CHAPTER 1 ELECTROSTATICS How charges interact? · PDF fileChapter 1 Electrostatics Page 5 ... 5 Electric Field ... Chapter 1 Electrostatics Page 11 Electric potential energy by charged

New Senior Secondary Physics Compulsory: Electricity and Magnetism Chapter 1 Electrostatics

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CHAPTER 1 ELECTROSTATICS

1 How charges interact?

Force between charges

Earthing

2 Conductors and Insulators

Difference between conductors and insulators

Conductors and Insulators under attraction


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3 Method of Charging

Charging by Sharing (Charge to same polarity) Given a positively charged conducting sphere, describe how you can charge another conducting sphere with same polarity.

Charging by Induction (Charge to opposite polarity) Given a positively charged conducting sphere, describe how you can charge another conducting sphere with opposite polarity.


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[HKCEE] [Charging] In the above diagram, X and Y are two insulated uncharged metal sphere in contact with each other. A negatively charged object C brought near X. X is touched momentarily with a finger, and the X and Y are separated. The charge on X and Y are Sphere X Sphere Y A. zero negative B. positive negative C. negative negative D. positive zero E. positive positive

[HKCEE] [Charging] A positively charged rod is brought near an insulated uncharged metal sphere as shown above. If the sphere isearthed with a finger, A. positive charges flow from the earth to the sphere. B. electrons flow from the earth to the sphere. C. electrons flow from the sphere to the earth. D. the sphere is completely discharged. E. there is no charge flow between the sphere and the earth.

[HKCEE] [Charging]

An insulated charged metal rod is brought near to insulated uncharged metal sphere as shown above, Which ofthe following can charge the sphere? (1) Touching the sphere momentarily with a finger (2) Touching the sphere momentarily with the rod (3) Touching the rod momentarily with a finger and then touching the sphere momentarily with the same finger A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3)


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4 Force on Charges – Coulomb’s Law

Coulomb’s Law– Electric Force by Point Charge

Calculation Example – Classical Hydrogen Model (a) Calculate the attraction force between nucleus and electron in a hydrogen atom. (b) Calculate the classical velocity of the electron orbiting the nucleus.

Daily life applications of electrostatics


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[HKALE] [Electrostatic Force]

The figure shows two small charged spheres Pand Q suspended by identical nylon threads from a fixedpoint O. At equilibrium, the threads OP and OQ makeangles α and β ( )α β< with the vertical.

Which of thefollowing conclusions must be correct? 1) Both P and Q carry positive charges. 2) The charge on P is greater than that on Q in magnitude. 3) The mass of P is greater than that of Q. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only

5 Electric Field

Concept of electric field

Definition of electric field


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Electric Field Pattern Draw the electric field patterns around the following configurations.

[HKALE] [Electric Field]

The straight lines in thediagram represent electricfield lines. Which of thefollowing statements aboutthis electric field is/arecorrect? 1) A stationary negativecharge placed at Qtends to move to P. 2) The electric fieldstrength at P isstronger than that at Q. 3) Work has to be done inmoving a negativecharge from R to P. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3)


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Electric field by point charge

Electric field by charged sphere


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Electric field by charged plate

Electric field by parallel charged plates


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[HKALE] [Electric Field] The electric field strength near the surface of acharged sphere is E. The charge on the sphereand its radius are doubled. What is the newelectric field strength near the sphere’s surface? A. E/4 B. E/2 C. E D. 2E


Two insulated parallel metal platesare connected to the terminals of anEHT. When a charged aluminum foilstrip is placed between the plates,deflection of the foil is observed asshown. Which of the following statementsis/are TRUE? 1) The charge on the foilis negative. 2) Deflection of the foilincreases if theseparation between theplates decreases. 3) When moving the foiltowards the positiveplate, the deflection ofthe foil increases. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3)


A negatively charged oil drop is kept stationary between twohorizontal metal plates connected to a d.c. supply as shown. Theoil drop then acquires an additional negative charge.Which ofthe following changes will be able to hold the oil drop stationary? 1) Disconnecting the plates from the supply and moving the plates closer 2) Keeping the separation between the plates unchanged and increasingthe p.d. between the plates 3) Keeping the p.d. between the plates unchanged and moving the platesfurther apart A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only


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6 Electric potential energy

Analogy from Gravitational Potential Energy

Electric potential energy by a point charge/charged sphere


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Electric potential energy by charged parallel plates

[HKALE] [Electric Potential Energy] Five identical point charges, each of charge Q, are fixed evenly on a circle of radius r. How much workhas to be done to bring another point charge Q frominfinity to the centre of the circle?

