EMF

Examples of electromotive force energy sources are

Cells and Batteries/ Generators/ Electromagnetic induction/ Photo-electric cells/ Thermocouples/ Piezo-electric cells/ Static electricity

The kilowatt hour is the unit of energy that

Energy authorities use as base to charge customers

If cost of energy is 9cents per kWh, much much to operate 1.5kW heater for 8hours?

Energy cost= Power x Time x Cost/Unit

How many joules if energy will motor produce when running full load for one hour?

Energy produced= 373 watts x (60 x 60) seconds Answer in Kilo Joules

If current flows for 5 hours how much energy expended

Energy= Power x Time

Example of dc ac circuits containing resistance are

Heating elements and incandescent lighting circuits

To measure current a 1kw draws from 230v supply

I=p/v

If an electric motor is rated at 1/2 h.p what is the power rating in kW

Power rating= h.p x 746 Then convert watts to Kilowatts

Energy cost

Power x time x cost/kWh Power= kilo Watts Time= Hours

What is the difference between primary and secondary cells in batteries?

Primary cannot be recharged, there are carbon zinc dry cells used in torches portable radios and lithium cells calculators and cameras. Secondary cells can be recharged as the chemical action is reversible and they may be charged by a DC source, there are lead acid car batteries and nickel cadmium alkaline cells used in heavy vehicles, boats and emergency lighting systems.

To find resistance of 230v 1kw heating element

R=v squared/power

Photo electric cells work by

Some materials produce EMF when light falls on them, one such material is selenium and one type of photo electric cell consists of a layer of selenium deposited on a metal base and covered by a very thin transparent gold film. Light when passing through gold film releases free electrons from the selenium to give the gold film a negative potential, and the metal base plate a positive potential. The EMF is 0.4v, so only small, but useful to measure light, its used in cameras, to measure lighting levels in buildings, and for counting objects on production lines when they break a light beam. A basic cell contains Metal plate/Selenium layer/ Transparent gold film/ collector ring/base.

Energy is

The ability to do work

Power product of voltage current in ac when

The circuits contain resistance

Production of electromotive force is defined as

The force that causes current to flow in an electric circuit. To produce EMF, some other form of energy must be converted into electrical energy.

Power is

The rate of doing work

Power is the product of

Voltage and current

To measure the voltage output in a thermocouple

Vout = V1- V2 the temperature difference between the two dissimilar junctions

Unit of power

Watt

Power in electric circuit is measured in

Watts

the charge a lead acid battery you will need

a DC power supply with a higher voltage than the battery voltage, the terminals are connected postitive to positive and negative to negative so that current is passed through the battery in the reverse direction to the discharging current.

if a dc motor supplied at 230v draws current of 8amps when producing output power of 1500watts, what is the efficiency of this motor?

find input power first Input power= V x I, then knowing input power calculate efficiency= output power/input power x 100%

how much current would an electric motor draw from 200v dc supply if its output power is 3/4h.p and its efficiency is 80%?

find output power first by converting h.p to watts. Then use the formula input power= output power/efficiency x 100%. THEN to find the current use the formula I= P(in)/V and the answer will be in Amps

Safety considerations when charging a battery are

hydrogen gas is released when charging a battery and this creates a danger of fire or explosion so charging areas are classed as hazardous. Since the electrolyte is highly corrosive sulphuric acid, if spilt it needs to be neutralised with a solution of baking soda, washing soda or ammonia in water. Take precautions when handling acid.

the find the amount of charge in a lead acid battery you use a

hydrometer, this device measures the gravity of the sample electrolyte taken from the battery, this amount of gravity is indicated within the device. A fully charged cell will have a specific gravity of 1250 and a terminal voltage of 2.2volts, and a discharged cell will have a specific gravity of 1180 and its voltage falls below 1.8volts.

batteries in parallel have

more Amp/hour but the voltage stays the same as a single battery voltage. This lasts longer.

Nickel Alkaline batterys are another type of secondary cell but are

more expensive, have a longer shelf life and are stronger in construction and lighter in weight. Used in marine applications and emergency lighting installations.

batteries in series have

more voltage (each battery voltage value is added) and one value Amperes/hour for all the batteries. Shorter life span but more power.

Batteries are made up of either

primary or secondary cells

Piezo-electric cells are crystals that

produce EMF when compressed, lead zirconate is common, when compressed with a sharp hammer type action it can produce voltage of over 1000volts. Which the energy doesnt pass current flow, it can produce a spark to ignite gas appliances, gas bbq and cookers use this principle.

the efficiency of a motor is defined as the

ratio between its output power and input power expressed as a percentage

The operation of a basic cell is

the chemical action between two disimilar electrodes in an electrolyte, one will transfer electrons to become negative whilst making the other positive. This causes EMF, when the cells stop producing a chemical action this means there is no more EMF, it is considered discharged. In primary cells this battery would be discarded, whereas in a secondary cell a DC power source can cause the chemical action to be reversed by passing current in the reverse direction.

what happens to a lead acid cell when discharging

the chemical action removes sulphur from the electrolyte and deposits it on the electrodes to form lead sulphate on both plate. This action dilutes the acidic electrolyte more. when the S.G drops to 1180 voltage 1.8v the cell is discharged.

