Form 5 Physics : Chapter 2 : ELECTRICITY

7.1 ANALYSING ELECTRIC FIELDS AND CHARGE FLOW

 A body is:

(a) neutral, if it has equal numbers of positive and negative charges.

(b) charged negative, if it has more negative than positive charges.  

     (atom gains electron)

(c) charged positive, if it has more positive than negative charges.

           (atom losses electron)
  

§ The force acting on two bodies of the same net charges will repel each other.

§ The force acting on two bodies of different net charges will attract each other.

§ The force causes movement of electrons or flow of charges. 

 Electric current 
       - is the rate of flow of electric charge 

• SI unit

             -  ampere (A) for current

             -  coulomb for electric charge

Activity 1 Aim :  To investigate the relationship between electric charges and electric            current.  Apparatus:               Van de Graaff generator, connection wires, microammeter  Procedure

1.    Start the motor of a Van de Graaff generator for a few minutes to produce positive charges on the metal dome of the generator. 2.    Bring your finger close to the dome of the generator. Observe what happens. 3.    Touch the dome of the generator with the free end of the wire that is connected to the microammeter. Observe the microammeter needle closely. 4.   Switch off the motor of the Van de Graaff generator

Observation

- Can feel a brief electric shock when your finger is brought close to the      dome of the generator. -  The microammeter needle is deflected when a wire is connected to the     dome of  the generator. - Can safely touch the metal dome with your finger. - The microammeter needle is returned to its zero position when the Van de     Graaff is switched off.

An electric field  

• is a region in which an electric charge experiences an electric force (attraction or repulsion). 
• is represented by a series of arrowed lines called electric field lines.

• The lines indicate both the magnitude and direction of the field.
• The direction of an electric field at any point is taken as the direction of theforce acting on a charged body placed at that point.
• Electric field lines never cross each other.
• Electric field lines are most dense around objects with the greatest amount of charge.

The effect of an electric field on a charge

(a) A charged ball in an electric field

Procedure:

1. Switch on the EHT power supply
2. Charge the ping-pong ball by contact with one of the electrodes. 
3. Observe what happen to the movement of the ping pong ball.

Observation:


When the EHT power supply is switched on, the ball oscillate to and fro between the two metal plates X and Y. 

  
Discussion:

1. The ping-pong ball is neutral it remains at the centre as the electric forces acting on it are balanced. 
2. When the EHT power supply is switched on, plate X is positively charged and plate Y is negatively charged.
3. When the ping-pong ball touches the positively charged plate X,  the ball receives positive charges from the plate and experiences a repulsive force. The ball will then pushed to the negatively charged plate Y. 
4.  When the ball touches plate Y, the positive charges are neutral by the negative charges. The ball then negatively charged and repels toward plate X.
5. The process is repeated and the ball oscillate to and fro between the two metal plates X and Y.
6. The rate of oscillation of the ping-pong ball can be increased by

• increasing the voltage of the EHT power supply and
• decrease the distance between the two plates X and Y.

  

(b) The effect of an Electric field on a Candle Flame

Observation

When the EHT power supply is switched on, the candle flame divided into two portions in opposite directions. 

1. The portion that is attracted to the negative plate P is very much larger than the portion that is attracted to the positive plate Q. 

Discussion

1. The hot flame of the candle ionized the air molecules in its surrounding into positive and negative ions.
2. The positive ions are attracted towards the negative plate P. At the same time, the negative ions are attracted to the positive plate Q.
3. The movement of the ions towards the plated P and Q caused the candle flame to spread out.
4. The bigger portion of the flame is attracted towards the negative plate as the mass of the positive ions is larger than of the negative ions.

  

Exercise:

Question 1

The current flows in a light bulb is 0.5 A.

(a) Calculate the amount of electric charge that flows through the bulb in 2 hours.

(b) If one electron carries a charge of 1.6 x 10^-19 C, find the number of electrons transferred 

     through the bulb in 2 hours.

