SLBS PHYSICS DEPARTMENT ELECTRIC FIELDS. PLEASE ANSWER ALL MULTIPLE CHOICE QUESTIONS (Section A) IN THE GRID PROVIDED BELOW

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1 NME:.. PHYSIS TEHER: TEHING GROUP:... ISSUE TE:.. HN IN TE:.. SLS PHYSIS EPRTMENT ELETRI IELS PLESE NSWER LL MULTIPLE HOIE QUESTIONS (Section ) IN THE GRI PROVIE ELOW TO HELP YOU REVISE ROM THIS WORK T LTER TE IT IS REOMMENE THT YOU NNOTTE THE QUESTIONS GIVING ETILS ON HOW YOU RRIVE T YOUR HOSEN NSWER NSWER THE REMINING QUESTIONS IN THE SPE PROVIE IN THIS OOKLET Instructions for the completion of Section or each question, 4 suggested answers are given on the question paper. HOOSE ONE LETTER ONLY for this question, and show your choice clearly on this sheet. EXMPLE: If you think is the correct answer to Question 1, SHE the letter like this:- 1 It is sensible to use a PENIL so that you can rub out a response if you wish to change your mind. If you shade TWO OR MORE letters for any question, your response will be marked wrong even if one of the answers is correct Page 1 of 15 Electric ields (v3) T onnolly (SLS Physics)

2 SETION Q1 The electric potential at a distance r from a positive point charge is 45 V. The potential increases to 50 V when the distance from the charge decreases by 1.5 m. What is the value of r? 1.3 m 1.5 m 7.9 m 15 m Q Q3 Q4 n electron and a proton are 1.0 x m apart. In the absence of any other charges, what is the electric potential energy of the electron? +.3 x J.3 x J +.3 x 10 8 J.3 x 10 8 J repulsive force acts between two positive point charges separated by a distance r. What will be the force between them if each charge is doubled and the distance between them is halved? 4 16 The repulsive force between two small negative charges separated by a distance r is. What is the force between the charges when the separation is reduced to 3 r? Q5 The force between two point charges is when they are separated by a distance r. If the separation is increased to 3r, what is the force between the charges? 3r 9r 3 9 Page of 15 Electric ields (v3) T onnolly (SLS Physics)

3 Q6 The diagram shows two particles at a distance d apart. One particle has charge +Q and the other Q. The two particles exert an electrostatic force of attraction,, on each other. Each particle is then given an additional charge +Q and their separation is increased to a distance d. Which one of the following gives the force that now acts between the two particles? an attractive force of 4 a repulsive force of 4 an attractive force of a repulsive force of Q7 The distance between two point charges of + 8.0n and +.0n is 60mm. t a point between the charges, on the line joining them, the resultant electric field strength is zero. How far is this point from the + 8.0n charge? 0 mm 5 mm 40 mm 45 mm Q8 The diagram shows two charges, +4µ and 16µ, 10 mm apart. What is the distance from the +4µ charge to the point between the two charges where the resultant electric potential is zero? 4 mm 40 mm 80 mm 96 mm Page 3 of 15 Electric ields (v3) T onnolly (SLS Physics)

4 Q9 The diagram shows four point charges at the corners of a square of side a. What is the electric potential at P, the centre of the square? Q pe a Q 0 0 pe a Q pe a 0 Q 4pe a 0 Q10 t a distance R from a fixed charge, the electric field strength is E and the electric potential is V. Which line, to, in the table gives the electric field strength and electric potential at a distance R from the charge? electric field strength E E E 4 E 4 electric potential V 4 V V V 4 Q11 Which one of the following cannot be used as a unit for electric field strength? Jm 1 1 J 1 s 1 m 1 N 1 s 1 J m 1 Page 4 of 15 Electric ields (v3) T onnolly (SLS Physics)

5 Q1 What is the acceleration of an electron at a point in an electric field where the field strength is 1.5 x 10 5 Vm 1? 1. x 10 6 ms 1.4 x ms.7 x ms.6 x ms Q13 Two protons are 10 x m apart. pproximately how many times is the electrostatic force between them greater than the gravitational force between them? (Use the ata and ormulae booklet) Q14 Two parallel metal plates separated by a distance d have a potential difference V across them. What is the magnitude of the electrostatic force acting on a charge Q placed midway between the plates? VQ d VQ d VQ d Qd V Page 5 of 15 Electric ields (v3) T onnolly (SLS Physics)

6 Q15 small object O carrying a charge +Q is placed at a distance d from a metal plate that has an equal and opposite charge. The object is acted on by an electrostatic force. Which one of the following expressions has the same unit as? e Q 0 d e0q d Q e d 0 Q e d 0 Q16 n a particle travels towards a gold nucleus and at P reverses its direction. Which one of the following statements is incorrect? The electric potential energy of the a particle is a maximum at P. The kinetic energy of the a particle is a minimum at P. The total energy of the a particle is zero. The total energy of the a particle has a constant positive value. Q17 Which one of the following statements about a charged particle in an electric field is correct? No work is done when a charged particle moves along a field line. No force acts on a charged particle when it moves along a field line. No work is done when a charged particle moves along a line of constant potential. No force acts on a charged particle when it moves along a line of constant potential. Q18 Which one of the following does not involve a centripetal force? an electron in orbit around a nucleus a car going round a bend an a particle in a magnetic field, travelling at right angles to the field an a particle in an electric field, travelling at right angles to the field Page 6 of 15 Electric ields (v3) T onnolly (SLS Physics)

