Download X CBSE Electricity -summary sheet

ELECTRICITY
Summary sheet
1. Current: If a total charge Q is crossing a surface area in time t, then current is defined
as:
𝑄
𝐼=
𝑡
Current has units of Amperes.
2. Direction of flow of current: It is the direction in which a positive charge would flow
(from outside the battery).
3. Potential difference: Usually generated by a cell or battery. Chemical reaction in the
cell maintains a potential difference across its terminals. This potential difference
drives the current. It is defined as Work done in moving a charge Q from one point to
another.
𝑊
𝑉=
𝑄
4. Volt: 1 volt is the potential difference generated between two points when a work of
IJ is done in moving a charge of 1C between the two points.
5. Ohm’s law: Voltage between two points in a circuit is directly proportional to the
current in the circuit provided temperature is constant.
𝑉 = 𝐼𝑅
6. Resistance: It is the proportionality constant in Ohm’s law that links Voltage and
current. Has units of ohms (Ω).
7. Resistivity: Resistance of a uniform metallic conductor depends on two main factor:
(i) Length of conductor and (ii) Cross section area of conductor.
𝑙
𝑙
𝑅∝ →𝑅=𝜌
𝐴
𝐴
The constant of proportionality 𝜌 is called Resistivity.
8. Resistivity is independent of geometry of material. It changes from material to
material.
9. Both resistance and resistivity vary with temperature.
10. Alloys have higher resistivity than the constituent metals. Alloys do not oxidise easily.
Hence preferred for practical uses.
11. Resistances in Series: Effective resistance is larger than the largest resistance in the
circuit.
𝑅 = 𝑅1 + 𝑅2 + 𝑅3 + ⋯
12. Resistance in Parallel: Effective resistance is smaller than the smallest resistance in the
circuit.
1
1
1
1
=
+
+
+⋯
𝑅 𝑅1 𝑅2 𝑅3
13. Never connect appliances with widely varying resistances in series. If one goes – they
all stop getting current. If connected in parallel, even if one goes, the rest continue to
get current.
14. Part of battery energy is used to do work (rotating fan, turning motor etc.). Remaining
part is used to generate heat in the load. For purely resistive loads, entire energy is
converted to only heat in the resistance.
15. Power in the circuit:
𝑊 𝑉𝑄
𝑃=
=
= 𝑉𝐼
𝑡
𝑡
16. Heat dissipated in resistor:
𝐻 = 𝐸 = 𝑉𝐼𝑡 = 𝐼 2 𝑅𝑡
17. Electric power in circuit:
𝑉2
𝑃 = 𝑉𝐼 = 𝐼 2 𝑅 =
𝑅
18. Commercial unit of electric power
1 𝑢𝑛𝑖𝑡 = 1 𝑘𝑊 − ℎ𝑜𝑢𝑟 = 1𝑣𝑜𝑙𝑡 × 1 𝑎𝑚𝑝𝑒𝑟𝑒 × 1 ℎ𝑜𝑢𝑟 = 3.6 × 106 𝐽𝑜𝑢𝑙𝑒𝑠