Section 14.2 Where Charges Come From? Section 14.2 Where Charges Come From?

Section 14.2 Where charges come from? • Charge in atom • Conservation of charge

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14.2 Where charges come from? (SB p. 6)

Charge in atom All substances are made up of atoms nucleus

neutrons - neutral

atom

electrons – negative charge

-

-

protons – positive charge

+++ +

-

-

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14.2 Where charges come from? (SB p. 6)

Class Practice 1: Fill in the blanks with correct terms: electrons, neutrons, nucleus, proton Ans wer nucleus proton neutron electron © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 6)

Charge in atom Unit of charge : coulomb (C) ++ + + -

-

1 proton: 1.6 × 10–19 C 1 neutron: 0 C 1 electron: –1.6 × 10–19 C

charge is negative © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 6)

Charge in atom -

-

++++

Normally in atoms, no. of protons = no. of electrons amount of +ve charges = amount of -ve charges no net charge ⇒ electrically neutral Thinking 2 © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 7)

Losing an electron charges proton electron net charge

4 × (1.6 × 10–19 C) 3 × (–1.6 × 10–19 C) +1.6 × 10–19 C

+ve charge > -ve charge -

positively charged

electron lost © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 7)

Gaining an electron charges proton electron net charge

4 × (1.6 × 10–19 C) 5 × (–1.6 × 10–19 C) –1.6 × 10–19 C

+ve charge < -ve charge electron gained

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-

negatively charged 7

14.2 Where charges come from? (SB p. 7)

Charge in atom -

charged neutral object object

charged

charged neutral object object

discharged

-

charging process → transfer of electrons

-

++ + +

-

Thinking 3 © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 7)

Conservation of charge total electric charges in isolated system → constant transfer of electron

-

-

++ + + -

-

-

++ + + -

-

-

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Activity 2

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To section 14.3

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14.2 Where charges come from? (SB p. 6)

Thinking 2

State one similarity and one difference of the charges of an electron and a proton. Equal in magnitude but opposite in sign.

Ans wer

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14.2 Where charges come from? (SB p. 7)

Thinking 3

Inside an atom, negative electrons are circulating around the positive nucleus. What are the forces between them?

Ans wer

Attractive electrostatic forces exist between them.

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14.2 Where charges come from? (SB p. 8)

Activity 2 Analogue of charges in atom

Let’s start: 1. Place a green plastic transparent sheet above a red one. Cut the sheets in the same shape. What is the colour of the combined sheets? Black © Manhattan Press (H.K.) Ltd.

Ans wer

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14.2 Where charges come from? (SB p. 8)

Activity 2 (Cont) Analogue of charges in atom

2. Consider the combined sheets to be an “atom”. Let the red sheet carries positive charges and the green sheet carries negative charges in the “atom”. What can be deduced from step 1? Inside an atom, there is an equal amountAns wer of positive and negative charges. Therefore, an atom is neutral. © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 8)

Activity 2 (Cont) Analogue of charges in atom

3. Now slowly shift the green sheet to the right slightly. (a) What is the relation between the size of the red colour part on the left and that of the green colour part on the right? Ans Red colour is seen on the left with an wer equal proportion of the green colour on the right. © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 9)

Activity 2 (Cont) Analogue of charges in atom

(b) What can be deduced from (a)? Is the “atom” charged? Ans When one side of an atom has more wer negative charges, the other side should have an equal amount of positive charges. The “atom” does not carry net charge overall and it is neutral. © Manhattan Press (H.K.) Ltd.

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14.2 Where charges come from? (SB p. 9)

Activity 2 (Cont) Analogue of charges in atom

4. Now cut a small piece from the green sheet. If an “atom” loses negative charges, what charges does it carry? It carries net positive charges. © Manhattan Press (H.K.) Ltd.

Ans wer 17

14.2 Where charges come from? (SB p. 9)

Activity 2 (Cont) Analogue of charges in atom

5. State any discrepancy between the plastic sheets model and a real atom. Ans In a real atom, the positive chargeswer are concentrated at the centre rather than distributed over the atom. The electrons carrying negative charges are orbiting around the positive charges. Return to © Manhattan Press (H.K.) Ltd.

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