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DYNAMIC ELECTRICITY

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ELECTRIC CURRENT The moving charges (the microscopic particles) from the electric supply constitute an electric current Conventionally: 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

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**THE DIRECTION OF THE FLOWING ELECTRIC CURRENT**

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**THE 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 conductor The instrument used to measure current is ammeter Formula: I = current, Q = charge, and t = time

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MEASURING CURRENT Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A

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**Ampere: 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

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circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires

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circuit symbols In circuit diagrams components are represented by the following symbols; cell battery switch lamp buzzer ammeter voltmeter motor resistor variable resistor

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**ELECTROMOTIVE 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 energy Such as : electrical cells, solar cells, generators, etc

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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.

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THE E.M.F OF A CELL The 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

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BATTERY A battery is a voltage source that converts chemical energy into electrical energy The way cells are connected, and the type of cells, determines the voltage and capacity of a battery

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**THE ARRANGEMENT OF BATTERIES**

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**THE FORMULA OF E.M.F The formula used to calculate e.m.f is: E = e.m.f**

W = converted energy Q = positive charge Therefore, the SI unit of e.m.f is joule/coulomb, which is volt (V)

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**POTENTIAL 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

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**THE FORMULA OF P.D The formula used to calculate p.d is: V = p.d**

W = converted energy Q = positive charge Therefore, the SI unit of p.d is joule/coulomb, which is volt (V)

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1 VOLT 1 volts is 1 joule of energy converted from electrical to other forms if 1 coulomb of positive charge passes through it Schematic Symbol for the DC Voltage Source Anode Cathode

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**THE 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 source The 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

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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

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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.

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VOLTMETER

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**Measuring Voltage Voltage:**

Probes connect to either side of the resistor

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**A circuit is the path that is made for an electric current.**

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**CIRCUITS Close circuit**

Circuit in which there is a continuous conducting path around which charge can flow continuously Open circuit Circuit in which there is a break in the circuit to stop the current from flowing

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Series Circuit A 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

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**If the path is broken, no current flows through the circuit.**

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Parallel Circuits A 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.

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If one part of the path is removed, the current continues to flow through the other paths of the circuit.

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**By Analogy: Series Vs Parallel**

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RESISTANCE Resistance is a property of a material that resist the movement of free electrons in the material It determines the size of the current which can pass in the material

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**Conductor is also known as a resistor **

Current through an ideal conductor is proportional to the applied voltage Conductor is also known as a resistor An ideal conductor is a material whose resistance does not change with temperature For an ohmic device,

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**THE 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)

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RESISTOR It is used to provide a certain value of resistance in a circuit Its function is to control the size of the current that flows in a circuit or divide voltage, and in some cases, generate heat There are 2 types of resistor: Fixed Resistor Variable Resistor (rheostat)

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**FIXED RESISTOR The types are: Carbon composition resistor**

Carbon film resistor Carbon composition resistor Tin oxide resistor Wire – wound resistor used in high power rating The common electric symbol is

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**Common Fixed Resistors**

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**Color-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 value 1st band is the first digit of the resistance value 2nd band is the second digit of the resistance value 3rd band is the multiplier (number of zeros) 4th band indicates the tolerance

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**Color-code Bands on a Resistor**

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**Color Coded Precision Resistors**

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**Alphanumeric Labeling**

Two or three digits, and one of the letters R, K, or M are used to identify a resistance value The letter is used to indicate the multiplier, and its position is used to indicate decimal point position

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VARIABLE RESISTOR Its function is to vary the current flowing in a circuit The common electric symbol is

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Variable Resistors Variable 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

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**Examples of Variable Resistors**

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Ohmic Resistors Metals obey Ohm’s Law linearly so long as their temperature is held constant Their resistance values do not fluctuate with temperature i.e. the resistance for each resistor is a constant Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.

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**Voltage and Current Relationship for Linear Resistors**

Voltage and current are linear when resistance is held constant.

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**Voltage 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.

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**Resistance 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 linearly As their temperature increases, the power dissipated by the bulb increases i.e. They are brighter when they are hotter

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Ohm’s Law continued

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**Series Circuit Current is constant Why?**

Only one path for the current to take

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**Parallel Circuit Voltage is constant Why?**

There are 3 closed loops in the circuit

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**Formula 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 configuration Series Circuit Parallel Circuit

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Series Resistors are connected such that the current can only take one path

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Parallel Resistors are connected such that the current can take multiple paths

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**Kirchhoff’s Current Law**

Current into junction = Current leaving junction The amount of current that enters a junction is equivalent to the amount of current that leaves the junction

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**THE RELATION BETWEEN RESISTANCE AND RESISTIVITY**

The resistance (R) of a conductor depends on: Its length Its cross-sectional area The material used in resistance Its temperature

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**EXPERIMENTAL RESULTS HAVE SHOWN THAT:**

The larger the cross-sectional area of a wire, the smaller the resistance The longer the wire, the larger the resistance

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**THE FORMULA OF RESISTANCE**

Combining the two equations, we find: is resistivity and its SI unit is Ω m

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**Electric Power Moving charges do work**

We can heat the filament in a light bulb We can turn the rotor in a motor The rate at which work is done is power Electric Power = current x voltage Units are watts = joules/sec = amps x volts

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Electric Power

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**Energy Loss in Power Lines**

Wires have some resistance To transport energy from Point A to Point B, we connect wires Each wire has resistance Energy Loss in one second is I2R Make I small to minimize Transformers convert AC Voltages

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**Fuses Limit the current that runs through wires in your house**

These wires have some resistance Energy loss by I2R converts to heat Hot wires can start a fire Limit the current with a fuse or circuit breaker

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S.MORRIS 2006 ELECTRICAL CIRCUITS More free powerpoints at www.worldofteaching.com.

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