A. 0 B. 0

5

4

Q

rπε C.

2

0

5

4

Q

rπε D.

2

20

5

4

Q

rπε E.

20

5

4

Q

rπε


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[HKALE] [Electric Potential Energy] Permittivity of free space = 8.85 x 10-12 F m-1. Aconducting sphere of radius 0.1 m carries a positivecharge of 10-4 C. A particle P of mass 2 x 10-5 kgcarrying a negative charge of 10-10 C is released fromrest at a distance of 1 m from the center of the sphere.If the force due to gravity is neglected, the velocity ofP when it strikes the surface of the sphere will be A.2.8 m s-1 B. 9.0 m s-1 C. 29.8 m s-1 D.81.0 m s-1 E. 890 m s-1

[HKALE] [Electric Potential Energy] In the figure, P, Q are two equal positivepoint charges. X, Y are two points on theperpendicular bisector of the line joining P andQ. Which of the following statements must becorrect? 1) The electric field at X is greater than that at Y. 2) The electric potential at X is higher than that atY. 3) Positive work must be done to bring a positivecharge from X to Y. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only


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7 Electric Potential Definition of Electric Potential

Electric potential by a point charge/charged sphere

Electric potential by a parallel charged plate


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[HKALE] [Electric potential]

In the above figure, a point charge +Q is placed at A.The resulting electric potential at B is V. If a pointcharge -2Q is now placed at C, the mid-point between Aand B, what is the electric potential at B produced byboth point charges? (Assume that the electric potentialat infinity is zero.) A. -4V B. -3V C. -2V D. 0

[HKALE] [Electric potential] A metal sphere is charged to a potential of 100 V. Ifthe charge density on its surface is 6.0 x 10-9 C m-

2,find the radius of the sphere.(Given : permittivity of free space = 8.85 x 10-12 F m-1) A. 0.04 m B. 0.15 m C. 0.21 m D. 0.35 m E. 0.60 m


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Charges +2 µ C and -2µ C are situated at pointsP and Q respectively. X is the mid-point of PQ. Whichof the following conclusions is/are correct? (Theelectric potential at infinity is taken to be zero.) 1) The electric field at X is pointing towards Q. 2) The electric potential at X is zero. 3) The electric field strength at X is the strongest amongany point between P and Q. A. (1) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3)


A positively-charged metal sphere A ofradius a is joined by a conducting wire toan uncharged metal sphere B of radius bplaced far away from the first sphere.The ratio of the surface charge density Ato that on sphere B is

A. b

a B.

2

2

b

a C.

a

b D.

2

2

a

b E.

b

a


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8 Equipotential Surfaces

Electric Field Pattern Draw the equipotential surfaces around the following configurations.


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[HKALE] [Equipotential Surface] In the figure, the solid curvesare concentric circlesrepresenting a set ofequipotential surfaces in anelectric field. The dotted curveABC represents the path of acharged particle moving in thefield. Which of the followingdeductions from the figure is/arecorrect? (Neglect the effects ofgravity.) 1) The charged particle isalways repelled from thecentre of the concentriccircles. 2) The speed of the chargedparticle at A is equal to thatat C. 3) The kinetic energy of thecharged particle at B isgreater than that at A. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only

9 Putting Everything Together

The theoretical connections


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[HKALE 2006]

In a vacuum, a beam of electrons with a horizontal velocity 3.7 × 107 m s-1 enters midway into a region of electric field between two horizontal square metal plates as shown in the figure below. The length of the side of the plates is 10 cm. A p.d. of 320 V is applied across the plates and the separation between them is 1.6 cm.

