Class 10 Science — Electricity
Quality Notes | Chapter 11
1. Electric Current and Electric Circuit
Electric current is the rate at which electric charge passes through a cross-section of a conductor.
I=\frac{Q}{t}
Where:
- (I) = current
- (Q) = charge
- (t) = time
SI unit: ampere (A)
1A=1C/s
In metallic conductors, electrons are the moving charges. However, conventional current is taken opposite to the direction of electron flow.
Electric Circuit
A closed, continuous conducting path through which current can flow is an electric circuit. Opening the switch breaks the path and stops the current.
Ammeter: measures current and is connected in series.
2. Electric Potential Difference
Charges do not normally flow through a conductor simply because the conductor exists. A potential difference is needed to drive charge through the circuit.
A cell or battery produces potential difference through chemical action.
V=\frac{W}{Q}
Where:
- (V) = potential difference
- (W) = work done
- (Q) = charge
SI unit: volt (V)
1V=1J/C
Thus, a potential difference of 1 V means that 1 J of work is done in moving 1 C of charge between two points.
Voltmeter: measures potential difference and is connected in parallel across the required component.
Important relation
W=VQ
3. Ohm’s Law
For a metallic conductor whose temperature remains constant, the potential difference across it is directly proportional to the current through it.
V\propto I
Therefore,
[boxed{V=IR}
where (R) is the resistance.
Resistance
Resistance is the property of a conductor that opposes the flow of electric charge.
\boxed{R=\frac VI}
SI unit: ohm ((\Omega))
1\Omega=1V/A
From Ohm’s law:
\boxed{I=\frac VR}
So, for a fixed voltage, greater resistance means smaller current.
V–I Graph
For an ohmic conductor at constant temperature, the (V-I) graph is a straight line through the origin. Its constant ratio (V/I) represents resistance.
4. Factors Affecting Resistance
The resistance of a conductor depends mainly on:
- Length (l)
- Area of cross-section (A)
- Nature of material
- Temperature also affects resistance/resistivity.
For a uniform conductor:
R\propto l
and
R\propto\frac1A
Combining:
\boxed{R=\rho\frac lA}
where (\rho) is the resistivity of the material.
Key conclusions
- Longer wire → greater resistance
- Thicker wire → smaller resistance
- Different materials → generally different resistances
- Resistivity is a characteristic property of the material.
SI unit of resistivity: (\Omega,m).
Practical applications
- Alloys generally have higher resistivity and can withstand high temperatures, so they are useful in heating devices.
- Tungsten is used for bulb filaments because it has a very high melting point.
- Copper and aluminium are used in transmission lines because of their low resistivity.
5. Combination of Resistors
Resistors can mainly be connected in:
- Series
- Parallel
A. Resistors in Series
Resistors connected end-to-end form a series combination.
Current
The same current passes through every resistor.
I=I_1=I_2=I_3
Potential difference
V=V_1+V_2+V_3
Equivalent resistance
boxed{R_s=R_1+R_2+R_3}
Therefore, series equivalent resistance is greater than any individual resistance.
Remember
Series → Same Current
B. Resistors in Parallel
In a parallel arrangement, the resistors are connected across the same two points.
Potential difference
The potential difference across every branch is the same:
V=V_1=V_2=V_3
Current
\boxed{I=I_1+I_2+I_3}
Equivalent resistance
\boxed{\frac1{R_p}
\frac1{R_1}+\frac1{R_2}+\frac1{R_3}
The equivalent resistance of a parallel combination is less than the smallest individual resistance.
Remember
Parallel → Same Voltage
Series vs Parallel — High-Value Comparison
| Series | Parallel |
|---|---|
| Same current through each component | Same potential difference across each branch |
| (R_s=R_1+R_2+…) | (\frac1{R_p}=\frac1{R_1}+\frac1{R_2}+…) |
| Equivalent resistance increases | Equivalent resistance decreases |
| Failure of one component can break the whole circuit | Other branches can continue operating |
| Not suitable for domestic appliances | Suitable for domestic circuits |
Domestic appliances are connected in parallel because different appliances can operate according to their individual current requirements, and failure of one does not necessarily stop the others.
