FLOW OF ELECTRIC CHARGE: ELECTRICITY
FLOW OF ELECTRIC CHARGE: ELECTRICITY WHAT WE NEED: A DEVICE THAT ALLOWS US TO HAVE A SUPPLY OF CHARGE WHENEVER WE NEED IT AN ELECTROCHEMICAL CELL (OR MANY CELLS CALLED A BATTERY)
Electric Current • An electric current I is a measure of the rate of flow of electric charge Q through a given cross section of a conductor. • Symbol of Electric Current = I • SI Unit of Electric Current = ampere (A)
I = Q/t where I = current in ampere (A) Q = amount of charges in coulombs (C) t = time in seconds (s) 3
Conventional Current and Electron Flow
Conventional current flows from the positive to the negative ends
Electric charges flow from the negative to the positive ends 4
Conventional Current and Electron Flow Measuring current • An ammeter is an instrument used for measuring electric current. • Ammeters must be connected in series in a circuit
A
ammeter symbol
Positive (negative) side of ammeter is connected to the positive (negative) terminal of the cell / battery. 5
Conventional Current and Electron Flow Measuring current The digital multimeter (DMM) is starting to replace the ammeter. has a wide range of between a few hundred A to several A can be used for direct current (D.C.) and alternating current (A.C.) able to read voltage and resistance too
Conventional Current and Electron Flow Measuring current Since the circuit consists of only one loop, the same current flows through the circuit; does not matter where the ammeter is placed on the circuit A1
-
+
A6
cell
A2
A5 resistor
A3
A4
Electromotive Force (e.m.f)
electric current is produced when there is a flow of charges a source of energy (provided by a cell, group of cells or generator) is needed to enable charges to be pumped or forced around a circuit electromotive force is the electric force that provides the pumping action for electric current to flow from the positive terminal to the negative terminal of the + battery I
cell
lamp
Electromotive Force (e.m.f) Electromotive Force (e.m.f) Definition • The electromotive force (e.m.f.) of an electrical source is the work done by the source in driving a unit charge round a complete circuit. – is the potential difference between the two terminals of the cell or battery. (From higher p.d. to lower p.d) – A point of high potential is a region where there is a large number of positive charges whereas a point of low potential has lesser positive charges (more negative charges) 9
Electromotive Force (e.m.f) Electromotive Force (e.m.f) • Symbol of Electromotive Force = • SI Unit of Electromotive Force = volts (V) or joules per coulomb (JC-1)
= W/Q where = e.m.f. (V) W = Energy converted from non–electrical forms to electrical form (J) [work done] Q = amount of charge in coulombs (C) 10
Potential Difference Potential Difference (p.d.) • The Potential Difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one coulomb of positive charge passes between the two points • Symbol of Potential Difference (p.d.) = V • SI Unit of Potential Difference (p.d.) = volts (V)
V = W/Q where V = Potential difference (V) W = Energy converted from electrical form to other forms (J) Q = amount of charge in coulombs (C)
11
Potential Difference Measuring p.d./e.m.f. • An voltmeter is an instrument used for measuring potential difference or electromotive force. • As charges flow round a circuit, they lose their P.E., transforming P.E. into other forms of energy. • It is connected in parallel to the circuit. • The SI unit for p.d. / e.m.f. is volt (V) V voltmeter symbol
Voltmeters will measure the potential difference across 2 points of the circuit, so we connect it in parallel with respect to those 2 points 12
Potential Difference Potential difference around a simple circuit sum of all the e.m.f.’s of the cells must be equal to the sum of potential differences across all the components in the circuit
V
+
-
1
2
V
V V1
V2
1 + 2 = V1 + V2 + V3
V3
Resistance In a circuit, the size of the current depends on the resistance in the circuit. Any component of a circuit resisting the flow of electricity is called a resistor The greater the resistance in a circuit, the lower the current. different types of resistors
Resistance Definition: • Resistance R of a component is the ratio of the potential difference V across it to the current I flowing through it. • Symbol of Resistance = R R I • SI Unit of Resistance = ohms () V
Where R = resistance in ohms () V = p.d. across the component in volts (V) I = current in ampere (A)
15
Ohm’s Law Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.
where I is the current through the resistance in units of amperes, V is the potential difference measured across the resistance in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
Resistance If a cell is connected to a resistance, the current gets smaller as the resistance increases.
