Current Electricity Concept Page - 13

Example

Example of a problem on Joule's Law

Example:
The internal resistance of a primary cell is 4$4$ ohm. It generates a current of 0.2$0.2$ ampere in an external resistance of 2$2$ ohm. Find the rate at which  chemical energy is consumed in providing the current.Solution:
The rate of consumption of chemical energy = Rate of generation of electrical energy.
The energy generated is calculated from the formula P=VI$P=VI$. Substituting V=IR$V=IR$ (Ohm's law) in the above equation we get, 

Power=VI=I2R$Power=VI=I^{2}R$. 

In this case, there are external and internal resistances. So, the equation is rewritten as I2(R+r)$I^2(R+r)$.
Therefore, Chemical energy = 0.22(4+2)=0.24J/s.${0.2}^2(4+2)=0.24J/s.$
Hence, the rate at which  chemical energy is consumed in providing the current is 0.24 J/s$0.24J/s$.

Result

Heating effect of current

Due to the collisions of electrons in the wire of  resistance, energy is lost causing the heating  of resistor.

Definition

Identify the appliances without heating coil

Microwave oven and induction cooker are appliances which do not use a heating coil. In a microwave oven, microwaves are used and induction cooker uses eddy currents.

Definition

CFL as a Power saver

CFL : CFL is the  Compact Fluorescent Lamp/Light ,which is designed to  save energy and electricity. It has worked as replacement to old incandescent bulbs because it saves 1/8th to 1/5th times of energy and last longer.
An electric bulb is used for light but it also gives heat. This is not desirable. This results in the wastage of electricity. This wastage can be reduced by using fluorescent tube lights in place of the bulbs. Compact fluorescent lamps (CFLs) also reduce wastage and can be fixed in the ordinary bulb holders.

Definition

Describe the construction and working of a compact fluorescent lamp(CFL)

Construction and working:Compact fluorescent lamps are designed to work at low power. The length of the tube is reduced considerably compared to that of a fluorescent lamp. The working of both the fluorescent lamps and the compact fluorescent lamps are almost similar. the glass tube of CFL is connected to that part which contains electronic circuits.

Definition

Describe the material used to make heating coils and state its advantages

Heating coils are made of nichrome. Nichrome is an alloy of nickel, chromium, iron and manganese.
The advantages of nichrome are made use of in electric heating appliances are as follows:

• high resistivity
• high melting point
• ability to remain in red hot condition for long time without getting oxidised

Result

Applications of heating effect of current

Some common applications of heating effects of electric current are electrical laundry iron, toaster, heater, electrical bulb and fuses.

Law

Joule's Law of heating

According to Joule's Law of heating, heat generated in a conductor is given by:
H=VIt=V2Rt=I2Rt$H=VIt={\displaystyle \frac{V^2}{R}}t=I^{2}Rt$

Definition

Experimental verification of Joule's Law of heating

Experimental setup for the verification of Joule's Law is shown in the attached figure. Before starting the experiment, the value of resistance is measured. A heating coil is dipped in a thermally insulated beaker filled with water of known mass and current of known constant amplitude (measured by ammeter and adjusted by variable resistance) is passed through it. A thermometer is dipped in the wire at time t=0$t=0$. The readings of thermometer are noted are regular intervals of time of 60$60$ seconds. 

The heat supplied to the resistor in time t$t$ is given by: Q1=I2Rt$Q_1=I^{2}Rt$
Heat absorbed by water is Q2=msΔT$Q_2=ms\mathrm{\Delta }T$
At all time instants, Q2$Q_2$ and Q1$Q_1$ are computed. Validity of Joule's Law is verified by the equality of Q1$Q_1$ and Q2$Q_2$.

Diagram

Battery and its combinations

Battery : Battery is the combination of two or more cells in series. The given diagram shows the combination of cells to form battery.

BookMarks

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