Name : Teacher : Cheng Wui LeapSubject : Physics Second Term Class : U6 Physics 1Chapter : 12 Electrostatics (1) Lesson no: 1

Date: 3/1/13Time: 10.30pm -12.30pm

Second Term

960/2 Physics Paper 2

I Electricity and Magnetism

Written Test

Section A15 compulsory multiple-choice questions to be answered.

Section B2 compulsory structured questions to be answered.

Section C2 questions to be answered outof 3 essay questions.All questions are based ontopics 12 to 18.

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11/2 hours Central asseeement

2nd TermChapter Topic

Electrostatics 12.1 Coulomb's Law12.2 Electric Field12.3 Gauss' Law12.4 Electric Potential

Capacitors 13.1 Capacitance13.2 Parallel-Plate Capacitors13.3 Dielectrics13.4 Capacitors in Series and in Parallel13.5 Energy Stored in a Charged Capacitor13.6 Charging and Discharging of a Capacitor

Electric Current 14.1 Conduction of Electricity14.2 Drift Velocity14.3 Current Density14.4 Electrical Conductivity and Resistivity

Direct Current Circuit

15.1 Internal Resistance15.2 Kirchhoff's Laws15.3 Potential Divider15.4 Potentiometer and Wheatstone Bridge

Magnetic Fields 16.1 Concept of a Magnetic Field16.2 Force on a Moving Charge16.3 Force on a Current-Carrying Conductor16.4 Magnetic Fields Due to Currents16.5 Force Between Two Current-Carrying Conductors16.6 Determination of the Ratio elm16.7 Hall Effect

Electromagnetic Induction

17.1 Magnetic Flux17.2 Faraday's Law and Lenz's Law

17.3 Self-Inductance17.4 Energy Stored in an Inductor17.5 Mutual Induction

Alternating Current Circuits

18.1Alternating Current Through a Resistor18.2Alternating Current Through an Inductor18.3 Alternating Current Through a Capacitor18.4R-C and R-L Circuits in Series

12.1 Coulomb’s law1. The SI unit for electric charge is coulomb (C). There are two types of charges, positive and

negative charges.2. Electric charges can be found on electrons and protons. A proton has a positive charge of

+1.6 x10-19 C and an electron has a negative charge of-1.6 x 10-19 C.3. Like charges repel each other and unlike charges attract each other.4. The magnitude of the charge of a proton or an electron is denoted as e, where

e = 1.6 x 10 -19 C . The charge of an electron is denoted as - e and the charge of a proton is denoted as +e.

5. A point charge is an electric charge carried by a very small particle. The charge is concentrated at a point.

6. Coulomb's law states that the magnitude of the electric force between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.

7.

8. Coulomb's law only applies to point charges.9. An electric force is a vector and it has magnitude and direction. The magnitude of an

electric force between two point charges can be found using Coulomb's law.10. The direction of the electric force acting on a charge can be found by inspecting the type of

force between the charges.

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11. When using Coulomb's law to find the magnitude of the electric force between two charges, there is no need to substitute the sign of the charges into the formula.

12. The electric force between two point charges is also called the coulomb's force.

(a) In Figure 12.2, the arrows indicate the directions of the forces acting on each of the charges.

(b) In each figure, the two forces are equal in magnitude but opposite in direction. The forces obey Newton's third law of motion.

13. Table 12.1 shows the comparison between electric and gravitational forces.Electric force Gravitational forceInversely proportional to the square ofseparation of the two charges

Inversely proportional to the square ofseparation of the two masses

Can be attractive or repulsive Attractive only STPM Practice 1

1. The following figure shows two point charges separated by a distance of 20.0 mm.

Calculate(a) the force acting on Q1,

(b) the force acting on Q2.

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2. Three point charges, Q1= +35 C, Q2 = +47 C and Q3 = -53 C, are arranged along a straight line in vacuum as shown in the following figure.

Determine the resultant force acting on Q1.

3. Three point charges, Q1= +2 C, Q2 = -3 C and Q3 = -4 C, are placed at the vertices of a right-angled triangle in vacuum as shown in the figure below.

Calculate the resultant force acting on Q3.

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4. The force acting between two point charges is F and the separation between the two charges is r.What is the force between the two point charges if(a) the separation of the two charges is increased to 2r?

