Effects of an Electric Current and Domestic Circuits Chapter 24

Heating Effect of Electric Circuits It was James Watt who experimentally investigated the effects of the heat, W, from a current carrying wire. It was found that the following equation may be used to find W: where W = amount of heat energy given I = current through the wire R = resistance of the conductor t = time that the current flows for

Joule’s law states that the rate at which heat is produced in a conductor is directly proportional to the square of the current provided its resistance is constant: By dividing both sides of the heat effect equation by ‘t’ we find: And by using Ohm’s law we get:

The advantage of using high voltage in transmission of electricity.

The Chemical Effect of an Electric Current An electric current may cause a chemical reaction when passed through a liquid, known as electrolysis. Applet… The liquid in which the current is passed is called the electrolyte, the plates that are in the electrolyte are called electrodes, the positive electrode is known as an anode, the negative electrode is the cathode. The container, electrodes and electrolyte are known as a voltameter.

Electrodes that are involved in the chemical reaction are called active electrodes, those that do not are called inactive electrodes. Examples of an electrolyte would be water with an acid, base or salt in it (i.e. a solution), or an ionic compound in it’s molten state. An ion is an atom or molecule that has gained or lost one or more electrons. The charge carriers in an electrolyte are positive and negative ions.

Applications of the Chemical Effect Electroplating, covering one metal with a thin plating of another. Extraction of minerals from their ores. In electrolyte capacitors electrolysis is used To purify metals.

Relationship between current and voltage for different conductors A metallic conductor Assuming we have constant temperature, the resistance of a conductor will not change as current increases, so we get a straight line graph through the origin, obeying Ohm’s law. Charge carriers in a metal are electrons.

A filament bulb As the voltage across the filament increases, so does the current and in turn the temperature. Since resistance α current, resistance increases. So as the bulb gets hotter a given increase in V doesn’t produce an increase of I when it was colder. Charge carriers in a filament bulb are negative electrons.

A semi conductor, e.g. a thermistor As the p.d. across the semiconductor is increased, the current increases, and in so doing the semiconductor gets hotter. This produces more holes and electrons for conduction and the resistance drops. Thus a further increase in V produces a larger increase of I when it was cold. Charge carriers are negative electrons and positive holes.

Active electrodes Electrolytes/ionic solutions As p.d. increases so will current. Active electrode takes part in the chemical reaction-also obey Ohm’s law and the graph is linear through the origin. If the electrodes are inactive, the voltameter behaves like a cell and has an emf that must be overcome before current will flow. Charge carriers are positive and negative ions. Inactive electrodes

A Gas An example would be a discharge tube A Gas An example would be a discharge tube. In region OA the positive ions in the tube are attracted to the negative electrode and the electrons move towards the positive electrode once a p.d. is applied, as number of ions crossing the tube increases so does the current. In region AB all the ions in the tube cross without recombination so no increases in current. In region BC, voltage increases to a stage where collision between fast moving ions and electrons produces more ions. Charge carriers are positive ions, negative electrons and a few negative ions. E.g. neon lamps.

A Vacuum No charge carriers in a vacuum, however if the cathode is heated sufficiently electrons will be produced by thermionic emission. A certain voltage is reached where all the electrons from the cathode are carried across the tube and the curve flattens out.

Why is electrical energy transmitted using high voltage power lines?   We have seen that the heat produced by an electric current is given by the formula P = I2R.

This means that a large current will produce a lot of heat energy This means that a large current will produce a lot of heat energy. This is a waste of energy and therefore should be minimised if possible. When power is transmitted through many kilometres of cable, the resistance (R) of the cable becomes significant. The power loss in the cable due to heating is given by I2R. Therefore, for a given cable resistance, the smaller the current, the smaller the power loss. Remember: P = VI

If the power, P, is delivered at a voltage, V, then the current through the cable is given by I=P/V. The power loss = I2R = (p/V)2R. Therefore, the larger the voltage V, the smaller the power loss and so electrical power is best transmitted at high voltages. In practice we make the current very small by making the voltage very large (e.g. 300 kVolts) when electrical energy is being carried across the countryside.

Transformers are used to change the voltage up or down

Transmission

Where does it come from?

