CURRENT ELECTRICITY

1. Static Electricity:-

The study of electric charges at rest is called static electricity.

2. Current Electricity:-

The study of various effects of electrical charges in motion is called current electricity.

3. Current:-

The net charge flowing through a cross-section of a conductor in unit time is called current.

Note:-

Current is measured in Amperes using an Ammeter.

4. Ampere:-

Ampere is an amount of current in a conductor when the net flow of charge per second through its cross-section is one coulomb.

5. Electric Potential:-

Electric potential at a point in space is defined as the work done in moving a single unit positive charge from infinity to that point.

Note:-

1. The electric potential is measured in units of ‘Volts’.

2. If W Joules of work is done in moving + q coulombs of charge to a point, then electrical potential ‘v’ at that point is given by

6. Volt:-

The electric potential difference between two points is said to be one volt, when one joule of work is done in carrying a coulomb of positive charge from one point to the another.

7. Electro motive force (e.m.f):-

The electromotive force (e.m.f) is defined as the amount of work done by the seat (cell) on charge carriers to force them to go to the point of higher potential.

8. What is Simple Electric Circuit ? Draw it.

A simple electric circuit is an arrangement consisting of a source of electrical energy, a device which utilizes this energy and conducting wires which connect them.

Figure: A Simple Electric Circuit (B- Source of Electric Energy, L- Torch Bulb)

9. What is the use of tap-key? Explain its working

A tap-key is used to make or break an electric circuit.

Working:-
A tap-key is connected in series with the battery (cell) as shown in figure.

When the tap-key is pressed, the P-end touches with Q that means the circuit gets closed and the bulb glows. This is said to be ‘make’ of the circuit. When the tap-key is released, again P gets separated with Q that means, the circuit gets opened and the bulb is put off. This is said to be break position of the circuit.

10. How do you connect cells in series? When they are connected in series what is the total e.m.f. of the combination.

Cells in series:-

When negative terminal of a cell is connected to the positive terminal of the next cell, then the cells are said to be connected in ‘series’.

When cells are connected in series, the total potential difference (pd) of the combination is the sum of the potential difference(p.d) of individual cells. Let the e.m.fs of three cells connected in series be E1, E2 and E3. Then the total e.m.f in the circuit,
E=E1+E2+E3.

11. How do you connect cells in parallel? What is the effective e.m.f of the combination

Cells in Parallel:-

When all positive terminals of two or more cells are connected to a single point and similarly all the negative terminals are connected to another single point, then the cells are said to be connected in parallel.
When cells of different e.m.fs are connected in parallel the effective p.d is equal to the e.m.f of that cell which has the greatest e.m.f.
Let three cells of e.m.fs E1,E2 and E3 such that E1>E2>E3 be connected in parallel. The effective e.m.f E is given by
E=E1.
12. Name some of the power sources and power consumers

Power sources:-

Battery, dry cells, power generators like dynamo, solar cells etc.

Power Consumers:-

Bulb, Electric heater, Washing machines, T.V, Radio, Computers, Electric motor etc.
Note:-

1. In an electric circuit, bulbs are said to be connected in series if the second terminal of the first bulb is connected to first terminal of the second bulb and so on.

2. Bulbs are said to be connected in parallel, if the first terminal of all the bulbs are connected to a common point and similarly all the second terminals are connected to another common point.

13. Electromotive force is measured in volts. Why

Since e.m.f is nothing but work done per unit positive charge, it is measured in volts.

14. A bulb connected to a series combination of cells glows with much higher brightness than a bulb connected to a single cell. Why

A bulb connected to a series combination of cells glows with much higher brightness than a bulb connected to a single cell because greater potential difference (e.m.f) is obtained in circuit when cells are connected in series.

15. When a number of bulbs are connected in series in a circuit, if one of them is removed or if it fails to work, the rest of the bulbs will not glow. Why

When a number of bulbs are connected in series in a circuit, and if one of them is removed or if it fails to work, the rest of the bulbs will not glow because removal of the bulb or its failure causes a break in the circuit.

16. When bulbs are connected in parallel and if one of them is removed or if it fails to work the rest of the bulbs will continue to glow. Why
When bulbs are connected in parallel and one of them is removed or if it fails to work the rest of the bulbs will continue to glow because only this particular part of the circuit remain unaffected even after the removal of the bulb or failure of the bulb.

17. Why are bulbs connected in parallel in house hold wiring

When a bulb in a room is put off, the bulbs in the other rooms continue to glow i.e., the other resistances in the circuit will have a continuous flow of current though one of them is detached. This is possible only in the parallel combination of resistances. Hence household bulbs are connected in parallel.

18. When three cells of e.m.fs 1 volt, 1.5 volts and 2 volts are connected in parallel. What would be the effective e.m.f

When cells of different e.m.fs are connected in parallel, then the effective potential difference is equal to the e.m.f of that cell which has the greatest e.m.f. Therefore e.m.f of above combination is equal to 2 volts.

19. Three dry cells each of the e.m.f 1.5 v are connected in parallel. What would be the effective e.m.f

When cells of equal e.m.f are connected in parallel, the effective e.m.f remains the same as that of any one of the cells. Therefore, e.m.f of the above combination is equal to 1.5 V.

