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# Static Electricity

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1. Static Electricity

2. Understand the nature of electron charge and field lines. Success Criteria • Can identify the symbol for electric charge. • Can describe the accumulation of negative charge in terms of loss or gain of electrons. • Can recall the unit and symbol of electric charge. • Can recall the amount of electrons in one Coulomb. • Can work out the magnitude of a charge when given the number of electrons present. • Can draw the direction of field lines when given point charges. • Can identify a situation where a uniform field is present and one where a radial field is present

3. Electric Charge • A positive electric charged is produced a point has lost electrons. • A negative charge is produced when there is a net gain in electrons. positive - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + loses electrons negative gains electrons

4. The Coulomb • The symbol for the quantity charge is q and it is measured in coulombs (C). • A charge (q) of -1.00 C is equal to the charge of 6.241 x 1018 electrons. • An electric charge can either be positive or negative. Exercise A balloon has a charge of - 3.50 μC. How many extra electrons are on the surface of the balloon? When charging a ruler on a silk cloth 6.52 x 1011 electrons leave the ruler. What its charge?

5. Field lines • Point charges produce field lines. These field lines flow from a positive charge to a negative charge. - - + +

6. Field lines • Draw the electric field lines present in the following situations. B A - - - + + + C

7. Uniform Electric Field • Two parallel plates form an electric field separated by a distance, d and attached to a battery with a voltage, V produce a Uniform Electric Field, E. • At any point between the two plates the strength of the electric field, E acting on a charge is constant. At the edges the electric field is no longer uniform, it is radial. V=12V d=5mm + + + + + + + E=2400Vm-1

8. Can work out the strength of an electric field Success Criteria • Can define what electric field strength is. • Can solve electric field problems involving the voltage, electric field strength and distance between two plates. • Can describe how an electric charge can be used to work out the strength of an electric field. • Can explain why the direction of the force on an electron is opposite to the direction of the electric field. • Can work out the strength of an electric field when given the force and a charge.

9. Electric Field Strength Voltage measured in V Electric field strength measure in Vm-1 or NC-1 distance measured in m For a constant voltage. The shorter the distance (d) between two the stronger the electric field(E). For plates kept at a constant distance. The larger the potential difference/voltage (V) the stronger the electric field(E).

10. Example Exercises 1 What is the strength of an electric field produced when 15 V is applied across two plates separated by 4.0 mm? 4.0mm 15V 2 What is the distance between two plate that produce a 2000 Vm-1 electric field when 3.5V is applied. + + + + + + +

11. 3 What voltage is needed to produce a 5200Vm-1 electric field between two plates 6.0mm apart? 4 If the voltage applied to two plates is kept constant what happens to the electric field strength if the two plates are pulled further apart? • Since the voltage stays constant, • The distance increases. • The Electric field strength is inversely proportional to the distance so it (E) deceases. + + + + + + +

12. Force on a charge in an electric field F F - + A positive charge has an electrostatic force acting on it in the direction of the electric field. This force, F is proportional to the charge, q of the particle and the strength of the electric field, E. The force acting on a negatively charged particle acts in the opposite direction to the electric field. + + + + + + +

13. Electric field strength measured in Vm-1 or NC-1 Force measured in N Charge measured in C If the charge is constant. If the electric field, E increases the force will increase proportionally. If the electric field, E is constant. The larger the charge, q the larger the force, F.

14. Example Exercises 1 What is the force acting on a particle with a +9.23 nC charge place in between two plates with a uniform electric strength of 275 Vm-1? q= +9.23 nC E = 275 Vm-1 2 What is the strength of an electric field that applies a 24.5μN force to a particle with a charge of 300 nC?. + + + + + + +

15. 3 If the electric field strength on a charge is doubled what happens to the electrostatic force the charge feels? • Since the charge stays constant, • The field strength increases. • The force is proportional to the electric field strength so it (F) increases. 4 What happens to the force on a charge in a uniform electric field if the distance between the two plates is halved? If we assume that the V stays constant, E ∝ 1/d ∴ when the distance halves, E doubles. If we also assume q stays constant, F ∝ E ∴ when the electric field strength doubles the force doubles.

16. Define electric potential in terms of work and energy.Use V = work/q and V = ∆Ep/q to calculate change in electric potential energy Success Criteria • Can work out the change in potential energy using • Recognise that when a charge does work(W) by moving against electric field lines it gains potential energy(∆Ep). • Recognise that when a charge moves with the field lines it loses potential energy and gains kinetic energy. • Can use to work out the potential energy a charge has • Can describe the relationship between work, Ek and Ep.