# Pizik

Pages: 24 (6007 words) Published: February 18, 2011
Discuss your physics questions online at www.physics4spm.com

7.1 ANALYSING ELECTRIC FIELDS AND CHARGE FLOW
State the • Matter is made up of tiny particles called atoms. relationship between • At the center of the atom electron is the nucleus which is and electric made up of protons and current neutrons. • Surrounding the Where nucleus are particles does called electrons. charge • A proton has a positive come charge. from? • An electron has an equal negative charge while a neutron is uncharged / neutral. Unit of electric charge • • • • Electric charge is denoted by the symbol Q. The unit of electric charge is the coulomb , C. Charge on one electron = - 1.6 x 10-19 C Charge on one proton = 1.6 x 10-19 C

A body is: (a) neutral, if it has equal numbers of positive and negative charges. (b) charged negative, if it has more negative than positive charges. (atom gains electron) (c) charged positive, if it has more positive than negative charges. (atom losses electron) • The force acting on two bodies of the same net charges will repel each other. • The force acting on two bodies of different net charges will attract each other. • The force causes movement of electrons or flow of charges. Electric • The rate of flow of electric charge 29

www.physics4spm.com

Discuss your physics questions online at www.physics4spm.com

Current • Current, I = charge, Q = Time, t • The SI unit = ampere, A Activity 1 Aim Apparatus

I=

Q t

To investigate the relationship between electric charges and electric current. Van de Graaff generator, connection wires, microammeter

• Start the motor of a Van de Graaff generator for a few minutes to produce positive charges on the metal dome of the generator. • Bring your finger close to the dome of the generator. Observe what happens. • Touch the dome of the generator with the free end of the wire that is connected to the microammeter. Observe the microammeter needle closely. • Switch off the motor of the Van de Graaff generator. Observation 1. You will feel a brief electric shock when your finger is brought close to the dome of the generator. 2. The microammeter needle is deflected when a wire is connected to the dome of the generator. 3. You can safely touch the metal dome with your finger. 4. The microammeter needle is returned to its zero position when the Van de Graaff is switched off. Procedure www.physics4spm.com

30

Discuss your physics questions online at www.physics4spm.com

Explanation 1. When the motor of the Van de Graaff generator is switched on, it drives the rubber belt. This cause the rubber belt to rub against the roller and hence becomes positively charged. The charge is then carried by the moving belt up to the metal dome where it is collected. A large amount of positive charge is built up on the dome. 2. The electric field around the metal dome of the generator can produced a strong force of attraction between the opposite charges. Electrons will suddenly accelerate from the finger to the dome of the generator and causes a spark. 3. When the wire touches the dome, the microammeter needle is deflected. This shows that a current is flowing through the galvanometer. 4. The electric current is produced by the flow of electrons from earth through the galvanometer to the metal dome to neutralize the positive charges on its surface. 5. The metal dome can be safely touched with the finger as all the positive charges on it have been neutralized. Conclusion A flow of electric charge (electrons) through a conductor produces an electric current.

Describe an An electric field is a region in which an electric charge electric experiences an electric force (attraction or repulsion). filed. • An electric field is created from a positively charged sphere in the spaces surrounding it. • A negative charged body when placed at any point in this region is pulled towards the charged sphere – attractive force • A positively charged body that is placed in the same region is pushed...

Please join StudyMode to read the full document