Current & Electromotive

Potential Force (emf) &

difference Internal Resistance

Electrical energy
Resistance & Power
Siti Asyuhra binti Rahman
Physics teacher
Kolej Yayasan Saad Melaka

Current and Potential

3.1 difference Source of electrical force
There are two kind of electric
Electric Charge charge, namely the positive
charge and the negative charge.
A neutral body can be attracted
by another body which has
either positive or negative
charge.
Like charge repel each other.
Unlike charge attract each
other.

- and - = REPEL + and + = REPEL Difference charge = ATTRACT

To study electric fields
Aim: To study electric fields by using an electric field kit
Apparatus: Electric field kit and extra high tension (E.H.T.) power supply
Materials: Olive oil and semolina powder
Instructions:
1. Set up the apparatus as shown.
2. Pour olive oil into a petri dish.
3. Sprinkle semolina powder on the
surface of the olive oil.
4. Switch on the E.H.T. power supply
and observe the movement of the
semolina powder.
5. Repeat steps 3 to 5 with a pair of
electrodes of different shapes.

Discussion:  Electric field is a the region around a charged particle where any electric charge in the
1. Why is the E.H.T. power supply used in this activity? To produce very high voltage region will experience an electric force.
2. What happens to the parallel plate electrodes when the E.H.T. power supply is  Electric field usually represented by arrow.
switched on? Become positively and negatively charged
3. Why does the sprinkled semolina powder form a certain pattern when the E.H.T. Properties of electric field lines of force:
The semolina powder aligns itself along the electric
power supply is switched on? lines of force due to the action of the electric field Olive oil is insulator which a) The electric field of a positively charge
4. What is the function of olive oil in this activity? does not conduct electricity.
represented by arrow lines pointing out of the object
5. Name other materials beside semolina powder that can be used in this activity.
Talcum powder b) The electric field of a negatively charge
represented by arrow lines pointing towards the
object
c) The electric field lines do not cross each other
d) The stronger of the electric field affected by:
-Electric field line
-Distance from the source of electric field
 


Electric field strength is electric force acting on a unit positive charge placed at the point.

𝐅
E=
𝐪
C–1


where

𝑽
E = electric field strength
V = potential difference between two parallel plates
E= d = distance between two parallel plates
𝒅
The S.I. unit for E in this formula is volt per metre (V m-1)

Effect of a Electric field on a charge

 The direction of current is from the positive (+) to
negative (-) terminal
 The direction of electron is from the negative (-) to
positive (+) terminal
 Electric current is caused by the flowing of charge
The ball will still If the ping pong ball When the ping pong The heat of the burning gases in the Charge Q
remain stationary is displaced to the ball touches the flame produces positives and negative Current = or I=
Time t
because the force right to touch the negative plate, it ions.
exert on the ball by positive plate, it will will be charged with The positive ions are pulled to the
the positive plate is then be charged with negative charge. negative plate while negative ions are
 We define current as the rate of flow of electric
Charge of an electron,
equal to the force positive charge. Since Like charge repel, pulled to the positive plate. charge
e = 1.6 × 10-19 C
exerted on it by the like charges repel, the ball will be The heavier positive ions occupy larger
negative plate the ball will be pushed towards the portion of the flame. Therefore, 1C 1 Cs-1 A = Ampere
1A= = C = Coulomb
pushed towards the positive plate The negative ions (electrons) which are 1C= 1 1s
negative plate. lighter, occupy smaller portion 1.6 × 10-19

= 6.25 × 1018 electrons

 The SI units for current is Ampere
 Potential difference, V between two points in an electric
field is the work done, W in moving one coulomb of
Ideas about Electric Potential Difference charge, Q from one point to another.
Work done,𝑊
Potential Difference, V =
A Charge,𝑄

B OR

A
W
B
V=
Electric potential at A (positive Q
Pressure at point A > pressure Gravitational potential energy terminal) > Electric potential at B
at point B at A > Gravitational potential (negative terminal)
energy at point B Unit: volt or JC-1
Water will flow from A to B The electric current flows from A to B
when the valve is open The stone will fall from A to B passing through an ammeter & resistor
because of the difference in because of the difference in because of the electric potential
the pressure of water gravitational potential energy. difference across the battery

Ammeter & Voltmeter in a circuit

10 -2

V V directly proportional to I

Answer
I
2.7 x 1021 e
 3.2 Resistance
Answer
720C > Ability of the conductor to resist the flow of an
V I
2160J
electric current

 Ohm’s law states that the electric current, I flowing

through a conductor is directly proportional to the
potential difference across the conductor, if
temperature and other physical quantities remain
constant.
 The resistance, R of a conductor is defined as the ratio
of the potential difference across the conductor to the 𝑉
Answer
1.5A R=
current, I flowing through a conductor 𝐼
A resistor is represented by the symbol Unit: Ohm (Ω) or VA-1
 SERIES CIRCUIT PARALLEL CIRCUIT

Diagram

I1 = I 2 = I3 Current I = I1 + I 2 + I3

V = V 1 + V 2 + V3 Voltage V1 = V2 = V3
The resistors connected in one non-branched
wire is said to be connected in series,
whereas resistors connected in a branched
R = R1 + R2 + R3 Resistance 1 1 1 1
= + +
wire is said to be connected in parallel. 𝑅 R1 R2 R3

Effective resistance
> Is a single resistor that replaces 2 or more
resistors in series or parallel and yet has the same
effect on the circuit by allowing the same of
amount of current to flow in the circuit.