Resistance Uses of high and low resistances materials. All metals have finite resistance. Materials Low resistance
High resistance
Uses
copper, gold, silver, aluminium
connecting wires, conductors or connectors
tungsten
used in light bulbs
nichrome (an alloy of nickel and chromium)
heaters, such as coils of electric kettles
carbon
resistors for radio and television sets
Resistance Resistors • Is a conductor that has a known value of resistance • Primary purpose is to control the size of the current flowing in the circuit. • Two types: fixed resistors & variable resistors (or rheostats) • Variable resistor (or rheostat) allows resistances to be changed easily
fixed resistor symbol
variable resistor symbol 19
Resistance Rheostats are variable resistors used for controlling the size of the current in a circuit are used as brightness controls for lights, volume controls on radio and television sets
Resistance Measuring Resistance • To determine the resistance of a metallic conductor, we use the following circuit: • We can find the current flowing through R from the ammeter reading. • We can find the potential difference across R from the voltmeter reading • R can be calculated from the equation: R=V/I
21
Resistance Experiment to Determine Resistance of a resistor 1. Set-up the apparatus as shown in the diagram. 2. As a safety precaution, adjust the rheostat to the maximum resistance so that a small current
battery
rheostat
flows in the circuit initially. 3. Record the ammeter reading (I) & voltmeter
R
reading (V).
ammeter
4. Adjust the rheostat to allow a larger current to flow in the circuit. Again record the values of I and V. 5. Repeat Step 4 for at least 5 sets of I and V readings. 6. Plot the graph of V(V) against I (A). Determine the gradient of the graph.
voltmeter
Note that: Always connect: Voltmeter in Parallel Ammeter in Series 22
Resistance Experiment to Determine Resistance of a resistor Result: The gradient of the graph gives the resistance of the load, R
V/V
Gradient = V / I = resistance
0
I/A
23
Resistance Factors Affecting Resistance There are several factors that affect the resistance of an object such as a wire: 1. Cross-sectional area of wire / thickness of wire thicker wire smaller resistance (R 1/A)
24
Resistance Factors Affecting Resistance 2. Length of wire longer wire larger resistance (R l)
25
Resistance Factors Affecting Resistance 3. Type of material Wires of the same length and thickness but made of different materials will have a different resistances. This is because they have different resistivities. (Units: Ωm)
26
Resistance • These factors can be placed together to find resistance
R = l /A Where R = resistance in ohms () = resistivity in ohm meter (m) l = length of wire (m) A = cross-sectional area in meter square (m2)
27
Resistance Example • The diameter of the copper wire used in a circuit is 2.0 mm. If the resistively for copper is 1.7 x 10-8 m, what is the resistance for 50 cm of the wire? Solution L = 50 cm = 0.5 m diameter = 2.0 mm = 0.002 m A = (d/2)2 = (0.002/2)2 = (0.001)2 m2 R = (1.7 x 10-8)(0.5) / (0.001)2 = 0.0027 28
Resistance resistors in series since resistors are in series, current I passing through each resistor is the same effective resistance I
R1
R2
R3
V1
V2
V3
is equivalent to
I
Rseries = R1 + R2 + R3
Rt V
Resistance resistors in parallel since resistors are in parallel, potential difference across each resistor is the same I1
I2
R1
R2
I I3
effective resistance
R3
is equivalent to
I
R
V V
Temperature Dependence Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material using the following formula: R = Ro[α(T-To)+1] -- Formula not in syllabus where T is its temperature, To is a reference temperature (usually room temperature), R0 is the resistance at T0, and α is the percentage change in resistivity per unit temperature. The constant α depends only on the material being considered.
Ohmic Conductors
V
The uniform gradient shows uniform resistance
I
O
(a) Pure metal
Pure metal, carbon and copper sulphate
V
O
I (b) Copper sulphate solution
Non-Ohmic Conductors At low temperature, the tungsten wire obey Ohm’s Law but at higher temperature it is not obeyed the Law.
