Electricity Test
1. What are the two types of electric charges?
2. What do you understand by potential difference?
3. The direction of current is the same as that of the flow of electrons. Explain with a circuit diagram. Is this statement true or false?
4. State Ohm’s law.
5. a) Calculate the energy transferred when 2 A current flows through a 10 Ω resistor
for 30 minutes.
b) Calculate the amount of charge that would flow in one hour through the
element of an electric iron drawing a current of 0.4 amps.
6. Draw a circuit diagram showing a cell, a bulb and a closed switch.
7. a) How much work is done in moving a charge of 3 coulumb from a point at the
volts 115 to a point at 125 volts?
b) Ammeter burns out when connected in parallel. Give reasons.
8. Given n resistors each of resistors Rs. How will you combine them to get the ( I )
maximum and (ii) minimum effective resistance? What is the ratio of the maximum
to minimum resistance?
9. a) A wire of length L and resistance R is stretched so that its length it’s doubled.
How will the (a) Resistance change (b) Resistivity change? b) In an
experiment the current flowing through a resistor and potential difference across it
are measured. The values are given below. Show that these values confirm Ohm’s
Law I (ampere) 1.0 1.0 2 1.5 2.0 2.0 2.5 2.5 3.0 3.0 V (volt) 4.0 4.0
6.0
6.0 8.0 8.0 10.0 10.0 12.0 12.0
10. a) A tube light draws 0.1 A current from a 220 V supply. What current will this tube
light draw when it is connected to a 110 V supply? a) An electric wire is
stretched to increase its length by 25%.By what % will the resistance be
increased and what will be increase in its resistivity?
11. a) Two resistances of 4 Ω and 8 Ω are connected in parallel. What would be the
combined resistance of the system? b) Two identical resistors each of
resistance 2 Ohm are connected in turn (1) in series (2) in parallel to a battery of 12
V. Calculate the ratio of power consumed in two cases.
12. a) A household uses the following electric appliances: (i) Refrigerator of
rating 400 W for ten hours each day. (ii) Two electric fans of rating 80 W
each for twelve hours each day. (iii) Six electric tubes of rating 18 W each
for 6 hours each day. Calculate the electricity bill of the household for the
month of June if the cost per unit of electric energy is Rs. 3.00. b) An
electric iron of resistance 20Ωtakes a current of 5 A. Calculate the heat developed
in 30sec.
13. a) Derive the expression for Joules� law of heating. b) Derive a formula
for four equivalent resistances connected in series.
14. (a) The electric power consumed by a device may be calculated by using either of the two expressions P = 12R or P =V2/R . The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. How can the seemingly different dependence of P on R in these expressions be explained?(b) (i) A 100 W electric bulb is connected to 220 V mains power supply. Calculate the strength of the electric current passing through the bulb. (ii) If the same bulb is taken to U.S.A where the main power supply is 110 V, how much electric current will pass through the bulb when connected to mains?
Answers
Electrons ( or negative charge) flow from negative potential to positive potential ,or we can also say that positive charge flow from positive to negative potential. Electric current or Conventional current is assumed to be flow to positive charge, Hence ,the direction of Electric current ( Conventional current) is opposite to the direction of electron.
But why use two conventions for the same thing. Actually the story began…In 1752 , Benjamin Franklin did a kite experiment in which he and his son flew a kite with a pointed, conductive wire attached to its apex ,It was flown near thunder clouds to collect electricity from the air. Electricity from the storm clouds transferred to the kite and electricity flowed down the string and gave him a little shock ,He called it charge or electric fluid ( basically a positive charge).
Being a pioneer in that field, his theory was adopted that flow of postive charge is called Electricity ( i.e. conventional current ) . But was Benjamin Franklin really the first person to discover electricity? Maybe not, but the idea got famous from here.
In 1897 ,Electron was discovered, however by 1897 , Franklin's convention had been in use for nearly 150 years, so appeared in thousands of books, tens of thousands of electrical circuit diagrams, and millions of minds by then.
Although Franklin's convention is not true in the context of conducting metals, it continues to be used in textbooks and circuits .
Finally , I would like to clarify that current flows in the conductor is due to flow of electrons , but since the conventional current direction which is assumed to be the flow of positive charge does not harm our main notion, this continues to be used.
Direction of conventional current can be seen as opposite charge particles, move in the opposite direction ( Double negation make our original fact to be true)