6Ω

1.5 Ω
Exercise
1. An ammeter in a circuit gives a reading of 2 A. Both 2. If each bulb has a resistance of 1 Ω, what will the ammeter
resistors have resistance of 1 Ω and 2 Ω respectively. What is reading be if the cell voltage is 3 V?
the voltage value?
 𝑉
From the formula, I = it can be seen that when V is held constant,
𝑅
the current, I is inversely proportional to the resistance, R

1
I
𝑅
When the value of R is large, the I will be small or vice versa

Factor influencing the resistance of a conductor:

a) The length of the conductor
b) The cross-sectional area of the conductor
c) The type of material of the conductor
d) The temperature of the conductor

R
a)The length of the conductor Formulae from the Results of Experiments
ℓ Resistance
greater collisions with atoms in the material

b)The cross-sectional area of the conductor ℓ

Resistance of a conductor, R ℓ Type of Resistivity (Ω m)
A Resistance materials
R 1 Silver 1.59x 10-8
More free electron to flow the current R
Resistance of a conductor, R
𝐴 Copper 1.68 x 10-8
Constantan 49 x 10-8
c)The temperature of the conductor 1 ℓ
T Resistance
A
A R Nichrome 100 x 10-8
R 𝐴
Atoms in conductor vibrate faster and
movement of electrons are obstructed
ℓ R = resistance
R=ρ ℓ = length
d)The type of material
Resistance
θ 𝐴 A = area
ρ = resistivity
Nichrome> Constantan> Copper > Silver
Conductivity Silver> Copper> Constantan > Nichrome
Structure of copper is small and more Resistivity is a measure of a conductor’s
free electrons ability to oppose the flow of electric current.




Element Critical Temperature (K)

Zinc, Zn 0.88
Aluminium, Al 1.14
Tin, Sn 3.69
Mercury, Hg 4.15
Lead, Pb 7.26
Niobium, Nb 9.2
 Electromotive force & At the end of this chapter,
3.3 Internal Resistance
you will be able to:
 Define emf
 Compare emf and
potential difference
 Determine emf and
internal resistance
 Solve problem involving
emf and internal
resistance
 Batteries arranged in
series and parallel
 Ω

Exercise:

1) The figure shows a dry cell, with an 2) A dry cell has emf of 1.5 V and an
electromotive force of 1.5 V, connected to a internal resistance of 0.5 Ω. This dry cell
resistor, R. The voltmeter and ammeter is connected to a resistor of 1.5 Ω.
readings are 1.2 V and 0.3 A respectively.

It is noticed that:
Vt < E
E = Vt + Vd
The ‘lost volts’ or voltage drop, Vd is the
difference between E and Vt. It cannot be
= IR + Ir
directly measure from the voltmeter. E = I (R+r)
Vd = E – Vt Calculate Calculate
E = Vt + Vd a) The resistance of R a) The circuit current
b) The internal resistance , r of the dry cell b) Potential difference across the terminals
c) The voltage loss within the cell
Vd is the potential difference required to drive the Answer:
4Ω,1Ω
current through the internal resistance. 0.75A, 1.125 V, 0.375 V
 To increase the amp-hour capacity
To increase the overall voltage
but the voltage remain the same
 3.4 Electrical energy & Power

At the end of this chapter, you will be able to:

 Define electrical energy
 Define electric power
 Solve problem involving electrical energy and power
 Compare power rating and energy consumption of
various electrical appliances.
 Compare various electrical appliances in terms of
efficient use of energy.


E
V= 
Q

E = Pt

E = VIt 
 Example:

A heater is labelled 240 V, 750 W.

a) What does it mean?

b) What is the resistive value?

P=IV E=VIt
Electrical energy
P=
time taken
Substituting V=IR P=I2R E=I2Rt Answer:
E a) When a source of 240 V is supplied to the
P = = VI heater, it will produce heat energy of 750 J in
t V2
V2 1 second
Substituting I=
V
P= E= t b) 76.8 Ω
R R R
The Power and Energy Consumption Rate for Various Electrical Devices

Answer:
1)(a) 800 J
(b) 576k J

Answer:
2)(a) RM 1.44
(b) RM 0.15