Question 2

Electric charges flow through a light bulb at the rate of 20 C every 50 seconds. What is the electric current shown on the ammeter? 

Question 3

When lightning strikes between two charged clouds, an electric current of 400 A flows for 0.05 s. What is the quantity of charge transferred?

7.2 RELATIONSHIP BETWEEN ELECTRIC CURRENT, I AND

      POTENTIAL  DIFFERENCE, V

1. When a battery is connected to a bulb in a circuit, it creates electric field along the wires.
2. The positive terminal P is at a higher potential and the negative terminal Q is at a lower potential.
3. The potential difference between the two terminals causes the charges to flow across the bulb in the circuit and lights up the bulb.
4. Work is done when electrical energy is dissipated as heat and light energy after crossing the bulb.

Potential difference, V, 

• is defined as the work done when 1 C of charge moves between two points in an electric field.

•  SI unit is Volt (V)
•  1 Volt = 1 joule per coulomb. 
• The potential difference across two points in a circuit is 1 Volt if 1 Joule of work is done in moving 1 Coulomb of charge from one point to the other.

How to measure potential difference and electric current?

• Voltmeter is using to measure the potential difference across two points in a circuit.   
• It must always be connected in parallel between the points concerned.
• Ammeter is using to measure current .
• It must always be connected in series with a resistor or other device 
• Ammeter has a low resistance so that its existence has little effect on the magnitude of current flowing.

• The potential difference is directly proportional to the current flowing through it.

Ohm’s Law: state that

The electric current, I  flowing through a conductor is directly proportional to the potential difference, V  across it if the temperature and other physical conditions are constant.

From Ohm’s Law, 
                   V ∝ I 

Ohmic conductor
Conductors that obey Ohm’s Law 




Non-ohmic conductor
Conductors that do not obey Ohm’s Law 

Resistance (R) of a conductor

• is defined as the ratio of the potential difference (V) across the conductor to the current (I) flowing through it.
• is a measure of how much a conductor resists the flow of electricity. A good conductor has a low resistance and a poor conductor has a high resistance.

Disadvantage of resistance

• Resistance causes some of the electrical energy to turn into heat , so some electrical energy is lost along the way if we are trying to transmit electricity from one place to another through conductor.

Advantage of resistance

• Resistance allow us to use electricity for heat and light. The heat is generated from electric heaters
• In a light bulb, the current flowing through a resistance filament causes it to become hot and then glow.

Plan & conduct an experiment to find the relationship between current and potential difference.


Aim: 
To determine the relationship between the potential difference and the electric current flowing through an ohmic and non ohmic conductor.


Hypothesis:


....................................................................................................................................


Variables :

• Manipulated variable: ..............................................................
• Responding variable: ..............................................................
• Controlled variables: ...............................................................

Apparatus: 

Rheostat, constantan wire, switch, connecting wire, batteries, ammeter, voltmeter

Procedure

1. Turn on the switch and adjust the rheostat until the ammeter reads the current, I = 0.2 A.
2. Read the value of the potential difference, V, from the voltmeter. Record the readings.
3. Repeat the experiment for I = 0.3 A, 0.4 A, 0.5 A, 0.6 A
4. Tabulate the data.
5. Plot a graph of V against I.

Repeat the experiment by replacing the constantan wire (ohmic conductor) with an electric bulb (non-ohmic conductor)

Graph V vs I

              Ohmic conductor              Non Ohmic conductor

        
Factors that affect resistance


Length, l

       For conductors of the same material and cross-sectional area,

       the resistance R is directly proportional to its length, l

                               R ∝ l
       This means that doubling the length doubles the resistance.

        

Cross-sectional area, A
      For conductors of the same material and length, 
      the resistance R is inversely proportional to its cross-sectional area, A.


      This means that doubling the cross-sectional area halves the resistance.