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7.1 ANALYSING ELECTRIC FIELDS AND CHARGE FLOW State the relationship between electron and electric current Where does charge come from? Unit of electric charge Matter is made up of tiny particles called atoms. At the center of the atom is the nucleus which is made up of protons and neutrons. Surrounding the nucleus are particles called electrons. A proton has a positive charge. An electron has an equal negative charge while a neutron is uncharged / neutral. 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

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Current Current, I = charge, Q = t Q I = Time, t The SI unit = ampere, A Activity 1 Aim To investigate the relationship between electric charges and electric current. Apparatus Van de Graaff generator, connection wires, microammeter Procedure 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. 30
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

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