Question

A lamp is labelled '230 V, 100 W'. How many joules of electrical energy is changed to thermal energy and light if the lamp is switched on for 2 hours?

Answer 1

Formula E(J) = P(W) × t(s)

3600 Seconds in an hour

7200 seconds in two hours.

watts = 100

100 x 7200 = 720000

Step-by-step explanation:

Watts to joules calculation

The energy E in joules (J) is equal to the power P in watts (W), times the time period t in seconds (s):

E(J) = P(W) × t(s)

Energy cannot be created or destroyed, only converted from one form to another. These conversions can be shown in Sankey diagrams. Efficiency is a measure of how much useful energy is converted. Attachment 1

Modern energy-saving lamps and LEDs (light-emitting diodes) work in a different way. They transfer a greater proportion of electrical energy as light energy. Attachment 2

From the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy from the energy-saving lamp. It's more efficient than the filament lamp.

Calculating efficiency

The efficiency of a device, such as a lamp, can be calculated:

efficiency = useful energy out ÷ total energy in (for a decimal efficiency)

or

efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)

The efficiency of the filament lamp is 10 ÷ 100 = 0.10 (or 10%). This means that 10 per cent of the electrical energy supplied is transferred as light energy (90 per cent is transferred as heat energy).

The efficiency of the energy-saving lamp is 75 ÷ 100 = 0.75 (or 75 per cent). This means that 75 per cent of the electrical energy supplied is transferred as light energy (25 per cent is transferred as heat energy).

Note that the efficiency of a device will always be less than 100 per cent. Occasionally the power is shown in W instead of the energy in J. The equations are the same – just substitute power for energy:

efficiency = useful power out ÷ total power in (for a decimal efficiency)

or

efficiency = (useful power out ÷ total power in) × 100 (for a percentage efficiency)

This is the Sankey diagram for a typical energy-saving lamp:

you can find the resistance using this formula

R = V ÷ I

where; V is the potential difference in volts, V

I is the current in amperes (amps), A

R is the resistance in ohms, Ω

Answer 2

Given;

V = 230V

Power, P = 100W

time, t = 2hrs = 7200s

from,

P = IV

and Energy, E= Pt

E = 100*7200

E = 720000 Joules

E = 720KJ