(b) the separation of the two charges is increased by 20%?

(c) the separation of the two charges is decreased by 30%?

(d) one of the charges is doubled?

5. The figure shows two spherical objects, each with a mass of 2.5 g. The two objects have equal charges and are suspended by insulated threads. The length of each thread is 15.0 cm and the two threads are suspended at an angle of 60° to each other.[Assuming the two objects behave like point charges]

(a) Calculate the tension in one of the threads.

(b) Show that the electrostatic force between the two objects is 1.4 x 10-2 N.

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(c) Calculate the charge on each object.

6. The figure below shows three point charges, Q1, Q2 and Q3, lying on a straight line where Q1= –30 C and Q3 = –70 C.

If the net force acting on Q2 is zero, find the value of y.

12.2 Electric Field 12.2.1 Electric Field

1. Electric field An electric field is a region in which an electric charge experiences a force. An electric field is represented by electric field lines.

2. The electric field strength at a point in an electric field is the force per unit charge.The symbol for the electric field strength is E and its unit is N C-1 (or V m-1). The electric field strength, E, is a vector quantity.

3. The direction of E at any point is the same as the direction of the force acting on a positive test charge placed at that point.

The direction of E at any point is the same as the direction of the field line at that point.4. If a charge Q is placed at a point which has an electric field intensity of E, then the force F

acting on the charge is given by

The equation, F = EQ, is only used to find the magnitude of the force F. The direction of the force F is determined by inspection.

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5. The direction of electric force F on a positive charge Q is the same as the direction of the field line. The direction of electric force F on a negative charge Q is opposite to the direction of the field line.

6. Figure 12.19 shows the direction of the electric force on an electron in an electric field.

7. Figure 12.20 shows the direction of the electric force on a proton in an electric field.

8. Figure 12.21 shows the direction of the electric force on an alpha particle in an electric field.

STPM Practice 21. A point charge of -75 C is placed at a point having an electric field strength of 4 400 N

C-1 directed in an upward direction. Determine the magnitude and direction of the force acting on the charge.

2. If the force acting on a point charge, Q = -4.0 X 10-7 C, is 0.02 N directed to the right, determine the direction and magnitude of the electric field strength at that point.

3. A point charge of +15 C experiences a force of 0.20 N to the right when placed at a point in an electric field. What is the magnitude and direction of the electric field strength at that point?

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4. A polystyrene ball coated with a metallic coating, has a mass of 4 g and carries a charge Q. It is suspended by a thread in a uniform electric field of 6 400 N C-1. When the system is in equilibrium, the thread is inclined at 15° with the vertical. Find the charge Q.

12.2.2 Electric Field by a Point Charge1. Figure 12.24 shows the electric field patterns produced by a positive point charge and a

negative point charge respectively.

2. Electric dipoleAn electric dipole is a positive charge and a negative charge, of the same magnitude, separated by a distance.

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3. Derivation of electric field strength formula at a point in an electric field produced by a point charge Q.

(a) E at a point has magnitude and direction.

(b) The magnitude of E is calculated using the formula (c) The direction of vector E at a point follows the direction of the field lines created by the point charge Q.

STPM Practice 31. Find the magnitude and direction of the electric field strength at a point P, 50 cm from a

point charge of -40 C.

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2. The figure below shows two point charges placed on a straight line.

(a) Find the magnitude and the direction of the resultant electric field strength at X.

(b) What is the magnitude and direction of the force experienced by a +5 C point charge placed at point X?

3. The figure below shows three charges placed at the three vertices of a right-angled triangle.

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4. What is the electric field strength that acts on an electron in a hydrogen atom? Orbital radius of the electron in a hydrogen atom is 0.052 nm. [e =1.6 x10-19 C]

5. The figure below shows two point charges Q1 and Q2, separated by a distance r.

Write an expression for(a) the electric field strength that acts on the charge Q1,(b) the electric field strength that acts on the charge Q2.

6. The figure shows two point charges, Q1 = +30 µC and Q2 = +90 C, placed 1.2 m apart.

Find a location along the line joining the two charges where the electric field is equal to zero.

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7. Figure 12.35 shows two point charges, Q1= +30 C and Q2 = +50 C, placed at 2.0 m apart.

Find the location of a point where the electric field strength is zero.

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