Fundamentals of Electricity

Question 1

Electric Charge

1. There are two kinds of electric charge, namely the positive charge and the negative charge.
2. Like charges repel each other.
3. Unlike charges attract each other.
4. A neutral body can be attracted by another body which has either positive or negative charge.
5. The SI unit of electric charge is the Coulomb (C)

Question 2

Electric Field

1. An electric field is a region in which an electric charged particle experiences an electric force.
2. Electric field is represented by a number of lines with arrows, called electric lines of force or electric field lines.
3. The direction of the field at a point is defined by the direction of the electric force exerted on a positive test charge placed at that point.
4. The strength of the electric field is indicated by how close the field lines are to each other. The closer the field lines, the stronger the electric field in that region.
5. The lines of force are directed outwards for a positive charge and inwards for a negative charge.
6. The electric line of force will never cross each other.
7. The figure shows a few examples of the field pattern that you need to know in the VET syllabus.

Question 3

Direction of Current

1. Conventionally, the direction of the electric current is taken to be the flow of positive charge.
2. The electron flow is in the opposite direction to that of the conventional current.
3. In a circuit, current flow from the positive terminal to the negative terminal.
4. In a circuit, electrons flow from the negative terminal to the positive terminal.

Question 4

Unit of Current

1. The SI unit for current is the ampere (A).
2. The current at a point is 1 ampere if 1 Coulomb of electric charge flows through that point in 1 second. Therefore, 1A = 1C

Example:

If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the wire?

Answer:
Charge flow, Q = 30C
Time taken, t = 2 minutes = 120s

Current,

Example:
Current of 0.5A flowed through a bulb. How many electrons had flowed through the bulb in 5 minutes? (The charge of 1 electron is equal to -1.6×10^-19 C)

Answer:
Current, I = 0.5A
Time taken, t = 5 minutes = 300s

Charge of 1 electron, e = -1.6×10^-19 C
Number of electrons, n = ?

Question 5

Potential and Potential Difference

1. The electric potential V at a point in an electric field is the work done to bring a unit ( 1 Coulomb) positive charge from infinity to the point.
2. The potential difference (p.d.) between two points is defined as the work done in moving 1 Coulomb of positive charge from 1 point in an electric field to another point.
3. In mathematics
   

Example, in the diagram above, if the work done to move a charge of 2C from point A to point is 10J, the potential difference between A and B,

Question 6

Arrangement of Voltmeter

To use the voltmeter in the measurement of potential difference across an object, the voltmeter must be connected in parallel to the circuit.

Question 7

Relationship Between Current and Potential Difference

Ohm’s Law

1. The relationship between the current passing through 2 points in a conductor and the potential difference between the 2 points is given by Ohm’s law.
2. Ohm’s Law states that the current flowing in the metallic conductor is directly proportional to the potential difference applied across it’s ends, provided that the physical conditions ( such as temperature ) are constant.

where k is a constant

Question 8

What is the current flow through an 800Ω toaster when it is operating on 240V?

Answer 8

Resistance, R = 800Ω
Potential difference, V = 240V
Current, I = ?

Question 9

Resistance

1. The resistance R of a material is defined as the ratio V : I, where V is the potential difference across the material and I is the current flowing in it.
   
2. The SI unit of resistance is the ohm (W). One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained.

Question 10

Finding Resistance from the Potential Difference - Current Graph

In the graph potential difference against current, the gradient of the graph is equal to the resistance of the resistor.

Resistance, R = Gradient of the Graph

Answer 10

Example:

Figure above shows the graph of potential difference across a wire against its current. Find the resistance of the wire.

Answer:
Resistance

Question 11

Ohmic Conductor

1. Conductors that obey Ohm’s law are said to be ohmic conductors.
2. Examples of Ohmic conductors:
   Metal, Copper sulphate solution with copper electrodes

Question 12

Non-Ohmic Conductor

1. Conductors which do not obey Ohm’s law are called non-ohmic conductors.
2. Example:
   Semiconductor Diode, Vacuum tube diode

Question 13

Factors Affecting the Resistance

The resistance R of a given conductor depends on:

1. Its length l,
2. Its cross-sectional area A
3. Its temperature and
4. The type of material.

Answer 13

Length

Resistance is directly proportional to the length of the conductor.

Cross Sectional Area

Resistance is inversely proportional to the cross sectional area of the conductor.

Temperature

A conductor with higher temperature has higher resistance.

Material

Difference materials have difference resistivity. The resistance of copper wire is lower than iron wire.

Since resistance is directly proportional to the length and inversely proportional to the cross sectional area of the conductor. If two resistors of same material have same temperature, we can relate the resistance of the two resistors by the following equation.

R1A1l1=R2A2l2