__Electric Charge__

1.

**What is an electric charge?**An**electric charge**is anything that**produces an electric field**in the space around it. It can be**positive**or**negative**.
2.
An

*

**electric charge**can be: an electron, a proton, a nucleus (an alpha particle i.e. helium nucleus), an ion, a charged molecule or any charged object such as a charged cloud, a plastic comb, an electrode or the charged dome of the Van de Graaff generator:*

**Like charges repel**
*

**Unlike charges attract**
3.
When there is a

**flow of electric charge**, there is an**electric current**– this is shown by the “**Van de Graaff Generator and Galvanometer**” experiment (pg. 43 of F5 textbook).
4.

**Electric charge Q**is a derived quantity and its unit of measurement**coulomb C**is a derived unit:- The
amount of
**electric charge Q**flowing through a point in a circuit is given by**the product of current, I, and time**of flow of the current,**t**, across the point, that is:

**Q = I x t …………….(Yr 2005 SPM P1 Q42 at pg. 12)**;

**(Yr 2007 SPM P1 Q38 at pg. 100)**

Alternatively,

**Q = ne**

(where n = number of electrons or protons; e = charge of an electron or proton)

- Accordingly
(from
**Q = I x t**),**electric current**is the**rate of charge flow of electric charge**:

**I = Q / t…………….. (Yr 2011 SPM P1 Q35 at pg. 288)**

- Notwithstanding
the foregoing, students must know that
**electric current**is a**base quantity**(i.e. a fundamental physical quantity). And,**SI**(International System of Units) defined the base unit for electric current**ampere, A**in terms of its**electromagnetic force effect**(rather than rate of flow of electric charge), as follows:

“

**One ampere**is the constant current which - if maintained in 2 straight parallel conductors of infinite length, of negligible cross-sectional area, and placed 1 metre apart in a vacuum -**produces a force**of**2 x 10**of length.”^{-7}newton per metre
5.
An electron carries an

**electric charge****e**=**- 1.6 x 10**^{-19}**C**. If a steady current of 1 A is flowing for 1 second past a point in a circuit:- the
**electric charge**flowing past is Q = I x t = 1 A x 1 s = 1 C. And, - therefore,
the
**number of electrons**flowing past the point to give**1 C**of**electric charge****=**1 / 1.6(10^{-19})**=****6.25 x****10**electrons.^{18}

__Electric Field__

1.

**What is an electric field?**An**electric field**is the**space around**a positive or negative**electric charge**in which any other electric charge would experience an electrical force acting on it.

__(Yr 2005 SPM P1 Q45 at pg. 13)__
2.
An

**electric field**can be represented by**arrowed lines**known as the**electric field lines**or the**electric lines of force**.
3.
By convention, the

**direction of electric field**lines is from**positive****to negative**– that is, the**path**that would be**taken by**a**positive test**charge if placed**in the electric field**.
4.
The

**closer**the**electric field lines**, the**stronger**the**electric force**.
5.
An

**electric field line**is a**vector quantity**because it has magnitude and direction of force.
6.
The

**pattern of electric field lines**depends on the**shape**and the**number**of the charged objects in the vicinity (Refer: “Semolina Powder on Olive Oil and Metal Electrodes” experiment for the various patterns – pg.349)**(Yr 2008 SPM P1 Q35 at pg. 147)****The Electric Field Pattern:**
7.
The effect of electric field on a charge can be
illustrated by

i.
“Polystyrene (Ping-Pong) Ball” Experiment (Click the link to see video showing this) (or, see pg.
350 of Fajar Oxford Reference Book); and,

**(Yr 2010 SPM P1 Q33 at pg. 239)**
ii.
“Candle Flame” Experiment (pg.351 or pg 46 of F5 textbook).

**(Yr 2011 SPM P1 Q37 at pg. 288)**

**(Yr 2010 SPM P2 CQ12(a) at pg. 264)**

**Candle Flame in the electric field**- Heat energy from the candle flame produces ionization of air molecules to form positive and negative ions.
- Movement of positive ions which are heavier towards the negative plate causes a bigger spread of the flame.
- Negative electrons which are lighter move towards the positive plate and causing a smaller spread of the flame. (E.H.T. voltage supply means "extra high tension" voltage supply)

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Updated by: tutortan1@gmail.com(23/06/16)

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