V Constant resistance
Higher resistance due to higher temperature
I
O filament bulb
Non-Ohmic Conductors Semiconductor diode A diode allows an electric current to pass in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a valve.
Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph). Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Bridge Rectifiers Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required.
References http://www.cartft.com/image_db/1n4001.jpg http://image.wistatutor.com/content/current-electricity/vacuum-diode-graph.gif http://cyberchalky.files.wordpress.com/2010/03/web_ohms_law_triangle.gif http://www.kpsec.freeuk.com/components/diode.htm
Electric Current • An electric current I is a measure of the rate of flow of electric charge Q through a given cross section of a conductor. • Symbol of Electric Current = I • SI Unit of Electric Current = ampere (A)
I = Q/t where I = current in ampere (A) Q = amount of charges in coulombs (C) t = time in seconds (s) 3
Conventional Current and Electron Flow
Conventional current flows from the positive to the negative ends
Electric charges flow from the negative to the positive ends 4
Conventional Current and Electron Flow Measuring current • An ammeter is an instrument used for measuring electric current. • Ammeters must be connected in series in a circuit
A
ammeter symbol
Positive (negative) side of ammeter is connected to the positive (negative) terminal of the cell / battery. 5
Conventional Current and Electron Flow Measuring current The digital multimeter (DMM) is starting to replace the ammeter. has a wide range of between a few hundred A to several A can be used for direct current (D.C.) and alternating current (A.C.) able to read voltage and resistance too
Conventional Current and Electron Flow Measuring current Since the circuit consists of only one loop, the same current flows through the circuit; does not matter where the ammeter is placed on the circuit A1
-
+
A6
cell
A2
A5 resistor
A3
A4
Electromotive Force (e.m.f)
electric current is produced when there is a flow of charges a source of energy (provided by a cell, group of cells or generator) is needed to enable charges to be pumped or forced around a circuit electromotive force is the electric force that provides the pumping action for electric current to flow from the positive terminal to the negative terminal of the + battery I
cell
lamp
Electromotive Force (e.m.f) Electromotive Force (e.m.f) Definition • The electromotive force (e.m.f.) of an electrical source is the work done by the source in driving a unit charge round a complete circuit. – is the potential difference between the two terminals of the cell or battery. (From higher p.d. to lower p.d) – A point of high potential is a region where there is a large number of positive charges whereas a point of low potential has lesser positive charges (more negative charges) 9
Electromotive Force (e.m.f) Electromotive Force (e.m.f) • Symbol of Electromotive Force = • SI Unit of Electromotive Force = volts (V) or joules per coulomb (JC-1)
= W/Q where = e.m.f. (V) W = Energy converted from non–electrical forms to electrical form (J) [work done] Q = amount of charge in coulombs (C) 10
Potential Difference Potential Difference (p.d.) • The Potential Difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one coulomb of positive charge passes between the two points • Symbol of Potential Difference (p.d.) = V • SI Unit of Potential Difference (p.d.) = volts (V)
V = W/Q where V = Potential difference (V) W = Energy converted from electrical form to other forms (J) Q = amount of charge in coulombs (C)
11
Potential Difference Measuring p.d./e.m.f. • An voltmeter is an instrument used for measuring potential difference or electromotive force. • As charges flow round a circuit, they lose their P.E., transforming P.E. into other forms of energy. • It is connected in parallel to the circuit. • The SI unit for p.d. / e.m.f. is volt (V) V voltmeter symbol
Voltmeters will measure the potential difference across 2 points of the circuit, so we connect it in parallel with respect to those 2 points 12
Potential Difference Potential difference around a simple circuit sum of all the e.m.f.’s of the cells must be equal to the sum of potential differences across all the components in the circuit
V
+
-
1
2
V
V V1
V2
1 + 2 = V1 + V2 + V3
V3
Resistance In a circuit, the size of the current depends on the resistance in the circuit. Any component of a circuit resisting the flow of electricity is called a resistor The greater the resistance in a circuit, the lower the current. different types of resistors
Resistance Definition: • Resistance R of a component is the ratio of the potential difference V across it to the current I flowing through it. • Symbol of Resistance = R R I • SI Unit of Resistance = ohms () V
Where R = resistance in ohms () V = p.d. across the component in volts (V) I = current in ampere (A)
15
Ohm’s Law Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.
where I is the current through the resistance in units of amperes, V is the potential difference measured across the resistance in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
Resistance If a cell is connected to a resistance, the current gets smaller as the resistance increases.
