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JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 7: Electricity

CHAPTER 7: ELECTRICITY
7.1 CHARGE AND ELECTRIC CURRENT

Van de Graaf

1. What is a Van de Graaff generator? Fill in each of the boxes the name of the part shown. A device that produces and store electric charges at high voltage on its dome

+ + + + roller

+

+

+ + + +

Metal dome dome

rubber belt roller

motor

-1-

JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 7: Electricity

2. How are electrical charges produced by a Van de Graaff generator? And what type of charges is usually produced on the dome of the generator?  When the motor of the Van de Graaff generator is switched on, it drives the rubber belt.  This causes the rubber belt to against the roller and hence becomes charged.  The charge is then carried by the moving belt up to the metal dome where it is collected.  A large amount of charge is built up on the dome  Positive charges are usually produced on the dome of the generator.

+ 3. What will happen if the charged dome of the Van de Graaff is connected to the earth via a micrometer? Explain.  There is a deflection of the pointer of the meter.  This indicates an electric current flow. +

+ + +

+ +

-2-

JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 7: Electricity

4. Predict what will happen if a discharging metal sphere to the charged dome. + +  When the discharging metal sphere is brought near the charged dome, + + + + +

sparkling occurs.  An electric current flow.

5.

Predict what will happen if hair of a student is brought near to the charged dome. Give reasons for your answer.  The metal dome attracts the hair and the hair stand upright.  This is because of each strand of hair receives positive charges and repels each other.

6. The flow of electrical charges produces electric current.

JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 7: Electricity

Electric Current

1. Electric current consists of a flow of electrons 2. The more charges that flow through a cross section within a given time, the larger is the current. 3. Electric current is defined as the rate of flow of electric charge Each second, 15 coulombs of charge cross the plane. The current is I = 15 amperes. One ampere is one coulomb per second.

4. In symbols, it is given as: where I = electric current

I=Q t

Q = charge t = time

(i) The SI unit of charge is (Ampere / Coulomb / Volt) (ii) The SI unit of time is (minute / second / hour) (iii) The SI unit of current is (Ampere / Coulomb / Volt) is equivalent to (Cs // C-1s // Cs-1) (iv) By rearranging the above formula, Q = ( It / t

I

t
/ I )

4. If one coulomb of charge flows past in one second, then the current is one ampere. 5. 15 amperes means in each second, 15 coulomb of charge through a cross section of a conductor. 6. In a metal wire, the charges are carried by electrons. 7. Each electron carries a charge of 1.6 x 10-19 C. 8. 1 C of charge is 6.25 x 1018 electrons.

JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 7: Electricity

Electric Field a) An electric field is a region in which an electric charge experiences a force. b) An electric field can be represented by a number of lines indicate both the magnitude and direction of the field c) The principles involved in drawing electric field lines are : (i) electric field lines always extend from a positively-charged object to a negatively-charged object to infinity, or from infinity to a negatively-charged object, (ii) electric field lines never cross each other, (iii) electric field lines are closer in a stronger electric field.

Demo 1 : To study the electric field and the effects of an electric field. Apparatus & materials Extra high tension (E.H.T) power supply (0 – 5 kV), petri dish, electrodes with different shapes (pointed electrode and plane electrode), two metal plates, talcum powder, cooking oil, polystyrene...
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