1. P.1 Book 4 Section 1.2 Electric field Electroreception What is an electric field? Electric field lines Electric field pattern Check-point 4 Electric field strength Check-point 5 1.2Electric field

2. P.2 Book 4 Section 1.2 Electric field Electroreception Sharks have special receptors inside the small pores around their heads. Do you know what an electric field is?  very sensitive to electric fields  can detect electric fields of prey’s muscle contractions / nerves signalling  locate the prey

3. P.3 Book 4 Section 1.2 Electric field 1 What is an electric field? Two charged objects close to each other experience electric force even if they are not in contact. Electric field:  Region around charged object.  When another charged object enters the field, it will experience electric force.  The size of force depends on field’s strength at that point.

4. P.4 Book 4 Section 1.2 Electric field 2 Electric field lines Use field lines to represent an electric field:  direction of field lines =direction of electric force on +ve test charges  closely-spaced  greater field strength widely-spaced  weaker field strength  go from a +ve charge to a –ve charge

5. P.5 Book 4 Section 1.2 Electric field 2 Electric field lines Also known as: lines of force  A test (+ve) charge in the field will experience a force directed along the field line. 1.2 Electric field Simulation

6. P.6 Book 4 Section 1.2 Electric field 3 Electric field pattern a Electric field around a point charge For an isolated +ve point charge, electric field lines always direct radially outwards from the charge. For an isolated –ve point charge, electric field lines always direct radially inwards into the charge.  the field becomes weaker as the distance from the charge  and the field lines become more widely-spaced.

7. P.7 Book 4 Section 1.2 Electric field 3 Electric field pattern a Electric field between two parallel charged plates For two oppositely charged parallel plates close to each other, the electric field between them is uniform.  parallel and evenly-spaced field lines  force on charge between them is constant Electric field patterns between objects of different shapes are different. Electric field patterns Expt 1c

8. P.8 Book 4 Section 1.2 Electric field Experiment 1c Electric field patterns 1.Set up the electric field apparatus on an overhead projector. 2.Pour castor oil into the dish and dip a pair of electrodes in the oil. 3.Connect the electrodes to the EHT supply. Sprinkle some semolina on the oil. Switch on the supply and set the voltage to 5 kV. Observe what happens.

9. P.9 Book 4 Section 1.2 Electric field Experiment 1c Electric field patterns 4.Repeat with electrodes of other shapes. 1.4 Expt 1c - Electric field patterns Video

10. P.10 Book 4 Section 1.2 Electric field 3 Electric field lines Different electric field patterns: Note: Field patterns are in fact 3-dimensional.

11. P.11 Book 4 Section 1.2 Electric field Check-point 4 – Q1 A point charge is placed at point P in a uniform electric field. (a) What is the direction of electric force acting on the charge if the charge is +ve? A Towards the right B Towards the left C No electric force

12. P.12 Book 4 Section 1.2 Electric field Check-point 4 – Q1 A point charge is placed at point P in a uniform electric field. (b) What is the direction of electric force acting on the charge if the charge is –ve? A Towards the right B Towards the left C No electric force

13. P.13 Book 4 Section 1.2 Electric field Check-point 4 – Q2 Which is the correct electric field pattern if the +ve charge is removed? A B

14. P.14 Book 4 Section 1.2 Electric field Check-point 4 – Q3 How do the field lines show the direction and the strength of an electric field? The arrows of the field lines show the direction of the force acting on a ( positive / negative ) point charge at that point. And the denser the field lines, the _____________ the electric field. stronger

15. P.15 Book 4 Section 1.2 Electric field 4 Electric field strength Electric field strength: the electric force experienced by a unit +ve test charge. If +ve charge q is in an electric field, it experiences an electric force F. Then, the electric field strength E at that point is defined as E =E = FqFq Unit: N C –1 Note: Electric field strength is a vector.