6. Heating Effect of Electric Current
When current passes through a resistance, electrical energy can be converted into heat energy.
For a current (I) through resistance (R) for time (t):
\boxed{H=VIt}
Using Ohm’s law:
\boxed{H=I^2Rt}
This is Joule’s law of heating.
Joule’s law tells us:
H\propto I^2
for fixed (R,t);
H\propto R
for fixed (I,t);
H\propto t
for fixed (I,R).
Applications
The heating effect is used in:
- electric iron
- toaster
- electric oven
- electric kettle
- electric heater
- electric bulb
- electric fuse
Electric Fuse
A fuse protects a circuit from excessively high current. It is connected in series. If excessive current flows, the fuse wire heats up, melts, and breaks the circuit.
7. Electric Power
Electric power is the rate at which electrical energy is consumed or converted.
\boxed{P=VI}
Using Ohm’s law:
\boxed{P=I^2R}
and
\boxed{P=\frac{V^2}{R}}
SI unit: watt (W)
1W=1V\times1A
A larger practical unit is:
1kW=1000W
8. Electrical Energy
Electrical energy consumed is:
\boxed{E=Pt}
Since (P=VI):
\boxed{E=VIt}
For a resistor, the same quantity can be expressed through the heating relation:
E=I^2Rt
Commercial Unit
Electrical energy supplied to homes is commonly measured in kilowatt-hour (kWh).
\boxed{1kWh=3.6\times10^6J}
One kWh is commonly called one unit of electricity.
Electricity Cost
\boxed{\text{Cost}=\text{Energy consumed in kWh}\times\text{Cost per kWh}}
9. Formula Sheet — Must Know
Current
\boxed{I=\frac Qt}
Potential difference
\boxed{V=\frac WQ}
Ohm’s law
\boxed{V=IR}
Resistance
\boxed{R=\frac VI}
Current
\boxed{I=\frac VR}
Resistance of a wire
\boxed{R=\rho\frac lA}
Series combination
\boxed{R_s=R_1+R_2+R_3}
Parallel combination
\boxed{\frac1{R_p}=
\frac1{R_1}+\frac1{R_2}+\frac1{R_3}}
Heat produced
\boxed{H=VIt=I^2Rt}
Electric power
\boxed{P=VI=I^2R=\frac{V^2}{R}}
Electrical energy
\boxed{E=Pt=VIt}
Energy conversion
\boxed{1kWh=3.6\times10^6J}
10. Exam-Focused Concept Triggers
If the question says…
- Rate of flow of charge → Current
- Work done per unit charge → Potential difference
- Opposition to charge flow → Resistance
- (V) directly proportional to (I) → Ohm’s law
- Longer wire → Resistance increases
- Thicker wire → Resistance decreases
- Same current → Series
- Same voltage → Parallel
- Heat due to current → Joule heating
- Rate of electrical energy consumption → Power
- Commercial unit of electrical energy → kWh
- Protective device against excessive current → Fuse
11. The Chapter in One Flow
Cell/Battery
↓ creates potential difference
Potential difference
↓ drives charge
Electric current
↓ encounters
Resistance
↓ obeys (V=IR) for an ohmic conductor at constant temperature
↓ produces
Heat (H=I^2Rt)
and electrical energy is consumed at a rate called
Power (P=VI).
Final Revision Priority
If you have limited time, learn these first:
- (I=Q/t)
- (V=W/Q)
- Ohm’s law: (V=IR)
- (R=\rho l/A)
- Series and parallel rules
- (H=I^2Rt)
- (P=VI=I^2R=V^2/R)
- (1,kWh=3.6\times10^6J)
- Ammeter → series
- Voltmeter → parallel
- Series → same current
- Parallel → same potential difference