve charge is that carried by protons -ve charge is that carried by electrons electrically neutral neutrons In atom. The protons and neutrons are packed tightly together in a central nucleus. ❖ Motion of electrons from one atom in an object to an other in other object produce electrically charged object and thus produce electricity. ❖ Hence electricity or electrical energy is the energy caused when moving electrons from one place to an other 12/6/2024 Dr. Khaled Ilaiwi 3 Properties of electric charge ◼ types of charges: ◼ Positive charge (+ve) ◼ Negative charge (-ve) Charged object When +ve and –ve charges are equal within object → electrically neutral object. If object charge is not balanced → charged object The charge manifest it self by attraction or repulsion force between charged materials → Like charges repel and Unlike charges attract attraction repulsion Properties of electric charge ◼ Electric charge is not created nor destroyed; Usually, negative charge is transferred from one object to the other. ➔ Electric charge is always conserved Charge is quantized ➔ Any charged object must have a charge of ±e, or ± 2e, or ± 3e, etc., but not say ± 1.5e. Hence the charge : q = ± ne (with n =1, 2, 3, …) is an integer Proton has a charge +1e. Electron has a charge –1e. The value of elementary charge e is : |e| = 1.6 x 10-19 C. A neutral atom has positive charges = negative charges. SI unit of electrical charge is coulomb (C) and usually denoted by q.  The Electric Force : Coulomb's Law ◼ Coulombs law: when two charges brought together and separated by a distance r, there exist an electric force between these charges ◼ Charles Coulomb discovered in 1785 the fundamental law of electrical force between two stationary charged particles. An electric force has the following properties: Inversely proportional to the square of the separation, r, between the particles, and is along a line joining them. Proportional to the product of the magnitudes of the charges |q1| and |q2| on the two particles. Attractive if the charges are of opposite sign and repulsive if the charges have the same sign. It is conservative force Mathematical Formulation of Coulomb's Law ◼ Mathematical formulation for q1 q2 coulomb’s law is → electrostatic r force has a magnitude of (N = Newton) ◼ Two charges act on each other same force but opposite in direction (Newton’s third law) Charges have Charges have same sign ➔ different sign ➔ repulsive force attractive force  8 Questions on Coulombs Law 1.Coulomb's law applies to which type of charges? a) Only positive charges b) Only negative charges c) Both positive and negative charges d) Neutral charges 2. According to Coulomb's law, the force between two charges is directly proportional to: a) The distance between the charges b) The product of the charges c) The square of the distance between the charges d) The sum of the charges 3. If the distance between two charges is doubled, the force between them becomes: a) Four times larger b) Two times larger c) Half as large d) One-fourth as large 4. What is the SI unit of the constant k in Coulomb's law? a) N m² C⁻² b) N m² kg⁻² c) N C⁻² d) C m² N⁻² 5.Two charges, q1=2 μC, and q2=−3 μC, are separated by a distance of 0.5 m. What is the magnitude of the electrostatic force between them? (Given k=9×109 N m2 C-2 ) a) 54 N b) 108 N c) 27 N d) 72 N 6. If the force between two charges is 20 N and the distance between them is halved, what will be the new force? a) 40 N b) 80 N c) 100 N100 d) 160 N 7. The force between two charges is 36 N when they are 2 m apart. What will be the force if the distance is increased to 6 m ? a) 12 N b) 9 N c) 4 N d) 1 N 8. Two charges, q1=5 μC and q2=10 μC, experience a force of 90 N. What is the distance between the charges? a) 0.1 m b) 0.2 m c) 0.3 m d) 0.4 m 12/6/2024 Dr. Khaled Ilaiwi 8 Electric Fields Electric force is a non-contact type force → there must be a field surrounding the charges. This field is called Electric Field If another charged object enters a region where an electrical field is present→ it will be subjected to an electrical force. Direction of E-field defined as the direction of the electrical force exerted on a small +ve test charge (q) placed at that location. ◼ Electric field points away ◼ Electric Field points towards a from a +ve charge -ve charge Electric Field due to a point charge ( SI unit is N/C) If q is +ve → E is directed away from q If q is -ve → E is directed towards q ◼ Hence the force on any charged particle, with charge q existing in any electric field E, is If q is +ve → force has same direction of E If q is -ve → force has opposite direction to E Electric Field Lines A convenient way to visualize electric field patterns is to draw lines in the direction of the electric field. ◼ Field lines start from positive charges an end at negative ones. ◼ E-field increase as spacing between lines decrease ◼ Field lines can not cross. Examples on E-field lines: 1) Electric field lines of single positive (a) and negative (b) charges. (a) (b) Electric Field Lines 2) two point charges Two opposite charges of equal magnitude Two opposite charges of different magnitude: number Two similar charges of equal of lines leaving +2q > magnitude (EA>EB>EC=0) number of lines entering -q Electric Field Lines 3) plates Positively charged plate: E-field lines away from plate two oppositly charge plates: parallel lines E-field away from +ve plate towards –ve plate negatively charged plate: E-field lines towards plate  6 Questions on Electric Filed 1. What is the SI unit of the electric field? a) Newton (N) b) Volt (V) c) Newton per Coulomb (N/C) d) Coulomb (C) 1. The electric field due to a positive charge at a point is directed: a) Towards the charge b) Away from the charge c) Parallel to the charge d) None of the above 2. Which of the following statements about the electric field is correct? a) Electric field is a scalar quantity. b) Electric field is independent of the magnitude of the charge producing it. c) Electric field is inversely proportional to the square of the distance from the charge. d) Electric field depends only on the medium, not the charge. 3. A point charge q=5 μC creates an electric field of magnitude 450 N/C at a certain point. What is the distance of this point from the charge? (Given k=9×109 N m2/C2) a) 1 m b) 2 m c) 3 m d) 4 m 5. Two charges, q1=2 μC and q2=4 μC, are placed 0.3 m. What is the magnitude of the electric field at the midpoint between the charges? a) 4×104 N/C b) 6×104 N/C c) 12×104 N/C d) 8×104 N/C 6. If the electric field at a point is 20 N/C and the charge placed at that point is 2 μC, what is the force experienced by the charge? a) 10 N10 b) 0.04 N c) 0.02 N d) 0.004 N 12/6/2024 Dr. Khaled Ilaiwi 14