(Given: mass of electron = 9.11 × 10-31 kg, electron charge = -1.60 × 10-19 C; permittivity in vacuum = 8.85 × 10-12 F m-1)

(a) Find the electric field strength between the plates. (2 marks) (b) The electron beam reaches one of the plates at a distance d from the plate's left edge.

(i) Find the distance d and sketch in the above figure the path of the electron beam between the plates. (Neglect the weight of the electron.) (5 marks)

Diagram NOT

drawn to scale 320 V 1.6 cm

10 cm

v = 3.7 ×××× 107 m s-1

electron

New Senior Secondary Physics CompulsoryChapter 1 Electrostatics

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(ii) Deduce the corresponding distance d if the applied p.d. is doubled.

(c) Calculate the charge on the upper plate.

[HKALE 1998] A sheet of carbon paper of uniform conductivity has been painted with two parallel, rectangular silver strips as shown in Figure 1 below. Points 6 V. The negative terminal of an ideal voltme

Figure 1 (a) What is the voltmeter reading when

moved along B1B2?

Compulsory: Electricity and Magnetism

Deduce the corresponding distance d if the applied p.d. is doubled.

Calculate the charge on the upper plate.

A sheet of carbon paper of uniform conductivity has been painted with two parallel, rectangular silver strips as shown in Figure 1 below. Points Ao and Bo on the two strips are connected to a battery of e.m.f.

V. The negative terminal of an ideal voltmeter is connected to Ao and its positive terminal is a probe

What is the voltmeter reading when P is connected to B1? What would be observed when

Deduce the corresponding distance d if the applied p.d. is doubled. (2 marks)

(2 marks)

A sheet of carbon paper of uniform conductivity has been painted with two parallel, rectangular silver on the two strips are connected to a battery of e.m.f.

and its positive terminal is a probe P.

? What would be observed when P is (2 marks)

New Senior Secondary Physics CompulsoryChapter 1 Electrostatics

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(b) Now the probe is moved along distance x of the probe from

(i) Explain the variation of (ii) Find the electric field strength at point (iii) Neglecting edge effect, sketch on Figure 1 the equipotential line at 1 V intervals between the

two silver strips. (c) An additional silver is now painted in between the two strips present as shown in Figure 2. Figure 2 (i) Sketch on the graph in (b) the variation of voltage (2 marks) (ii) State and explain the change, if any, in the electric field strength at point

FINAL REMARKS

The study of electrostatics forms the basic theory of all sense that it is an inverse square law with simple charge interaction. However, the theoretical terms of force field and potential can be a huge difficulty for students. Students are advised to think more and try to interpret the phenomena in correlation with Newton

Compulsory: Electricity and Magnetism

Now the probe is moved along AoBo and the voltage reading V recorded is plotted against the of the probe from Ao ?

Explain the variation of V along AoBo.

Find the electric field strength at point X. State its direction.

Neglecting edge effect, sketch on Figure 1 the equipotential line at 1 V intervals between the

An additional silver is now painted in between the two strips present as shown in Figure 2.

Sketch on the graph in (b) the variation of voltage V with distance

State and explain the change, if any, in the electric field strength at point

study of electrostatics forms the basic theory of all electric phenomena. The force law itself is simple in the sense that it is an inverse square law with simple charge interaction. However, the theoretical

ial can be a huge difficulty for students. Students are advised to think more and try to interpret the phenomena in correlation with Newton’s law.

recorded is plotted against the

(2 marks)

(3 marks)

Neglecting edge effect, sketch on Figure 1 the equipotential line at 1 V intervals between the (2 marks)

An additional silver is now painted in between the two strips present as shown in Figure 2.

with distance x of the probe from Ao.

State and explain the change, if any, in the electric field strength at point X. (2 marks)

phenomena. The force law itself is simple in the sense that it is an inverse square law with simple charge interaction. However, the theoretical understanding in

ial can be a huge difficulty for students. Students are advised to think more and try

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CHAPTER 1 ELECTROSTATICS How charges interact? · PDF fileChapter 1 Electrostatics Page 5 ... 5 Electric Field ... Chapter 1 Electrostatics Page 11 Electric potential energy by charged