CURRENT IN ELCETRIC CIRCUITS • Electric circuits transfer energy from battery or power supply to circuit components. • Circuit components then transfer this electrical energy to other forms of energy. • For an electric current to flow we need a complete circuit and a source to supply energy to make the current flow. • The energy is supplied by a cell, a battery or a power supply. • The complete circuit (or path) is provided by metals like copper or steel which are electrical conductors. FLOW OF CURRENT IN A CIRCUIT • In a closed circuit, current flows from the positive terminal of the cell or battery through the switch and electrical component(s) and back to the negative terminal. • The current flowing in a fixed direction is called a direct current (d. c.). • The current flowing in two alternating directions is called alternating current (a. c.). Alternating current reverses its direction every half a cycle. • Current does not flow in an open circuit where the switch is open. WHERE DOES ELECTRICITY COME FROM? • The free electrons in a conductor need to be replaced when they move through the conducting medium. • The electrons in the conductor are replaced by the electrons from the source such as a cell or a battery. CELL & BATTERY • A cell is an electrochemical device that can convert chemical energy to electrical energy. • A battery is a combination of two or more electrochemical cells that can change chemical energy to electrical energy. • The chemicals inside the battery pull all the electrons to one of the terminals making it charged negatively. • This leaves the other terminal charged positively. • Thus the positive and negative charges are separated. CIRCUIT DIAGRAM • Circuit diagram is used to represent an actual circuit. • A circuit diagram shows how the components of a circuit are connected to one another. • Different components of a circuit are represented by different symbols. CURRENT AND CHARGE • Current is defined as the rate of flow of charge. • The amount of charge in Coulomb flowing through a particular point per unit time is equal to current in Ampere. • The unit of electrical current is Ampere (A). • 1 Ampere = 1 Coulomb per second. • 1 A = 1C / s CONDUCTORS AND INSULATORS • Conductors are substances that allow the flow of electricity through them. They have free electrons. • Example: metals, salt solution, acids, alkalis, carbon (graphite). • Insulators are substances that do not allow the flow of electricity through them. They have no free electrons. • Examples: wood, glass, plastic, distilled water, carbon (diamond). • Semiconductors are substances that allow the flow of electricity through them under certain conditions. • Example: silicon, germanium, gallium, arsenic, indium. MEASURING CURRENT IN CIRCUIT • Current in an electric circuit is measured using ammeter connected in series with the component. • There are two types: analogue and digital. • Analogue ammeter has a needle which moves across a scale. The reading has to be taken by making judgement of the position of the needle against the scale. • The digital ammeter gives a direct read-out in figures (including decimal values) with better precision. There is no judgement required. • Current is measured in A, mA (10-3A) and µA (10-6A). ACTUAL AND CONVENTIONAL CURRENT • When a circuit is connected, the electrons are pulled by the positive terminal of the battery as there is a deficiency of electrons. • The electrons flow from the negative terminal of the battery through the conductor to the positive terminal. • Initially the current was considered as the flow of positive charges which is incorrect. • For many scientific reasons the direction of current in a circuit is shown as the flow of charge from the positive terminal of the battery to the negative terminal of the battery which is termed as conventional current. • Actual current (the flow of electrons) is therefore opposite to the conventional current. VOLTAGE IN ELECTRIC CIRCUITS • A force is needed to make the charged particles move around a circuit. • The source of this force is the chemical reaction inside the cell, battery or power supply. • This force given to the charged particles changes the chemical energy to electrical energy by doing work on the electrons as they move around the circuit. • Work is done on the charged particles by the source. • The charged particles deliver this energy as they move through the electrical components. • Work is done by the charged particles on the components. • The electrical energy gets converted to other forms of energy. VOLTAGE IN ELECTRIC CIRCUITS • The work done per coulomb of charge by the source is defined as the electromotive force (e.m.f.). • The work done per coulomb of charge on the component is defined as potential difference (p.d.) • Both e.m.f and p.d. are measured in unit called voltage (V). • Voltage is therefore defined as work done per coulomb of charge. • Voltage = Work done ÷ Amount of charge • 1 Volt = 1 Joules / Coulomb • 1 V = 1 J/C MEASURING VOLTAGE IN CIRCUIT • Voltage in an electrical circuit is measured using voltmeter connected in parallel across the component. • There are two types: analogue and digital. • Analogue voltmeter has a needle which moves across a scale. The reading has to be taken by making judgement of the position of the needle against the scale. • The digital voltmeter gives a direct read-out in figures (including decimal values) with better precision. There is no judgement required. • Voltage is measured in V, mV (10-3V) and µV (10-6V). CELLS IN SERIES AND PARALLEL • The cells in a circuit can be connected in series or parallel. • In series connection the voltages of individual cells are added or subtracted. The same current flows through each cell. • In parallel connection the voltages across the cells stays the same. The current flowing through each cell are added. • In series connection of cells the charged particles get more energy but the battery runs for short time. • In parallel connection of cells the charged particles get same energy but the battery runs for long time. RESISTANCE • Electrical resistance is the characteristic property of a material that slows down the flow of electrons through it. • Resistance is represented by the symbol R. It is measured in Ohm (Ω). • Different components have different resistances to the flow of current for the same potential difference applied across them. • The resistance of a conductor is defined as the ratio of the potential difference across it to the current flowing through it. • Resistance = Potential difference ÷ Current OR R = V ÷ I • 1 Ohm = 1 Volt / Ampere OR 1 Ω = 1 V/A • For constant potential difference across a component, the current through it decreases if the resistance of the component increases. • For constant resistance of the