(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) Silver and copper are good conductors of electricity because they have free electrons available for conduction.
Give two reasons why nichrome alloy is used for making the heating elements of electrical appliances.
How should the two resistances of 2 ohms each be connencted so as to produce an equivalent resistance of 1 ohm ?
(a) How many milliamperes are there in 1 ampere ?
(b) How many microamperes are there in 1 ampere ?
The electrical resistivities of four materials A, B, C and D are given below :
Which material is : (a) good conductor (b) resistor (c) insulator, and (d) semiconductor ?
Why are the coils of electric irons and electric toasters made of an alloy rather than a pure metal ?
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.
How many electrons are flowing per second past a point in a circuit in which there is a current of 5 amp ?
Which of the following statements correctly defines a volt ?
(a) a volt is a joule per ampere.
(b) a volt is a joule per coulomb.
What is the general name of the substances having infinitely high electrical resistance ?
Keeping the potential difference constant, the resistance of a circuit is halved. By how much does the current change ?
(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.
What do you understand by the term “electric potential” ? (or potential) at a point ? What is the unit of electric potential ?
(a) What do you understand by the term “potential difference” ?
(b) What is meant by saying that the potential difference between two points is 1 volt ?
(c) What is the potential difference between the terminals of a battery if 250 joules of work is required to transfer 20 coulombs of charge from one terminal of battery to the other ?
(d) What is a voltmeter ? How is a voltmeter connected in the circuit to measure the potential difference between two points. Explain with the help of a diagram.
(e) State whether a voltmeter has a high resistance or a low resistance. Give reason for your answer.
(a) In which direction does conventional current flow around a circuit ?
(b) In which direction do electrons flow ?
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 ?
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 resistance of 20 ohms has a current of 2 amperes flowing in it. What potential difference is there between its ends ?
How does the resistance of a conductor depend on :
(a) length of the conductor ?
(b) area of cross-section of the conductor ?
(c) temperature of the conductor ?
Show how you would connect two 4 ohm resistors to produce a combined resistance of
(a) 2 ohms
(b) 8 ohms.
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.
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 current of 200 mA flows through a 4 kΩ resistor. What is the p.d. across the resistor ?
How does the resistance of a wire vary with its :
(a) area of cross-section ?
(b) diameter ?
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.
The electrical resistivities of five substances A, B, C, D and E are given below :
A 5.20 x l0-8 Ω m
Two resistors, with resistances 5 Ω and 10 Ω respectively are to be connected to a battery of emf 6 V so as to obtain :
(i) minimum current flowing (ii) maximum current flowing
(a) How will you connect the resistances in each case ? .
(b) Calculate the strength of the total current in the circuit in the two cases.
The atoms of copper contain electrons and the atoms of rubber also contain electrons. Then why does copper conduct electricity but rubber does not conduct electricity ?
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 is an ammeter ? How is it connected in a circuit ? Draw a diagram to illustrate your answer.
Why should the resistance of :
(a) an ammeter be very small ?
(b) a voltmeter be very large ?
In 10 s, a charge of 25 C leaves a battery, and 200 j of energy are delivered to an outside circuit as a result.
(a) What is the p.d. across the battery ?
(b) What current flows from the battery ?
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 ?
Name the law which relates the current in a conductor to the potential difference across its ends.
Keeping the resistance constant, the potential difference applied across the ends of a component 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) Define the unit of resistance (or Define the unit “ohm”).
What happens to the resistance as the conductor is made thinner ?
Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change ?
(a) Why do electricians wear rubber hand gloves while working with electricity ?
(b) What p.d. is needed to send a current of 6 A through an electrical appliance having a resistance of 40 Ω ?
(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 ?