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Transcript
ELECTRICITY Summary sheet 1. Current: If a total charge Q is crossing a surface area in time t, then current is defined as: π πΌ= π‘ 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. π π= π 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. Ohmβs law: Voltage between two points in a circuit is directly proportional to the current in the circuit provided temperature is constant. π = πΌπ 6. Resistance: It is the proportionality constant in Ohmβs law that links Voltage and current. Has units of ohms (Ξ©). 7. Resistivity: Resistance of a uniform metallic conductor depends on two main factor: (i) Length of conductor and (ii) Cross section area of conductor. π π π β βπ =π π΄ π΄ The constant of proportionality π 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. π = π 1 + π 2 + π 3 + β― 12. Resistance in Parallel: Effective resistance is smaller than the smallest resistance in the circuit. 1 1 1 1 = + + +β― π π 1 π 2 π 3 13. Never connect appliances with widely varying resistances in series. If one goes β 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: π ππ π= = = ππΌ π‘ π‘ 16. Heat dissipated in resistor: π» = πΈ = ππΌπ‘ = πΌ 2 π π‘ 17. Electric power in circuit: π2 π = ππΌ = πΌ 2 π = π 18. Commercial unit of electric power 1 π’πππ‘ = 1 ππ β βππ’π = 1π£πππ‘ × 1 ππππππ × 1 βππ’π = 3.6 × 106 π½ππ’πππ