Effects of an Electric Current and Domestic Circuits Chapter 24

Experiment to show the Heating Effect of an Electric Current Use the equipment shown. Allow the current to flow through the coil for a few minutes. Note the rise in temperature on the thermometer. This shows the heating effect.

Why does an Electric Current in a metal produce Heat? As the electrons move through the metal they collide with the fixed metal atoms. When an electron collides with an atom it gives it Energy. This energy causes the atom to vibrate. Vibrational energy of atoms in a solid is Internal Energy or Heat.

State three factors on which the Heating Effect of an electric current depends. The Heating Effect depends on: The size of the Current I. The Resistance R of the conductor through which the current flows. The Time t for which the current flows.

State Joule’s Law 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 the resistance is constant. i.e. P    I 2​ Where P is the power and I is the current.

Experiment to verify Joule’s Law The graph is a straight line through the origin. This shows that:   I 2 It follows that: P  I 2 i.e. Joule’s Law is verified

Why High Voltages are used in the Transmission of Electrical Energy The heat produced per second in the long distance power lines is directly proportional to the current squared ( ​I​ 2). The smaller the current the less heat energy wasted in power lines. Since P = V I, for a given power ( P ) the bigger the voltage ( V ) the smaller the current ( I ). Therefore high voltages are used to minimise energy wasted.

What is the Chemical effect of an electric current? The Chemical Effect of an Electric Current is when an electric current passes through a liquid and causes a chemical reaction in the liquid. Electrolysis is another name for the Chemical Effect of an Electric Current.

Give three uses of the chemical effect of an electric current. Electroplating Extracting some metals from their ores Purifying metals Breaking water up into hydrogen and oxygen

What is an Ion? An atom or a molecule that has lost or gained one or more electrons is called an Ion. If a substance breaks up into positive and negative ions when it dissolves in water, the solution formed is called an Ionic Solution.

(negative electrode) (positive electrode) ( the electrolyte ) Examples of Electrolytes: A solution of an acid, base or salt in water or an ionic compound if it is in the molten state are electrolytes.

Experiment to show the Chemical Effect of an Electric Current Use the equipment shown. Turn on the current. Observe that the anode gradually gets eaten away and the cathode becomes coated with copper. Turn off the current and the reaction stops. Conclusion: An electric current can cause a chemical reaction.

Electrolysis of Water (Also experiment to show the chemical effect of an electric current.) Use the equipment shown. Turn on the current. Observe that bubbles of gas are given off at both cathode and anode. These gasses are Hydrogen and Oxygen. Turn off the current and the reaction stops. Conclusion: An electric current can cause a chemical reaction.

Charge Carriers when electric current flows in different conductors A Metal An Electrolyte (an Ionic Solution) A Gas A Semiconductor A Vacuum The charge carriers in a Metal are Electrons. The charge carriers in an Ionic Solution are Positive Ions and Negative Ions. The charge carriers in a Gas are Positive Ions, Negative Ions and Electrons. The charge carriers in a Semiconductor are Electrons and Positive Holes. A Vacuum will not conduct electricity because there are no charge carriers present. If two electrodes are placed in a vacuum and the negative one heated sufficiently, Electrons are released from it. These will become the charge carriers if a pd is placed across the electrodes.

Graph showing the relationship between Current and Voltage for a Metal at Constant Temperature Graph showing the relationship between Current and Voltage for a Filament Bulb

Graph showing the relationship between Current and Voltage for a Semiconductor Graph showing the relationship between current and voltage for a Gas

Graphs showing the relationship between Current and Voltage for an Electrolyte (Ionic solution) Electrodes that take part in the chemical reaction in an electrolyte called Active Electrodes. Electrodes that do not take part in the chemical reaction called Inactive Electrodes.

An appliance that takes a large current (e. g An appliance that takes a large current (e.g. an electric cooker) has a separate live and neutral wires coming to it from the distribution box. The live wire has a fuse or MCB in it. Such a circuit is called a Radial Circuit. A number of sockets in a room have their live terminals connected together in a loop which is then connected back to the live at the distribution box. The neutrals and earths are connected similarly. Such a circuit is called a Ring Circuit.

Fuses A Fuse consists of a thin piece of wire, usually in a fire proof case. Current flows through the wire and it heats up. If the current exceeds the fuses rating the wire melts and breaks the circuit.

A fuse should always be Connected in the Live Wire. As well as switching off the current, when the fuse blows, it also disconnects the appliance from the live. Circuit symbol for a fuse

A Miniature Circuit Breaker (MCB) is a safety device that breaks a circuit if too large a current flows. It thus protects the equipment and the people using it. It has the same function as a fuse. It operates faster than a fuse. A blown fuse must be replaced but an MCB can be reset by flicking a switch once the fault has been located and fixed.

Residual Current Devices (RCDs) A Residual Current Device is a safety device that disconnects a circuit from the mains and prevents electrocution. When there is a difference in the value of the current flowing in the live wire and the neutral wire, it disconnects the circuit from the mains. In domestic circuits an RCD disconnects the appliance if the value of the current in the live wire and the neutral wire differ by 30 mA or more.

Wiring a Plug Brown is connected to the Live. Blue is connected to the Neutral. Green-Yellow is connected to the Earth.

A fault develops - the live wire comes in contact with the metal of the kettle. A man touches the kettle. Electric current passes through him, electrocuting him. If the metal of the kettle was Connected to Earth, current from the live flows through the kettle to Earth. This path has low resistance so a large current will flow. This will blow the fuse (MCB or RCD) and disconnect the kettle from the live. If the current is not large enough to blow the fuse, it is still safe to touch the kettle, since being connected to Earth its potential is zero. All electrical equipment with exposed metal parts should be earthed.

Bonding is a safety precaution used in domestic electric circuits. All metal water pipes, metal taps, metal water tanks etc. in a house must be Connected to Earth (Bonded to Earth). If they then become accidentally connected to the live they will not be at 230 V but will remain at zero potential and will not pose a danger of electrocution. Fuses, MCBs or RCDs should also trip or blow disconnecting the system from the live.

What is a kilowatt-hour? The electrical energy supplied to houses in Ireland is measured in a unit called the kilowatt-hour (kW h). One kilowatt-hour (kW h) is the amount of energy used by a 1 000 W appliance in one hour. The Electricity meter in a house records the number of units (kilowatt-hours) of electrical energy used by the customer.