20. Three dry cells each of e.m.f 1.5V are connected in series. What would be the effective e.m.f

When the cells are connected in series, the effective e.m.f is equal to the sum of the e.m.fs of the individual cells.
Effective e.m.f.=1.5+1.5+1.5
=4.5 Volts.

II. Problems:-

1. The total charge of 90 coulumbs flows in a conductor during a time of 5 minutes. What is the strength of the current in the conductor

Given,
Charge=q=90 coulumbs.
Time=t=5 minutes=5×60=300 seconds.
Current=i=?
We know that,

2. What is the total e.m.f when three cells of voltages 1V, 1.5 V and 2 Volts are connected:
a) In series .
b) In parallel.

 

3. What is the total quantity of charge which flows in 8 minutes when a current of 2 Ampere exist in a conductor

Given,
Current=i=2 Amperes.
Time=t=8 minutes=8×60=480 seconds.
Charge=q=?
We know that,

4. If the total e.m.f of the series combination of three cells of equal e.m.f is 4.5 V. What is the e.m.f of each cell

Let e.m.f. of each cell be E
Given,
Total e.m.f=4.5 Volts
→E+E+E=4.5 V
→3E=4.5 V
→E=1.5 V

ELECTRICAL RESITANCE-OHM’S LAW AND ITS VERIFICATION

1. Resistance:-

The electrical property of a conductor which opposes the flow of electrons is called electric resistance.
Or

The electrical property of a conductor by virtue of which opposition is offered to the free flow of electrons in a conductor is called ‘resistance’.

2. Ohm’s Law:-

The electric current(i) in a conductor is directly proportional to the potential difference(V) between its ends, at a constant temperature(T)
V α i or V=iR.

Note:-
The unit of resistance is Ohm (Ω).

3. Ohm(Ω):-

Resistance of a conductor is said to be one ohm if a potential difference of one volt between its ends causes a current of 1 ampere in it.

4. Describe an experiment to verify Ohm’s law.

Parts:-
A-ammeter, V-volt meter, R-load resistance, Rh-rheostat, B-battery.
1. Connect a battery (B) an Ammeter (A) a resistance (R) and a Rheostat (Rh) in series as shown in the figure.

2. Connect a voltmeter across R, let R be an unknown resistance and its value should be determined.

3. The current in the circuit can be varied with the help of the Rheostat (Rh).

4. Adjust the position of the Rheostat such that a maximum current flows in the circuit.

5. Then note the readings in the voltmeter and Ammeter.

6. By changing the position of Rheostat gradually, repeat the experiment and note down the corresponding values of voltage ‘V’ and current ‘i’ in the voltmeter and the Ammeter.

7. Tabulate the results as follows

S.no. Voltmeter reading

V  Volts

Ammeter reading

 i amperes

 
1

2

3

4

5

     

8. From the last column, it will be clear that is constant, which verifies Ohm’s law.

5. Ohmic conductors:-

1. The conductors which obey ohm’s law are called Ohmic conductors.

2. For Ohmic conductors, the relation between current ‘i’ and potential difference ‘v’ is linear.

3. Ohmic conductors are called Linear conductors.

4. All metallic conductors are called Ohmic conductors.

Examples:-

Sliver, Copper, Aluminium etc.,.

6. Non-Ohmic Conductors:-

1. The conductors which do not obey Ohm’s law are called Non-Ohmic conductors.

2. For Non-Ohmic conductors the relation between ‘i’ and potential difference ‘v’ is non linear.


3. Non-Ohmic conductors are also called non-linear conductors.

4. Semi conductors and electrolytes are examples for non-Ohmic conductors.

Note:-

1. The symbol of resistance is

2. Rheostat is used to regulate the value of current in a circuit.

3. The symbol of Rheostat is

7. Differences between Ohmic and Non-Ohmic conductors:-

Ohmic conductors Non-Ohmic conductors
1.  The conductors which obey Ohm’s law are called Ohmic conductors (or) linear conductors. 1.  The conductors which do not obey Ohm’s law  are called Non-Ohmic conductors (or) Non-linear conductors.
2.  Examples:-  Metals 2.  Examples:-semiconductors and electrolytes.
3.  Resistance of Ohmic conductors increases with increase in temperature. 3.  Resistance of Non-Ohmic conductors such as Semi conductors decreases with increase in temperature.
4.  For these conductors the graphs between V and  i is a straight line (linear). 4.  For these conductors the graph between V and  i  is curved line (non-linear).

II. Problems:-

1. The potential difference across a bulb is 240 volts and a current of 3 Ampere flows through it. Find the Resistance of the bulb?

Given,
Voltage=v=240 volts.
Current=i=3 Ampere.
Resistance=R=?
We know that,

2. An immersion heater of resistance 23 Ω is connected to a mains of 230 v supply. How much current flows through it

Given,
Resistance=R=23Ω
Voltage=V=230 v
Current=i=?
We know that,


3. A 1.5 v battery is connected across a small bulb. Calculate the resistance of the filament if the current flowing through it is 0.15A.

Given,
Voltage=V=1.5v
Current=i=0.15Amperes.
Resistance=R=?
We know that,

4. Calculate the current through a Resistance of 30Ω across which a potential difference of 4.5v is applied

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