1.

a. A complete circuit consists of both the

**Electromotive force**V(e.m.f.) of an**electrical source**is defined as the**work done**(W) by the source**in driving per unit charge****around a complete circuit**:**V(e.m.f.) = W/Q = E/Q**.a. A complete circuit consists of both the

**internal**(within the source) and the**externa**l circuits (outside the source):
b.
An

**electrical source**can be a**cell,**a**battery**or any other source of electricity.
c.
A

**cell**uses**chemical reaction**to**produce current –**it converts chemical energy to electrical energy. A cell produces one directional current known as the DC current.
d.
A

**battery**is a combination of**2 or more cells in series.**
2.
Since it is energy that enables work to be done,

**electromotive force V(e.m.f.)**can be alternatively viewed as the**electrical energy E**supplied or used by**the source to drive****per unit charge**around a complete circuit:
V(e.m.f.)
= W/Q = E/Q

V(e.m.f.)
= W/It (since, Q = It as I = Q/t)

**V(e.m.f) = P/I**(since power P = W/t)

Therefore,

**electromotive force**may also be defined as the ratio of**the total power supplied**to the**whole circuit****to**the**current**flowing through it.
3.

**Electromotive force**V(e.m.f.) may be measured by:
i.
A high-resistance

**voltmeter**:**By measuring the****potential difference**of the cell or electrical source in an**open circuit**– this is however**not the true value**because a small current still flows through the voltmeter and part of the electromotive force becomes part of the potential difference across the voltmeter itself;
ii.
A cathode ray oscilloscope; or

iii.
A potentiometer.

4.
Experimental evidence (pg. 379) shows that:

i.

**Electromotive force V(e.m.f.)**is (approximately) the**potential difference**across the cell or source of electricity in an**open circuit**– when**no current**flows through the**external circuit**or through any external components__.__**(Yr 2005 SPM P1 Q38 at pg. 11)**
ii.

**Electromotive force V(e.m.f.)**is**not the same**as**terminal potential difference Vt**– i.e. it is**not equal to**the**potential difference**across a**closed external circuit**through which current is flowing:
V(e.m.f.)
>

**terminal potential difference Vt:****V(e.m.f) > Vt or Vt < V(e.m.f) …(**

__Yr 2011 SPM P1 Q36 at pg. 288)__
5

**.****Lost volt**or voltage drop**(Vd)**refers to:**The****difference**between electromotive force**E**(or, V(e.m.f.) when circuit is open and terminal potential difference**Vt**(when circuit is closed) (Vd = V(e.m.f.) – Vt). Lost volt (Vd) is due to the internal resistance r of the cell or electrical source:
V(emf) – Vt = + Vd; or

V(emf) = Vt + Vd; or

Vt = - Vd + V(emf); or

Vt = -rI + V(emf)...(This linear equation is analogous to y = mx + c, y = Vt; x = I; -r is the gradient m and V(e.m.f.) is the y-intercept of the linear graph)

Vt = -rI + V(emf)...(This linear equation is analogous to y = mx + c, y = Vt; x = I; -r is the gradient m and V(e.m.f.) is the y-intercept of the linear graph)

6.
The

**internal resistance r**of a cell is the**resistance within the cell**or**within the internal circuit**– that is, the resistance against the moving charge due to the electrolyte.
7.
To show the

**existence of internal resistance:**A torch is switched on for, say, 20 minutes and the dry cell in the torch becomes**hot**–**due to internal resistance**of the cell.
8.

**Internal resistance r**can be found:
i.
By finding the voltage drop Vd over the current
I flowing when the circuit closed; or

**; or**

__(Yr 2007 SPM P1 Q37 at pg. 100)__/__(Yr 2010 SPM P1 Q41 at pg. 241)__

ii.
By finding the gradient of the Vt-I graph,

where

**Vt = -rI + V(emf)**
(as
in y = mx + c,

Where, V(emf) is the y-intercept; and

gradient m = internal resistance -r;

see the experiment to determine V(emf) and r using
the formula, Vt = -rI + V(emf) at pg. 381)

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Updated By: tutortan1@gmail.com (8/4/12)

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