Chapter 9 Science

1. Current, Potential Difference
and Resistance
Jurongville Secondary School
Secondary 3A
Science

2. Topic Overview
1. Introduction
2. Electric Current
3. Potential Difference
4. Resistance
5. Factors Affecting Resistance

3. INTRODUCTION

4. Who Discovered Electricity?
The history of electricity goes back more than two thousand years, to the
time the Ancient Greeks discovered that rubbing fur on amber caused an
attraction between the two. By the 17th century, many electricity-related
discoveries had been made, such as the invention of an early
electrostatic generator, the differentiation between positive and negative
currents, and the classification of materials as conductors or insulators.
In the year 1600, English physician William Gilbert conned the term
electric, from the Greek elektron, to identify the force that certain
substances exert when rubbed against each other.
While many believe Benjamin Franklin to be the father of electricity,
current findings seem to show otherwise. In 1752, Franklin is said to
have performed the famous experiment of flying a kite during a
thunderstorm, which led to the discovery that lightning and electricity
were somehow related. Modern scientists know this to be something of a
tall tale, since being hit by lightning would have been fatal. It's likely that
Franklin was actually insulated, away from the path of lightning.

5. Who Discovered Electricity?
The kite experiment helped Franklin establish a relationship
between lightning and electricity, which led to the invention of the lightning
rod. Benjamin Franklin went on to observe other phenomena related to
electricity, but many believe that he didn't actually discover its true nature.
In 1800, Italian-born physicist Alessandro Volta constructed the
voltaic pile, later known as the electric battery, the first device to produce a
steady electric current. It was Volta, not Franklin, who discovered that
certain chemical reactions could produce electricity. Volta also created the
first transmission of electricity by linking positively-charged and negatively-
charged connectors and driving an electrical charge, or voltage, through
them.
It wasn't until 1831 that electricity became viable for use in
technology. English scientist Michael Faraday created the electric dynamo,
a crude precursor of modern power generators. This invention opened the
door to the new era of electricity. A few decades later, in 1879, Thomas Alva
Edison invented the light bulb.

6. Famous Scientists
- Click on the links below to read more about them…
Coulomb Volta Ampere
Charles Augustin de Coulomb 70yrs
Alessandro Giuseppe Antonio Anastasio Volta 82yrs
André Marie Ampère 61yrs

7. 1. Electric Current

8. Electric Current
• Electric current, I is defined as the rate of flow
of charge.
• The SI unit is the ampere (A)
• Mathematically,
Amount of charge
Q flowing past a point
Current I
t Time taken for
charges to flow

9. C
Charges flowing slowly – small current
Charges flowing quickly – large current

10. Video on Electric Current
• Click here to view a video on current.

11. Electric Current
1000 1000000
• 1 A = ___________ mA = ____________ μA.
• Conventional current describes the flow of
electric charges from the positive terminal to
the negative terminal of a battery or power
supply.

12. Electric Current
+ -
cell
Conventional Flow of
current electrons
switch bulb

13. Electric Current
Conventional
current
e-
Flow of
electrons

14. Measuring Electric Current

15. Measuring Electric Current
We use an ammeter to measure the size of an electric
current. The ammeter must be connected in series to
the circuit as shown below.
A

16. Measuring Electric Current
Negative Positive
terminal terminal

17. Measuring Electric Current
The positive terminal of ammeter must be connected
to the positive terminal of the battery.

18. Measuring Electric Current
“+” to “+”
“-” to “-”

19. Example 1
What is wrong with the electrical circuit shown
below?
– +
The ammeter is connected to the battery
wrongly. i.e. positive terminal of the battery is
connected to negative terminal of the ammeter.

20. Example 2
What is the reading on the ammeter shown
below?
10 divisions  0.2 A
1 division  0.2 / 10 = 0.02 A
Ammeter reading = 14 divisions
= 0.02 x 14 = 0.28 A

21. 2. POTENTIAL DIFFERENCE
(p.d.)

22. 2. Potential Difference (p.d.)
In an electric circuit, electric charges require energy
to move around the circuit.
The energy is given by a electrical energy source such
as a battery.
battery
bulb

23. Potential Difference (p.d.)
The electric cell has positions of high potential and
low potential.
The difference between the high and low potentials
provides the energy that moves the charges out of
the battery through the wires in a closed circuit.
High
potential
Low
potential

24. Potential Difference (p.d.)
• When there is a potential difference across a
conductor, a current will flow through the
conductor.
• The SI unit is the volt (V)

25. Measuring Potential difference.
• We use a voltmeter to
measure the
– voltage of a electrical
source or
– potential difference
across an electric
component.

26. Measuring Potential Difference
Negative Positive
terminal terminal

27. Measuring Potential Difference
• The voltmeter must be
connect in parallel.
• The positive terminal of
voltmeter must be
connected to the
positive terminal of the
battery.

28. Example 3
What is wrong with the electrical circuit shown
below?
voltmeter
– +
The voltmeter is connected wrongly. i.e. it
should be connected in parallel with the bulb.

29. Example 4
What is the reading on the voltmeter shown
below?
10 divisions  1 V
1 division  1 / 10 = 0.1 V
Voltmeter reading = 12 divisions
= 0.1 x 12 = 1.2 V

30. 3. RESISTANCE

31. What is resistance?
• Resistance is a measure of how difficult it is
for an electric current to pass through a
material.
• It is a property of the material that restricts
the movement of charges in the material.

32. Optional: Video on Resistance
Click on links to view videos on
resistance.
• Video 1
• Video 2

33. Resistance
Resistance is defined as the ratio of the
potential difference across the resistor to the
current flowing through it.
Volt, V
V
R
I Ampere, A
The SI Unit of resistance is ohm (Ω).

34. Example 5
A coil of wire is connected across a 6 V batter. If the
current in the wire is 0.3 A, what is the resistance of
the wire?
V 6
R 20
I 0.3
The resistance of the wire is 20 Ω