1kw is how much horsepower

1.34hp

mercury primary cells have

1.35v/size-small/shape- disk shape/advantage- suitable for higher current loads/App- Watches, cameras (is toxic)

alkaline primary cells have

1.5v/size- small med/shaped- cylinder block/advantage- suitable for higher current loads, longer life than carbon zinc/ App- remote controlled toys, CD players.

carbon zinc primary cells have

1.5v/sized- small med/ shaped- cylinder block/ advantage-relatively cheap/application- torches, radio,small toys, clocks

K W h

1000watts x 60 x 60 seconds= 3,600,000 joules

lithium primary cells have

3.0v/size- very small/shape- disc shape/adv- higher voltage, smaller size, longer life/ digital cameras/calculators/video games

One horsepower equals

746 watts or 0.746 kW

The difference between EMF and terminal voltage of a battery.

A battery EMF is the set value of volts a battery is given. Battery has internal resistance so value drops so terminal voltage always less than EMF. A voltmeter connected across a cell or battery draws no significant current because of its very high resistance so it measures the open circuit voltage or EMF of the cell or battery, but when the cell or battery is delivering current to an external load the same voltmeter will indicate the terminal voltage of the cell or battery.

How does a battery work?

A battery is a collection of electrochemical cells connected together, each of the cells produce EMF when two electrodes transfer electrons between one another, the electrons become either positive or negative charges when immersed in an electrolyte solution which causes current to flow in a circuit when a battery is connected to a circuit.

Thermocouples

A thermocouple consists of a welded junction between two different metal wires, when the junction is heated, the electons move away from the heat in one of the metals and towards the heat in the other metal. This produces small EMF, because the EMF is proportional to the heat applied , thermocouples are used for temperature measurement in industrial control processes. The two main types are Copper-constantan, used up to 300degrees and at that temperature produces 20milli volts(0.02v) Iron-constantan, used up to 800 degrees and at that temperature produces about 40 milli volts (0.04v) Constantan is an alloy of copper and nickel.

A joule is

A watt second/joules per second

Why cant Nickel alkaline batteries be tested with a hydrometer?

Because the specific gravity of the electrolyte does not change during charge and discharge

Voltage current can be out of phase

Power factor

Unit of energy

Joule

Work is what is mechanical energy

Joules

Work is what in electrical energy

Kilo watt hours

Whats the difference between lead acid batteries and nickel alkaline.

Lead-Acid have cell voltage of 2.2v, smaller size, higher discharge currents, deteriorates if left discharged for long periods, S.G of electrolytes indicates the state of charge. Nickel-Alkaline have cell voltage of 1.2v, more cells needed for same voltage, lighter in weight, more expensive, stronger construction, can be left discharged without harm, S.G of electrolyte.

What are the two main types of Nickel alkaline batteries

Nickel Cadmium and Nickel Iron.

One watt is produced when

One volt causes one amp to flow

One joule is expended when

One watt is applied to circuit for one second

Find power in 4.35amps from 230 v supply

P= V times I

230v applies to resistor current in resistor 5amps, how much power in resistor

P=VI

Solar cells are

Photo voltaic cells, this cell contains negative and positive type silicone and between a pn interface layer, this has an outer layer which is connected to a circuit which causes current to flow. These also produce 0.4v but when made with large enough surface area and connected in banks containing several cells they can produce enough energy to charge batteries on boats and in remote areas.

static electricity is

the effect that electrons can have when they remain stationary. It can be produced by friction between materials , it can build up on insulating materials because insulators do not allow electron movement to occur within them, these are called electrostatic charge. Fuel trucks must connect an earthing strap to prevent electrostatic build up causing sparking which could cause a fire. Walking across carpet can also cause electrostatic charge. Electrostatics is used in spray painting cars as the car and paint are at opposite polarities. Electrostatics are great for filtration as they use electrostatic charge to remove tiny peices of matter from flowing gases such as air. This can help remove dust or smoke from an air stream. It is very efficient in its consumption of energy.

the most common secondary cell is

the lead acid cell, it is used as a starting and general battery functions for cars and trucks. Each cell in these batteries produces a nominal EMF of 2-2.2 volts, a battery will normally contain 6 cells so the battery is 12-13.2 volts. When charged the positive plate is lead peroxide and the negative plate is spongy lead. the electrolyte is diluted in sulphuric acid. The density of the battery is the most accurate indication of the amount of charge available.

Electrochemistry is

the study of chemical effects of electric current, when current flows between electrodes in a liquid electrolyte, a chemical change occurs in the electrodes and the electrolyte. This action is called electrolysis and two common examples of electrolysis are electroplating and electrolytic corrosion.

what happens to a lead acid cell when charging

when passing current from source in opposite direction from the discharge current recharges the cell, lead sulphate is removed from the plates. The plates are restored to lead peroxide for positive plates and spongy lead for the negative plates. The sulphur is restored to the electrolyte to increase its density. When the S.G reached 1250 voltage 2.2v the cell is fully charged.