16. P.16 Book 4 Section 1.2 Electric field 4 Electric field strength a Point charge Consider a +ve point charge Q and a test charge q. Since the force F acting on q is, electric field strength E due to a point charge Q is Qq 4  0 r 2 E =E = Q40r 2Q40r 2 Electric field strength of the hydrogen nucleus Example 6

17. P.17 Book 4 Section 1.2 Electric field Example 6 Electric field strength of the hydrogen nucleus In a hydrogen atom, an e – moves around the proton in a circular orbit of radius 5.29  10 –11 m. Find the electric field strength due to the proton at the position of the e –. Charge of a proton = 1.6  10 –19 C  0 = 8.85  10 –12 C 2 N –1 m –2

18. P.18 Book 4 Section 1.2 Electric field Example 6 Electric field strength of the hydrogen nucleus Electric field strength = = = 5.14  10 11 N C –1 (away from the proton) Q 4  0 r 2 1.6  10 –19 4  (8.85  10 –12 )(5.29  10 –11 ) 2

19. P.19 Book 4 Section 1.2 Electric field 4 Electric field strength a Point charge Electric field strength of two point charges Example 7

20. P.20 Book 4 Section 1.2 Electric field Example 7 Electric field strength of two point charges Point charges A (2  10 –8 C) and B (–2  10 –8 C) are separated by 30 cm. X and Y are points on the perpendicular bisector of the line joining the charges. (Given  0 = 8.85  10 –12 C 2 N –1 m –2 ) (a) State the directions of electric field at X and Y. The electric field points to the left at X and Y.

21. P.21 Book 4 Section 1.2 Electric field Example 7 Electric field strength of two point charges (b) Find the electric field strength at Y. AY = BY = cm = 0.15 m 30 2 For test charge q placed at Y, electric force due to A = Q 1 Q 2 4  0 r 2 = (7.99  10 3 )q (towards left) 2  10 –8  q 4  (8.85  10 –12 ) (0.15) 2 =

22. P.22 Book 4 Section 1.2 Electric field Example 7 Electric field strength of two point charges Similarly, force due to B = (7.99  10 3 )q(towards left) Total electric force on q = (7.99  10 3 )q + (7.99  10 3 )q = (1.60  10 4 )q(towards left)  Electric field strength = = 1.60  10 4 N C –1 (towards left) FqFq

23. P.23 Book 4 Section 1.2 Electric field Example 7 Electric field strength of two point charges (c) If charge on B becomes 2  10 –8 C, (i) find the directions of the electric field at X and Y. At X, electric field due to A points left at 45  to the horizontal, and that due to B points right at 45 . Q A = Q B and AX = BX  same electric field strength  resultant field points upwards No electric field at Y ( ∵ neutral point)

24. P.24 Book 4 Section 1.2 Electric field Example 7 Electric field strength of two point charges (ii) how will a neutral particle move at X ? The neutral particle carries no charge.  Electric field exerts no force on it. By Newton’s 1st law of motion, the neutral particle remains at rest or in uniform motion. (c) If charge on B becomes 2  10 –8 C,

25. P.25 Book 4 Section 1.2 Electric field 4 Electric field strength b Parallel charged plates If the separation between two oppositely charged parallel plates is small, the electric field between them is uniform. The electric field strength E depends on the surface charge density . (i.e. charge per unit surface area = Q /A ) E =E = 00 Unit of  : C m –2

26. P.26 Book 4 Section 1.2 Electric field Check-point 5 – Q1 A charge q (−2  10 –8 C) experiences an electrostatic force of 1.2  10 –3 N towards the left. Electric field strength at the position of q = ? The electric field strength is ________ N C –1 pointing to the _______. 6  10 4 right Electric field strength FqFq = = 1.2  10 –3 −2  10 –8 = –6  10 4 N C –1

27. P.27 Book 4 Section 1.2 Electric field Check-point 5 – Q2 P is 30 cm away from a point charge (4  10 –8 C). Electric field strength at P due to the charge = ? (Given  0 = 8.85  10 –12 C 2 N –1 m –2 ) Q 4  0 r 2 Electric field strength = = = 4  10 –8 4  (8.85  10 –12 )(0.3) 2 4.00  10 3 N C –1

28. P.28 Book 4 Section 1.2 Electric field Check-point 5 – Q3 There are two oppositely charged parallel metal plates. The quantity of charge on P is 5  10 –8 C. Surface area of each plate is 0.01 m 2. quantity of charge surface area = (a) Find the surface charge density of P. Surface charge density  = = 5  10 –8 0.01 = 5  10 –6 C m –2

29. P.29 Book 4 Section 1.2 Electric field Check-point 5 – Q3 There are two oppositely charged parallel metal plates. The quantity of charge on P is 5  10 –8 C. Surface area of each plate is 0.01 m 2. = (b) Find the electric field strength between the plates. (Given  0 = 8.85  10 –12 C 2 N –1 m –2 ) Electric field strength = 5  10 –6 8.85  10 –12 = 5.65  10 5 N C –1 00

30. P.30 Book 4 Section 1.2 Electric field The End