Electricity generated from renewable energy (normalised) reached 19 Electricity generated from renewable energy (normalised) reached 19.6% of gross electricity consumption (RES-E) in 2012. The national target for 2010 was 15% of electricity consumption generated by renewables and the EU target for Ireland was 13.2%. Ireland’s target for 2020 is 40%. Ref: SEAI.ie

VOLTAGE SUPPLY

In a plug the live wire is brown, neutral is blue and earth is green/yellow.

Plugs Brown – Live wire Blue – Neutral wire Green/Yellow – Earth wire Current comes in on the live

Many electrical appliances, including cookers, washing machines and refrigerators, have metal cases. The earth wire creates a safe route for the current to flow through, if the live wire touches the casing. You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing. However, the earth terminal is connected to the metal casing, so the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.

Some appliances, such as vacuum cleaners, hair dryers, radios and electric drills, do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire can not touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside become loose. These appliances have double insulation and carry a symbol:

Fuses Consists of a metal wire which melts if too large a current flows through it, breaking the circuit and preventing damage The fuse is always on the live wire

A table lamp has a power rating of 100 W A table lamp has a power rating of 100 W. What is the most suitable fuse for the lamp? The fuse in the plug of an electric kettle was replaced with a 5 A fuse. The kettle has a power rating of 2 kW when connected to the ESB mains voltage of 230 V. Calculate the current that flows when the kettle is first plugged in.

Bonding is when metal taps, metal water tanks, etc. in a house are earthed in case they ever come into contact with a live wire, thus nobody is electrocuted. Earthing is where an appliance is earthed so that if a fault develops where the live wire came in contact with the outer metal casing, for example a metal kettle, nobody would be electrocuted if they touched the casing.

MCBs Miniature Circuit Breakers have the advantage that they do not need to be replaced when they trip. They can contain an electromagnet and/or a bimetallic strip. Either way, when the current exceeds a preset value, the circuit should break.

Residual current devices(RCDs) operate by detecting a difference between the current in the live and the neutral-which could arise if somebody came in contact with a live wire.

RCDs Residual Current Devices operate by detecting a difference between the current on the live wire and the current on the neutral wire. These should be the same, but if they are not this indicates a problem, and so the RCD trips. Note that RCBs act quicker and are therefore safer than MCBs.

Domestic Electric Circuits Appliances that take a large current , like an electric cooker, electric shower or immersion heater have a separate live and neutral wire coming from the distribution box. Such a circuit is called a radial circuit. In a ring circuit, the live terminals of each socket are connected together. Power is thus fed along both sides of the ring to each socket. The neutrals are also connected together and connected back to the neutral at the distribution box.

Radial Circuits Appliances that take a large current have their own live wire going in, and neutral wire coming out of, the appliance (from the fuse box). Examples would include a cooker or a washing machine.

Ring Circuits In a ring circuit the live wire from each of the sockets are connected to a common line, as is the case with the neutral wire from each socket (on that particular ring).

The Kilo-watt Hour The kilo-watt hour is the amount of energy used by a 1000W (1kW) appliance in one hour. Remember that: 1kW = 1000W = 1000 joules per second and energy=power x time The ESB sells electricity units called kilowatt-hours (kWh). No. of kilowatt-hours = No. of kilowatts x No. of units To find the cost we multiply the number of kWh by the cost per kilowatt-hour/unit. Cost = No. of kWh x price per unit

Example: If a unit electricity costs 12c Example: If a unit electricity costs 12c. How much does it cost to run a 5kW electric heater for 2 hours? No. of kilowatt-hours = No. of kilowatts x No. of units = 5 x 2 = 10 kWh Cost = No. of kWh x price per unit = 10 x 12 = 120c = €1.20

RMS The root mean square (RMS) voltage or current is the time-averaged voltage or current in an AC system.

Unlike DC current and voltage, which are constant, AC current and voltage vary over time. This is called alternating current because the direction alternates.

The root mean square (abbreviated RMS or rms) is a statistical measure of the magnitude of a varying quantity. We use the root mean square to express the average current or voltage in an AC system. The RMS current and voltage are the peak current and voltage over the square root of two.

RMS CURRENT the root mean square of the current, Irms= I0/√2 , where I0 is the peak current, in an AC system

RMS VOLTAGE the root mean square of the voltage, Vrms= V0/√2 , where V0 is the peak voltage, in an AC system

Why not give it a go? The voltage in Irish homes has a peak voltage of 325 V. What is the corresponding value of the root-mean-square voltage?