4_Work and power of current. Joule-Lenz's law

Start. Energy transformation in electric circuits

Teacher asks learners to draw a simple circuit consisting of electric source lamp, switch and wiring. Question:

• What kind of energy transformation in electric circuit when switch is close?

(W) Whole class discussion. Energy transformations

Electric energy is useful to us because it can be easily transformed into other forms of energy. Motors transform electric energy into mechanical energy. In other devices such as electric heaters, stoves, toasters, and hair dryers, electric energy is transformed into thermal energy in a wire resistance known as a “heating element.” And in an ordinary lightbulb, the tiny wire filament becomes so hot it glows; only a few percent of the energy is transformed into visible light, and the rest, over 90%, into thermal energy. Lightbulb filaments and heating elements in household appliances have

resistances typically of a few ohms to a few hundred ohms.

Electric energy is transformed into thermal energy or light in such devices, and there are many collisions between the moving electrons and the atoms of the wire. In each collision, part of the electron’s kinetic energy is transferred to the atom with which it collides. As a result, the kinetic energy of the wire’s atoms increases and hence the temperature of the wire element increases.

The increased thermal energy can be transferred as heat by conduction and convection to the air in a heater or to food in a pan, by radiation to bread in a toaster, or radiated as light.

(T) Teacher explanation. Joule-Lenz's law

Joule-Lenz's law shows the relationship that exists among:

• The current that flows through a wire.
• The heat generated by the current that flows through a wire.
• The resistance of the wire and …
• The time the current passes through the wire.

Joule’s law formula: Q = I² × R × t

where:

Q is the amount of heat, in Joules (J)

I is the electric current that flows through the wire, in amperes (A)

R is the value of the electrical resistance of the wire, in ohms (R)

t is the amount of time that current passes through the wire, in seconds (s).

Heating effect

Joule’s law can be established as the amount of heat (Q) generated in a wire with resistance (R), when a current (I) passes through it for a period of time (t).

This heat is directly proportional to:

• The square of the current.
• The resistance of wire.
• The time the current flows through the wire.

(I) Individual work (differentiation)

• What are the heating effects caused by electric current?

Able students can answer for this question and teacher asks them to other learners.

Worked example

Calculate the amount of energy supplied by a 4.5 V battery when:
(a) a charge of 20 C passes through it
(b) a current of 25 mA flows through is for

      3 minutes

Solution:

Energy = potential difference x   

     Q = 4.5 x 20 = 90 J
(b) Energy = potential difference x Q =

     p.d x current x time 

Therefore:
Energy = 4.5 x 25x10-3 x 180 = 20.25 J

(f) Formative assessment.

Set of calculations for electrical energy

1.  Calculate the electrical energy produced in 5 minutes when a current of 2 A is sent through a conductor by a potential difference of 500 volts.

                                                                                                                                                   [1]

2.  A current of 5 ampere is passed through a conductor of 12 ohms for 2 minutes. Calculate the amount of heat produced.                                                                                                     [1]

3. What is heating effect of current? List two electrical appliances which work on this effect.

                                                                                                                                                    [2]

Answers

1.  W=3 ∙ 10⁵ J                                                                                                                               [1]

2.   3.6  10⁴ J                                                                                                                                 [1]

3. When electric current passes through a high resistance wire, the wire becomes and produces heat. This is called heating effect of current. electric Iron, Electric Heater are two appliances which work on this effect