Electric Circuits PDF

ELECTRICAL CIRCUITS  The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery....

ELECTRICAL CIRCUITS  The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.  What is an electric current? An electric current is a flow of microscopic particles called electrons flowing through wires and components. + - In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell. AMPERE Is proportional to the number of electrons flowing in the electric circuit. Where electric current is measured. 1 ampere is equal to an electric charge of 1 C flowing through a conductor each second. Electric potential is measured in VOLTS. Electric resistance is measured in OHMS.  OHM’S LAW The voltage across the total circuit on any portion of the circuit is equal to the current times the resistance. V=IR Where; V= is the electric potential in volts I= is the electric current in amperes R= is the electric resistance in ohms  simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. cell wires switch lamp To make the circuit, these components are connected together with metal connecting wires.  simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around. If there were any breaks in the circuit, the current could not flow.  circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires  circuit diagrams In circuit diagrams components are represented by the following symbols; cell battery switch lamp buzzer ammeter voltmeter motor resistor variable resistor  types of circuit There are two types of electrical circuits; SERIES CIRCUITS PARALLEL CIRCUITS  SERIES CIRCUITS The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. RULES FOR SERIES CIRCUIT 1. The total resistance is equal to the sum of the individual resistance. 2. The current through each circuit elements is the same and is equal to the circuit current. 3. The sum of the voltages across each circuit element is equal to the total circuit voltage.  SAMPLE PROBLEM 1. A series circuit contains 3 resistance elements having the values of 8,12, and 15Ω. If the voltage is 110 V a. What is the total resistance and current and voltage across the resistor?  PARALLEL CIRCUITS The components are connected side by side. The current has a choice of routes. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.  RULES FOR PARALLEL CIRCUIT 1. The sum of the currents through each circuit element is equal to the total current. 2. The voltage across each circuit elements is the same and is equal to the total circuit voltage. 3. The total resistance is the inverse of the sum of the reciprocals of each individual resistance.  SAMPLE PROBLEM  3 resistors are connected parallel with resistance of 2Ω,5Ω&7Ω, the voltage is 220V. a. What is the total resistance? b. What is the current across the 7Ω resistance? c. What is the total current? d. What is the voltage across the 2 resistor? e. What is the current across the 5Ω resistance?  measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A  measuring current This is how we draw an ammeter in a circuit. A A SERIES CIRCUIT PARALLEL CIRCUIT  measuring current SERIES CIRCUIT current is the same 2A 2A at all points in the circuit. 2A PARALLEL CIRCUIT 2A 2A current is shared between the 1A components 1A copy the following circuits and fill in the missing ammeter readings. 4A ? 3A ? 3A 1A ? 4A ? 4A 1A 1A ?  measuring voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V  measuring voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter a voltmeter is connected across the components Scientist usually use the term Potential Difference (pd) when they talk about voltage.  measuring voltage This is how we draw a voltmeter in a circuit. V V SERIES CIRCUIT PARALLEL CIRCUIT  measuring voltage V V V V  series circuit voltage is shared between the components 3V 1.5V 1.5V  parallel circuit voltage is the same in all parts of the circuit. 3V 3V 3V  measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.  measuring current & voltage a) 6V 4A A V V A measuring current & voltage b) 6V 4A A V A V A  answers a) b) 6V 6V 4A 4A 6V 4A 4A 3V 3V 2A 4A 6V 2A