7 Q19 Which one of the following statements about electric field strength and electric potential is incorrect? Electric potential is a scalar quantity. Electric field strength is a vector quantity. Electric potential is zero whenever the electric field strength is zero. The potential gradient is proportional to the electric field strength. Q0 The diagram shows the path of an a particle deflected by the nucleus of an atom. Point P on the path is the point of closest approach of the a particle to the nucleus. Which one of the following statements about the a particle on this path is correct? Its acceleration is zero at P. Its kinetic energy is greatest at P. Its speed is least at P. Its potential energy is least at P. Page 7 of 15 Electric ields (v3) T onnolly (SLS Physics)

8 SETION Q1 igure 1 shows an electron at a point in a uniform electric field at an instant when it is stationary. igure 1 (a) (i) raw an arrow on igure 1 to show the direction of the electrostatic force that acts on the stationary electron. (a) (ii) State and explain what, if anything, will happen to the magnitude of the electrostatic force acting on the electron as it stalls to move in this field. ( marks) (b) igure a shows a stationary electron in a non-uniform electric field. igure b shows a stationary proton, placed in exactly the same position in the same electric field as the electron in igure a. igure a igure b (b) (i) State and explain how the electrostatic force on the proton in igure b compares with that on the electron in igure a. ( marks) Page 8 of 15 Electric ields (v3) T onnolly (SLS Physics)

9 (b) (ii) Each of the particles starts to move from the positions shown in igure a and igure b. State and explain how the magnitude of the initial acceleration of the proton compares with that of the electron. ( marks) (b) (iii) escribe and explain what will happen to the acceleration of each of these particles as they continue to move in the electric field. (c) ( marks) The line spectrum of neon gas contains a prominent red line of wavelength 650 nm. (c) (i) Show that the energy required to excite neon atoms so that they emit light of this wavelength is about ev. (3 marks) (c) (ii) n illuminated shop sign includes a neon discharge tube, as shown in igure 3. pd of 4500 V is applied across the electrodes, which are 180mm apart. igure 3 ssuming that the electric field inside the tube is uniform, calculate the minimum distance that a free electron would have to move from rest in order to excite the red spectral line in part (c). answer =. m (3 marks) Page 9 of 15 Electric ields (v3) T onnolly (SLS Physics)

10 Q (a) efine the electric potential at a point in an electric field (3 marks) (b) igure 1 shows part of the region around a small positive charge. igure 1 (b) (i) The electric potential at point L due to this charge is V. alculate the magnitude Q of the charge. Express your answer to an appropriate number of significant figures. (b) (ii) Show that the electric potential at point N, due to the charge, is +1.0 V. answer =... (3 marks) (b) (iii) Show that the electric field strength at point M, which is mid-way between L and N, is.5 Vm 1. Page 10 of 15 Electric ields (v3) T onnolly (SLS Physics)

11 (c) R and S are two charged parallel plates, 0.60m apart, as shown in igure. They are at potentials of V and V respectively. igure (c) (i) On igure, sketch the electric field between R and S, showing its direction. ( marks) (c) (ii) Point T is mid-way between R and S. alculate the electric field strength at T. answer =... Vm 1 (c) (iii) Parts (b)(iii) and (c)(ii) both involve the electric field strength at a point mid-way between potentials of V and V. Explain why the magnitudes of these electric field strengths are different. Page 11 of 15 Electric ields (v3) T onnolly (SLS Physics)

12 Q3 igure 1 shows a small polystyrene ball which is suspended between two vertical metal plates, P1 and P, 80 mm apart, that are initially uncharged. The ball carries a charge of 0.17 µ. igure 1 (a) (i) pd of 600 V is applied between P1 and P when the switch is closed. alculate the magnitude of the electric field strength between the plates, assuming it is uniform. answer = Vm 1 ( marks) (a) (ii) Show that the magnitude of the electrostatic force that acts on the ball under these conditions is 1.3 mn. Page 1 of 15 Electric ields (v3) T onnolly (SLS Physics)

13 (b) ecause of the electrostatic force acting on it, the ball is displaced from its original position. It comes to rest when the suspended thread makes an angle q with the vertical, as shown in igure. igure (b) (i) On igure, mark and label the forces that act on the ball when in this position. ( marks) (b) (ii) The mass of the ball is 4.8 x 10 4 kg. y considering the equilibrium of the ball, determine the value of q. answer = degrees (3 marks) Page 13 of 15 Electric ields (v3) T onnolly (SLS Physics)

14 Q4 small negatively charged sphere is suspended from a fine glass spring between parallel horizontal metal plates, as shown in igure 1. (a) Initially the plates are uncharged. When switch S is set to position X, a high voltage dc supply is connected across the plates. This causes the sphere to move vertically upwards so that eventually it comes to rest 18 mm higher than its original position. (a) (i) State the direction of the electric field between the plates.. (a) (ii) The spring constant of the glass spring is 0.4 N m 1. Show that the force exerted on the sphere by the electric field is 4.3 x 10 3 N. (a) (iii) The pd applied across the plates is 5.0kV. If the charge on the sphere is 4.1 x 10 8, determine the separation of the plates. (b) Switch S is now moved to position Y. answer =.m (3 marks) (b) (i) State and explain the effect of this on the electric field between the plates. ( marks) Page 14 of 15 Electric ields (v3) T onnolly (SLS Physics)

15 (b) (ii) With reference to the forces acting on the sphere, explain why it starts to move with simple harmonic motion. (3 marks) Page 15 of 15 Electric ields (v3) T onnolly (SLS Physics)