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.

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.

Ohmic Conductor

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

Non-Ohmic Conductor

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

(Examples of characteristic of non-Ohmic conductor)

 

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

 

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
   
   or
   
   

4. 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,
5. 

Arrangement of Ammeter

To use the ammeter in the measurement of an electric current, the ammeter must be connected in series to the circuit.

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.

 

 

Effect of Electric Field on a Ping Pong Ball Coated with Conducting Material

1. A ping ball coated with conducting material is hung by a nylon thread.
2. When the ping pong ball is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, opposite charges are induced on the surface of the ball. The ball will still remain stationary. This is because the force exert on the ball by the positive plate is equal to the force exerted on it by the negative plate.
3. If the ping pong ball is displaced to the right to touch the positive plate, it will then be charged with positive charge. Since like charges repel, the ball will be pushed towards the negative plate.
4. When the ping pong ball touches the negative plate, it will be charged with negative charge. Again, like charge repel, the ball will be pushed towards the positive plate. This process repeats again and again, causes the ping pong ball oscillates to and fro continuously between the two plates.

Candle Flame in an Electric Field

1. Normally, with absent of wind, the flame of a candle is symmetry.
2. The heat of the candle flame removes electrons from the air molecules around it, and therefore ionised the molecule. As a result, the flame is surrounded by a large number of positive and negative ions.
   
3. If the candle is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, the positive ions will be attracted to the negative plate while the negative ions will be attracted to the positive plate.
4. The spreading of the flame is not symmetry. This is because the positive ions are much bigger than the negative ions; it will collide with the other air molecule and bring more air molecule towards the negative plate.
   

 

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.
   Example
   
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 SPM syllabus.

 

 

1. An electric current I is a measure of the rate of flow of electric charge Q through a given cross-section of a conductor.
2. In other words, current is the measure of how fast the charge flow through a cross section of a conductor.

Equation

or

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.

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, 1 A = 1C/s.

 

1. There are two kind of electric charge, namely the positive charge and the negative charge.
2. Like charge repel each other.
3. Unlike charge 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 Coulomb (C).
   Example
   Charge of 1 electron = -1.6 x 10^-19C
   Charge of 1 proton = +1.6 x 10^-19C

Charge and Relative Charge

Sum of Charge

Sum of charge
= number of charge particles × charge of 1 particle
$$Q=ne$$