UEENEEE003BSolve Problems in Single Path DC Circuits

Energy, Work and Torque

Refers to an activity involving a force and movement in the direction of the force.

A force of 20 newtons pushing an object 5 meters in the direction of the force does 100 joules of work.

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Work

Is the rate of doing work or the rate of using energy, which are numerically the same.

If you do 100 joules of work in one second (using 100 joules of energy), the power is 100 watts.

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Power

Is the capacity for doing work You must have energy to accomplish work – it is

like the ‘currency’ for performing work. To do 100 joules of, you must expend 100 joules

of energy.

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Energy

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Energy cannot be created or destroyed but can beconverted from one form to another. When energyis converted, work is done. To produce electrical

energy one of the following forms of energy mustbe converted:

1. mechanical energy (friction, pressure);

2. chemical energy (chemical action);

3. magnetic energy (magnetism);

4. heat energy (heat);

5. light energy (light).

Energy

◦Work◦Work is done when a body is moved through a distance by a force acting on it.

◦Power◦The rate of doing work◦Energy◦The potential to do work

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Work, power and energy

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Torque

Torque is the turning effort a motor uses to turn something.

This motor develops torque to turn the gears.

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Measurement of torque

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Torque

Power = 2 x π x N x T / 60

T = torque N = RPM

How do we find the Torque ?

If

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Kilowatt

Kilowatt- hour meter to measure

electrical energy.

Kilowatt hours is a convenient unit rather than

watt-seconds

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Efficiency of a Machine

Motors

The Output power of a motor is mechanical. This is calculated using the speed (RPM) and the force of rotation.

The Input power is electrical. This is calculated using Voltage, and Current.

Power = Current x Voltage (watts) P = IV

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Watts loss area

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Efficiency

Output

= ------------ x 100

Input

Always as a percentage

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An electric drive motor takes 48kW from the supply and delivers 42kW out of a gearbox into the load.

What is the efficiency of the machine ?

If a similar machine took 56kW from the supply and delivered energy to the load at 85% , what is the output of the unit ?

Student exercise

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Whenever electric current flows, heat is generated. Nearly all electric circuits produce some heat.

This heat is “lost” power. P = I² x R We will look at this later.

Losses are an indication of efficiency