2ELECTRIC CURRENTThe moving charges (the microscopic particles) from the electric supply constitute an electric currentConventionally:The direction of the electric current is taken to be the flow of positive charge (from negative terminal to positive terminal)Whereas, the electrons flow in the opposite direction to that of the electric current
4THE MEASUREMENT OF CURRENT An electric current (I) can be stated as the rate of the flowing electric charge (Q) through a certain cross-section of a conductorThe instrument used to measure current is ammeterFormula:I = current, Q = charge, and t = time
5MEASURING CURRENTElectric current is measured in amps (A) using an ammeter connected in series in the circuit.A
6Ampere: The Unit of Current One ampere is the amount of current that exists when a number of electrons having a total charge of one coulomb pass a given cross-sectional area in one second
7circuit diagramScientists usually draw electric circuits using symbols;celllampswitchwires
8circuit symbolsIn circuit diagrams components are represented by the following symbols;cellbatteryswitchlampbuzzerammetervoltmetermotorresistorvariable resistor
9ELECTROMOTIVE FORCE (E.M.F) A source of e.m.f is a device in which non-electrical energy such as chemical, mechanical, etc, is converted into electrical energySuch as : electrical cells, solar cells, generators, etc
10The CELLThe 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.
11THE E.M.F OF A CELLThe e.m.f of a cell is defined as the energy converted from non- electrical forms to electrical form when one coulomb of positive charge passes through the cell
12BATTERYA battery is a voltage source that converts chemical energy into electrical energyThe way cells are connected, and the type of cells, determines the voltage and capacity of a battery
14THE FORMULA OF E.M.F The formula used to calculate e.m.f is: E = e.m.f W = converted energyQ = positive chargeTherefore, the SI unit of e.m.f is joule/coulomb, which is volt (V)
15POTENTIAL DIFFERENCE (p.d) The potential difference between two points can be defined as the energy converted from electrical energy to other forms when one coulomb of positive charge passes through the cell
16THE FORMULA OF P.D The formula used to calculate p.d is: V = p.d W = converted energyQ = positive chargeTherefore, the SI unit of p.d is joule/coulomb, which is volt (V)
171 VOLT1 volts is 1 joule of energy converted from electrical to other forms if 1 coulomb of positive charge passes through itSchematic Symbol for the DC Voltage SourceAnode Cathode
18THE MEASUREMENT OF E.M.F and P.D The electromotive force (e.m.f) can be measured by a voltmeter connected directly across the terminals of the sourceThe potential difference (p.d) between 2 points separated by a load (such as resistor) can be measured by a voltmeter connected in parallel to the load
19MEASURING VOLTAGEThe ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeterV
20MEASURING VOLTAGEDifferent 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 componentsScientist usually use the term Potential Difference (pd) when they talk about voltage.
22Measuring Voltage Voltage: Probes connect to either side of the resistor
23A circuit is the path that is made for an electric current.
24CIRCUITS Close circuit Circuit in which there is a continuous conducting path around which charge can flow continuouslyOpen circuitCircuit in which there is a break in the circuit to stop the current from flowing
25Series CircuitA circuit that only has one path for current to flow through is called a series circuit.Current is the same at all points in a series circuit
26If the path is broken, no current flows through the circuit.
27Parallel CircuitsA type of circuit that has multiple paths for current is called a parallel circuit.Different paths may contain different current flow. This is also based on Ohms Law.
28If one part of the path is removed, the current continues to flow through the other paths of the circuit.
30RESISTANCEResistance is a property of a material that resist the movement of free electrons in the materialIt determines the size of the current which can pass in the material
31Conductor is also known as a resistor Current through an ideal conductor is proportional to the applied voltageConductor is also known as a resistorAn ideal conductor is a material whose resistance does not change with temperatureFor an ohmic device,
32THE FORMULA OF RESISTANCE The formula used to calculate resistance is:V = p.d (V)I = electric current (A)R = resistance (Ω)Therefore, the SI unit of resistance is volt/ampere which is ohm (Ω)
33RESISTORIt is used to provide a certain value of resistance in a circuitIts function is to control the size of the current that flows in a circuit or divide voltage, and in some cases, generate heatThere are 2 types of resistor:Fixed ResistorVariable Resistor (rheostat)
34FIXED RESISTOR The types are: Carbon composition resistor Carbon film resistorCarbon composition resistorTin oxide resistorWire – wound resistor used in high power ratingThe common electric symbol is
36Color-code Bands on a Resistor This coding is used for 5%, 10%, and 20% tolerance resistors• The first three bands are used to denote the rated value• The fourth denotes how much the device may vary from the rated value1st band is the first digit of the resistance value2nd band is the second digit of the resistance value3rd band is the multiplier (number of zeros)4th band indicates the tolerance
39Alphanumeric Labeling Two or three digits, and one of the letters R, K, or M are used to identify a resistance valueThe letter is used to indicate the multiplier, and its position is used to indicate decimal point position
40VARIABLE RESISTORIts function is to vary the current flowing in a circuitThe common electric symbol is
41Variable ResistorsVariable resistors are designed so that their resistance values can be changed• A potentiometer is a three terminal variable resistor used to divide voltage• A rheostat is a variable resistor used to control current
43Ohmic ResistorsMetals obey Ohm’s Law linearly so long as their temperature is held constantTheir resistance values do not fluctuate with temperaturei.e. the resistance for each resistor is a constantMost ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.
44Voltage and Current Relationship for Linear Resistors Voltage and current are linear when resistance is held constant.
45Voltage versus Current for Constant Resistance The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.
46Resistance is affected by temperature Resistance is affected by temperature. The resistance of a conductor increases as its temperature increases.Light bulbs do not obey Ohm’s Law linearlyAs their temperature increases, the power dissipated by the bulb increasesi.e. They are brighter when they are hotter
48Series Circuit Current is constant Why? Only one path for the current to take
49Parallel Circuit Voltage is constant Why? There are 3 closed loops in the circuit
50Formula of a number of resistors in series and in parallel The total resistance of a circuit is dependant on the number of resistors in the circuit and their configurationSeries CircuitParallel Circuit
51SeriesResistors are connected such that the current can only take one path
52ParallelResistors are connected such that the current can take multiple paths
53Kirchhoff’s Current Law Current into junction = Current leaving junctionThe amount of current that enters a junction is equivalent to the amount of current that leaves the junction
54THE RELATION BETWEEN RESISTANCE AND RESISTIVITY The resistance (R) of a conductor depends on:Its lengthIts cross-sectional areaThe material used in resistanceIts temperature
55EXPERIMENTAL RESULTS HAVE SHOWN THAT: The larger the cross-sectional area of a wire, the smaller the resistanceThe longer the wire, the larger the resistance
56THE FORMULA OF RESISTANCE Combining the two equations, we find:is resistivity and its SI unit is Ω m
57Electric Power Moving charges do work We can heat the filament in a light bulbWe can turn the rotor in a motorThe rate at which work is done is powerElectric Power = current x voltageUnits are watts = joules/sec = amps x volts
59Energy Loss in Power Lines Wires have some resistanceTo transport energy from Point A to Point B, we connect wiresEach wire has resistanceEnergy Loss in one second is I2RMake I small to minimizeTransformers convert AC Voltages
60Fuses Limit the current that runs through wires in your house These wires have some resistanceEnergy loss by I2R converts to heatHot wires can start a fireLimit the current with a fuse or circuit breaker