Physics Chapter 8 Electricity

Physics Chapter 8  Electricity

1. ELECTRICITY: “Is the rate of flow of electrons”. i = q/t

2. To keep electrons move, potential difference is needed. This is done by a cell or battery.

3. Potential difference “is the amount of work done in moving a unit positive charge from one point to other.”

Potential difference V = Work done (W) / charge moved (Q)  Or W = QV but charge = it  Hence, W = VIt or Electrical energy = VIt Power is “Rate of doing work” P = W/t = VIt/t = VI

Power is measured in watt or J S-1

4.1 Watt = 1 Volt × 1 Ampere

S.I. unit of charge is coulomb (C).

S.I. unit of current is Ampere (A).

S.I. unit of P.D. is volt.

S.I. unit of electrical energy is Joule (J) and of power is watt (W)

1 kWh-3600000J = 3.6 x 106 J

5. ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as these stations are at very far off place from areas where it is to be used. The voltage (A.C.) is of 50 HZ frequency.

6. AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 132 kV to minise the loss of energy in transmission line wires.

7. At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and transmitted to city SUB-STATION.

8.  At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to hourses for consumers.

9.  Colour coding: Live wire — Red or Brown

Neutral—Black or light blue

Earth wire — Green or yellow

10.  1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when worked on a 220 V will consume 100 W electricity power

11. OVER LOADING: is the condition of Electric circuit, when it draws more current than it is designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. Electric motor etc. or a large number of plugs are put in the same socket.

12.  EARTHING: is done in a house near the kWh meter. Earthing is a safety device which puts the appliance at zero potential.

 

13 .SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used with an appliance) breaks. The LIVE WIRE

COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING

14.  FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse wire should have

(i) High resistance

(ii) Low melting point.

These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when current flowing excess.

15.  Appliances in a house are connected in parallel.