INTRODUCTION
Electricity is one of the most useful forms of energy in
modern life. It is used in:
- Homes
- Schools
- Hospitals
- Industries
- Transportation
- Communication
systems
Electricity provides us:
✔ Light
✔ Heat
✔ Mechanical energy
✔ Sound
✔ Communication facilities
This chapter explains:
- Electric
current
- Electric
circuits
- Potential
difference
- Ohm’s
law
- Resistance
- Combination
of resistors
- Heating
effect of current
- Electric
power
11.1 ELECTRIC CURRENT AND CIRCUIT
ELECTRIC CURRENT
The flow of electric charge through a conductor is called:
Electric Current
In metallic conductors, electric current is due to the
movement of:
Electrons
DIRECTION OF CURRENT
Electrons are negatively charged and move from:
- Negative
terminal → Positive terminal
But conventionally, current is considered to flow from:
- Positive
terminal → Negative terminal
Thus:
Direction of current is opposite to direction of electron
flow.
ELECTRIC CIRCUIT
A continuous and closed conducting path through which
current flows is called:
Electric Circuit
Examples:
- Torch
circuit
- Bulb
circuit
- Fan
circuit
If the circuit breaks anywhere:
- Current
stops flowing
- Bulb
does not glow
ELECTRIC CURRENT FORMULA
Current is defined as the rate of flow of charge.
Where:
SI UNIT OF CURRENT
SI unit of current:
Ampere (A)
Named after:
Andre-Marie Ampere
IMPORTANT RELATION
Meaning:
If 1 coulomb of charge flows in 1 second, current is 1
ampere.
ELECTRIC CHARGE
SI unit of charge:
Coulomb (C)
Charge on one electron:
1 coulomb contains approximately:
SMALLER UNITS OF CURRENT
|
Unit |
Value |
|
1 milliampere (mA) |
|
|
1 microampere (μA) |
|
AMMETER
Instrument used to measure electric current.
Important Rule
- Ammeter
is always connected in:
Series
with the circuit.
EXAMPLE 11.1
A current of 0.5 A flows for 10 minutes.
Given:
Using:
Answer:
Charge = 300 C
11.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
WHY DO CHARGES FLOW?
Charges flow only when there is a difference in electric
pressure between two points.
This difference is called:
Potential Difference
POTENTIAL DIFFERENCE
Definition:
Work done to move unit charge from one point to another.
Where:
SI UNIT OF POTENTIAL DIFFERENCE
SI unit:
Volt (V)
Named after:
Alessandro Volta
DEFINITION OF 1 VOLT
Meaning:
If 1 joule work is done to move 1 coulomb charge, potential
difference is 1 volt.
VOLTMETER
Instrument used to measure potential difference.
Important Rule
- Connected
in:
Parallel
across the component.
EXAMPLE 11.2
Charge
Potential difference
Using:
Answer:
Work done = 24 J
11.3 CIRCUIT DIAGRAM
Electric circuits are represented using symbols.
Important circuit symbols:
|
Component |
Symbol Meaning |
|
Electric cell |
Source of energy |
|
Battery |
Combination of cells |
|
Open switch |
Circuit broken |
|
Closed switch |
Circuit complete |
|
Ammeter |
Measures current |
|
Voltmeter |
Measures voltage |
|
Resistor |
Opposes current |
|
Rheostat |
Variable resistance |
11.4 OHM’S LAW
German scientist:
Georg Simon Ohm
discovered relationship between current and potential
difference.
OHM’S LAW
At constant temperature:
Current through conductor is directly proportional to
potential difference across it.
Thus:
Where:
OHM’S LAW GRAPH
The graph between:
- Potential
difference (V)
and - Current
(I)
is a:
Straight line passing through origin
This shows:
RESISTANCE
The property of conductor that opposes flow of current is
called:
Resistance
SI unit:
Ohm (Ω)
DEFINITION OF 1 OHM
If:
- Potential
difference = 1V
- Current
= 1A
then:
Resistance = 1Ω
FORMULAS FROM OHM’S LAW
RHEOSTAT
A device used to change resistance and regulate current.
ACTIVITY 11.1
Nichrome wire experiment.
Observation:
- As
voltage increases, current also increases.
Conclusion:
Ohm’s law verified.
ACTIVITY 11.2
Different components connected in same circuit show
different current values.
