(a) Define electric current. What is the SI unit of electric current.
(b) One coulomb of charge flows through any cross-section of a conductor in 1 second. What is the current flowing through the conductor ?
(c) Which instrument is used to measure electric current ? How should it be connected in a circuit ?
(d) What is the conventional direction of the flow of electric current ? How does it differ from the direction of flow of electrons ?
(e) A flash of lightning carries 10 C of charge which flows for 0.01 s. What is the current ? If the voltage is 10 MV, what is the energy ?
(a) Electric current is the flow of electric charges (electrons) in a conductor such as a metal wire.
SI unit of electric current is ampere.
(b) 1 ampere.
(c) An ammeter is used to measure electric current. It should be connected in series with the circuit.
(d) Conventional direction of flow of electric current is from positive terminal of a battery to the negative terminal, through the outer circuit. The direction of flow of electrons is opposite to the direction of conventional current, i.e. from negative terminal to positive terminal.
A resistance of 20 ohms has a current of 2 amperes flowing in it. What potential difference is there between its ends ?
If the charge on an electron is 1.6 x 10-19 coulombs, how many electrons should pass through a conductor in 1 second to constitute 1 ampere current ?
A student made an electric circuit shown here to measure the current through two lamps.
(a) Are the lamps in series or parallel ?
(b) The student has made a mistake in this circuit.
What is the mistake ?
(c) Draw a circuit diagram to show the correct way to connect the circuit.
Use the proper circuit symbols in your diagram.
(a) Define resistivity. Write an expression for the resistivity of a substance. Give the meaning of each symbol
which occurs in it.
(b) State the SI unit of resistivity.
(c) Distinguish between resistance and resistivity.
(d) Name two factors on which the resistivity of a substance depends and two factors on which it does not depend.
(e) The resistance of a metal wire of length 1 m is 26 Ω at 20 °C. If the diameter of the wire is 0.3 mm, what will be the resistivity of the metal at that temperature ?
(a) How many milliamperes are there in 1 ampere ?
(b) How many microamperes are there in 1 ampere ?
An electric heater is connected to the 230 V mains supply. A current of 8 A flows through the heater.
(a) How much charge flows around the circuit each second ?
(b) How much energy is transferred to the heater each second ?
Calculate the area of cross-section of a wire if its length is 1.0 m, its resistance is 23 Ω and the resistivity of the material of the wire is 1.84 x 10-6 Ωm.
Why are the coils of electric irons and electric toasters made of an alloy rather than a pure metal ?
(a) How does the resistance of a pure metal change if its temperature decreases ?
(b) How does the presence of impurities in a metal affect its resistance ?
(a) Name a device that helps to measure the potential difference across a conductor.
(b) How much energy is transferred by a 12 V power supply to each coulomb of charge which it moves around a circuit ?
Why should the resistance of :
(a) an ammeter be very small ?
(b) a voltmeter be very large ?
(a) What is an electric current ? What makes an electric current flow in a wire ?
(b) Define the unit of electric current (or Define ampere).
(a) What is the ratio of potential difference and current known as ?
(b) The values of potential difference V applied across a resistor and the corresponding values of current I
flowing in the resistor are given below :
(c) Name the law which is illustrated by the above V-I graph.
(d) Write down the formula which states the relation between potential difference, current and resistance,
(e) The potential difference between the terminals of an electric iron is 240 V and the current is 5.0 A. What is the resistance of the electric iron ?
The p.d. across a lamp is 12 V. How many joules of electrical energy are changed into heat and light when :
(a) a charge of 1 C passes through it ?
(b) a charge of 5 C passes through it ?
(c) a current of 2 A flows through it for 10 s ?
A piece of wire of resistance 20 Ω is drawn out so that its length is increased to twice its original length. Calculate the resistance of the wire in the new situation.
(a) In which direction does conventional current flow around a circuit ?
(b) In which direction do electrons flow ?
A current of 4 A flows around a circuit for 10 s. How much charge flows past a point in the circuit in this time ?
What do you understand by the term “electric potential” ? (or potential) at a point ? What is the unit of electric potential ?
Keeping the potential difference constant, the resistance of a circuit is halved. By how much does the current change ?
(a) What is meant by the “resistance of a conductor” ? Write the relation between resistance, potential
difference and current.
(b) When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Calculate the value of the resistance of the resistor.
A p.d. of 10 V is needed to make a current of 0.02 A flow through a wire. What p.d. is needed to make a current of 250 mA flow through the same wire ?
What would be the effect on the resistance of a metal wire of :
(a) increasing its length ?
(b) increasing its diameter ?
(c) increasing its temperature ?
How does the resistance of a wire vary with its :
(a) area of cross-section ?
(b) diameter ?
If 3 resistances of 3 ohm each are connected in parallel, what will be their total resistance ?
Show how you would connect two 4 ohm resistors to produce a combined resistance of
(a) 2 ohms
(b) 8 ohms.
A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω, 0.5 Ω and 12 Ω. How much current would flow through the 12 Ω resistor ?
Draw a circuit diagram to show how 3 bulbs can be lit from a battery so that 2 bulbs are controlled by the same switch while the third bulb has its own switch.
A current of 5 amperes flows through a wire whose ends are at a potential difference of 3 volts. Calculate the resistance of the wire.
Fill in the following blank with a suitable word :
Ohm’s law states a relation between potential difference and……………………
Distinguish between good conductors, resistors and insulators. Name two good conductors, two resistors and two insulators.
The graph between V and 1 for a conductor is a straight line passing through the origin.
Which law is illustrated by such a graph ?
What should remain constant in a statement of this law ?
(a) Give two examples of substances which are good conductors of electricity. Why do you think they are
good conductors of electricity ?
Calculate the resistance of a copper wire 1.0 km long and 0.50 mm diameter if the resistivity of copper is 1.7 × 10-8 Ωm.
(a) Give one example to show how the resistance depends on the nature of material of the conductor.
(b) Calculate the resistance of an aluminium cable of length 10 km and diameter 2.0 mm if the resistivity of aluminium is 2.7 x 10-8Ωm.