component, the current flowing through it increases if the potential difference across the component increases. • A component having fixed or variable resistance is called a resistor. DETERMINING THE RESISTANCE OF A RESISTOR DETERMINING THE RESISTANCE OF A RESISTOR • The circuit is set up as shown the diagram that follows the description. • With the switch closed the resistance of the rheostat is adjusted to set the current flowing through the circuit. • The potential difference across the resistor is measured using a voltmeter. • The current through the resistor is measured using an ammeter. • The resistance is calculated using R = V ÷ I GRAPICAL METHOD OF DETERMINING THE RESISTANCE OF A RESISTOR GRAPICAL METHOD OF DETERMINING THE RESISTANCE OF A RESISTOR • The circuit is set up as shown in the diagram following the description. • Using the rheostat different values of potential difference are applied across the resistor and measured using the voltmeter. • The corresponding values of current are measured using the ammeter. • The values of p.d and current are recorded and a graph of voltage against current is plotted. • Resistance is calculated from the gradient of the graph. RESISTANCE, LENGTH & AREA • The resistance of a conductor is directly proportional to the length of the conductor. • The electrons undergo more collisions with the positive ions in the lattice as the length increases. • For example, longer the wire higher the resistance. • The resistance of a conductor is inversely proportional to the cross sectional area of the conductor. • The electrons undergo fewer collisions with the positive ions in the lattice as the area increases. • For example, thicker the wire lower the resistance. RESISTOR COMBINATION • There are two basic connections of resistors in a circuit. • Resistors can be connected in series. • The total resistance of the circuit containing resistors in series is given by: • RTotal = R1 + R2 + R3 + …… + RN • Resistors can be connected in parallel. • The total resistance of the circuit containing resistors in parallel is given by: • RTotal-1 = R1-1 + R2-1+ R3-1 + …… + RN-1 RESISTORS IN SERIES & PARALLEL DIFFERENCE SERIES PARALLEL There is only one path for the charged particles to flow. There are more than one path for the charge particles to flow. If one of the resistors becomes inactive then all the other resistors do not work. If one or more resistors becomes inactive even then the active resistors work. Total potential difference (voltage drop) in the circuit is equal to the sum of the potential difference across each resistors. Total current flow in the circuit is equal to the sum of the current through each resistors. Current is the same through each resistors. Potential difference is the same across each of the resistors. Voltage drop varies according to the resistance values; higher the resistance higher the voltage drop. Current flow varies according to the resistance values; higher the resistance lower the current flow. Total resistance of the circuit is higher than the maximum resistance value. Total resistance of the circuit is lower than the minimum resistance value. CURRENT – VOLTAGE CHARACTERISTICS CURVE • Current – Voltage curve for different circuit shows how the current flowing through the component varies according to the potential difference across it. • I – V curves help us to determine how the resistance of the component varies as the voltage or current through it changes. • The basis circuit components are: resistor, filament lamp, diode (simple or light emitting), light dependent resistor & thermistor. I – V CURVE OF A RESISTOR (WIRE) • A metal wire is an Ohmic conductor. • The current increases proportionally with the potential difference across it. • The I – V graph is a straight line passing through the origin. • The resistance of the wire stays constant. I – V CURVE OF A FILAMENT LAMP • A (tungsten) filament lamp is not an Ohmic conductor. • The current does not increase proportionally with the potential difference across it. • The I – V graph is a curve where current increases with potential difference at a decreasing rate. • The resistance increases with increasing current. I – V CURVE OF A DIODE OR LED • A diode or LED is not an Ohmic conductor. • It allows current flow in one direction only when it is forward biased. • The current does not increase proportionally with the potential difference across it. • The I – V graph is a curve where current increases with potential difference at an increasing rate. • The resistance decreases with increasing voltage. I – V CURVE OF AN LDR • An LDR is an Ohmic conductor. • It conducts current by generating charge particles. • The number of charge particles generated increases with the increase in light intensity. • The current increases proportionally with the potential difference across it. • The I – V graph is a straight line where current increases with potential difference and also with light intensity. • The resistance decreases with increasing intensity of light. I – V CURVE OF A THERMISTOR • A thermistor is a non Ohmic conductor. • It conducts current by generating charge particles. • The number of charge particles generated increases with the increase in thermal energy. • The current does not increase proportionally with the potential difference across it. • The I – V graph is a curve line where current increases with potential difference at an increasing rate and also with thermal energy. • The resistance decreases with increasing thermal energy. ELECTRICAL ENERGY, WORK & POWER • Electricity is used because it is a good way of transferring energy. • Cells, batteries and power supplies give energy to the charges in a circuit. • Work is done on the charge particles as they move around the circuit. • This electrical energy is delivered by the electrons to the components as they move through them. • The rate at which the components convert electrical energy to other forms of energy is defined as power. ELECTRICAL ENERGY, WORK & POWER • Power rating of an electrical appliance is the rate at which it transfers energy and is measured in watts (W) or kilowatts (kW). • Power = Energy transferred ÷ Time taken • 1 watt = 1 joules per second • 1 W = 1 J/s • Electrical power = Current × Voltage • P=I×V • Electrical energy = Power × Time • E=I×V×t ELECTRICAL ENERGY, WORK & POWER • The rate at which heat energy is lost during the flow of current through a component is given by: • P = I2 × R or P = V2 ÷ R • In domestic and industrial scale the energy supplied by the mains electricity is measured in unit called kilowatt-hour. • 1 kilowatt-hour is the unit of electricity. • It is also equal to the unit of energy consumed by an electrical appliance in 1 hour at a power rating of 1 kilowatt. • 1 kWh = 3.6 × 106 J = 3600000 J.