Reason:
Different materials offer different resistances.
11.5 FACTORS AFFECTING RESISTANCE
Resistance depends on:
- Length
of conductor
- Area
of cross-section
- Nature
of material
- Temperature
EFFECT OF LENGTH
Resistance increases with length.
EFFECT OF AREA
Resistance decreases with area.
RESISTIVITY FORMULA
Combining both:
Where:
SI unit of resistivity:
Ωm
RESISTIVITY OF MATERIALS
Good Conductors
- Silver
- Copper
- Aluminium
High Resistivity Materials
- Glass
- Rubber
- Ebonite
IMPORTANT TEXTBOOK EXAMPLES
Nichrome
Used in:
- Electric
iron
- Heater
- Toaster
Reason:
- High
resistivity
- High
melting point
- Does
not oxidise easily
Tungsten
Used in bulb filaments because:
- Very
high melting point
Copper and Aluminium
Used in transmission wires because:
- Low
resistivity
EXAMPLE 11.3
Electric bulb resistance =
Voltage =
Using:
11.6 RESISTANCE OF SYSTEM OF RESISTORS
Two types:
- Series
combination
- Parallel
combination
RESISTORS IN SERIES
Resistors connected end-to-end.
Important Features
✔ Same current through each
resistor
✔ Total voltage divides
Equivalent resistance:
Total resistance increases.
ACTIVITY 11.4
Observation:
Current remains same at all positions in series circuit.
ACTIVITY 11.5
Observation:
Total voltage equals sum of individual voltages.
EXAMPLE 11.7
20Ω lamp and 4Ω resistor connected in series.
Equivalent resistance:
Current:
RESISTORS IN PARALLEL
Resistors connected between same two points.
Important Features
✔ Same voltage across each
resistor
✔ Current divides
Equivalent resistance:
Total resistance decreases.
ACTIVITY 11.6
Observation:
Total current equals sum of branch currents.
ADVANTAGES OF PARALLEL COMBINATION
✔ Appliances work independently
✔ Different appliances get required current
✔ One appliance failure does not stop others
DISADVANTAGES OF SERIES COMBINATION
✔ Same current for all devices
✔ Failure of one device stops whole circuit
Example:
Fairy lights
11.7 HEATING EFFECT OF ELECTRIC CURRENT
When current flows through resistor:
- Electrical
energy converts into heat energy.
This is called:
Heating Effect of Electric Current
Applications:
- Electric
iron
- Heater
- Toaster
- Kettle
JOULE’S LAW OF HEATING
Heat produced:
Using Ohm’s law:
Heat produced depends on:
- Square
of current
- Resistance
- Time
EXAMPLE 11.10
Electric iron:
Power = 840W
Voltage = 220V
Current:
PRACTICAL APPLICATIONS OF HEATING EFFECT
Electric Heater and Iron
Use heating effect to produce heat.
Electric Bulb
Uses tungsten filament.
Reason:
- High
melting point
Bulb filled with:
- Nitrogen
- Argon
gas
to increase filament life.
ELECTRIC FUSE
A safety device protecting circuits from excessive current.
Working
- Excess
current heats fuse wire
- Fuse
melts
- Circuit
breaks
FUSE MATERIALS
Made from:
- Aluminium
- Copper
- Iron
- Lead
alloys
Domestic fuse ratings:
- 1A
- 2A
- 3A
- 5A
- 10A
11.8 ELECTRIC POWER
Electric power is the rate of consumption of electrical
energy.
Formula:
Using Ohm’s law:
SI UNIT OF POWER
Watt (W)
Definition:
1 watt is power consumed when:
- Current
= 1A
- Voltage
= 1V
COMMERCIAL UNIT OF ELECTRIC ENERGY
Kilowatt-hour (kWh)
Also called:
Unit
EXAMPLE 11.12
Voltage = 220V
Current = 0.5A
Power:
EXAMPLE 11.13
Refrigerator:
- 400W
- 8
hours/day
- 30
days
Energy:
Cost at Rs 3/unit:
IMPORTANT DEFINITIONS
|
Term |
Definition |
|
Electric current |
Flow of electric charge |
|
Potential difference |
Work done per unit charge |
|
Resistance |
Opposition to current |
|
Resistivity |
Characteristic property of material |
|
Electric power |
Rate of electrical energy consumption |