Resistance Uses of high and low resistances materials. All metals have finite resistance. Materials Low resistance
High resistance
Uses
copper, gold, silver, aluminium
connecting wires, conductors or connectors
tungsten
used in light bulbs
nichrome (an alloy of nickel and chromium)
heaters, such as coils of electric kettles
carbon
resistors for radio and television sets
Resistance Resistors • Is a conductor that has a known value of resistance • Primary purpose is to control the size of the current flowing in the circuit. • Two types: fixed resistors & variable resistors (or rheostats) • Variable resistor (or rheostat) allows resistances to be changed easily
fixed resistor symbol
variable resistor symbol 19
Resistance Rheostats are variable resistors used for controlling the size of the current in a circuit are used as brightness controls for lights, volume controls on radio and television sets
Resistance Measuring Resistance • To determine the resistance of a metallic conductor, we use the following circuit: • We can find the current flowing through R from the ammeter reading. • We can find the potential difference across R from the voltmeter reading • R can be calculated from the equation: R=V/I
21
Resistance Experiment to Determine Resistance of a resistor 1. Set-up the apparatus as shown in the diagram. 2. As a safety precaution, adjust the rheostat to the maximum resistance so that a small current
battery
rheostat
flows in the circuit initially. 3. Record the ammeter reading (I) & voltmeter
R
reading (V).
ammeter
4. Adjust the rheostat to allow a larger current to flow in the circuit. Again record the values of I and V. 5. Repeat Step 4 for at least 5 sets of I and V readings. 6. Plot the graph of V(V) against I (A). Determine the gradient of the graph.
voltmeter
Note that: Always connect: Voltmeter in Parallel Ammeter in Series 22
Resistance Experiment to Determine Resistance of a resistor Result: The gradient of the graph gives the resistance of the load, R
V/V
Gradient = V / I = resistance
0
I/A
23
Resistance Factors Affecting Resistance There are several factors that affect the resistance of an object such as a wire: 1. Cross-sectional area of wire / thickness of wire thicker wire smaller resistance (R 1/A)
24
Resistance Factors Affecting Resistance 2. Length of wire longer wire larger resistance (R l)
25
Resistance Factors Affecting Resistance 3. Type of material Wires of the same length and thickness but made of different materials will have a different resistances. This is because they have different resistivities. (Units: Ωm)
26
Resistance • These factors can be placed together to find resistance
R = l /A Where R = resistance in ohms () = resistivity in ohm meter (m) l = length of wire (m) A = cross-sectional area in meter square (m2)
27
Resistance Example • The diameter of the copper wire used in a circuit is 2.0 mm. If the resistively for copper is 1.7 x 10-8 m, what is the resistance for 50 cm of the wire? Solution L = 50 cm = 0.5 m diameter = 2.0 mm = 0.002 m A = (d/2)2 = (0.002/2)2 = (0.001)2 m2 R = (1.7 x 10-8)(0.5) / (0.001)2 = 0.0027 28
Resistance resistors in series since resistors are in series, current I passing through each resistor is the same effective resistance I
R1
R2
R3
V1
V2
V3
is equivalent to
I
Rseries = R1 + R2 + R3
Rt V
Resistance resistors in parallel since resistors are in parallel, potential difference across each resistor is the same I1
I2
R1
R2
I I3
effective resistance
R3
is equivalent to
I
R
V V
Temperature Dependence Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material using the following formula: R = Ro[α(T-To)+1] -- Formula not in syllabus where T is its temperature, To is a reference temperature (usually room temperature), R0 is the resistance at T0, and α is the percentage change in resistivity per unit temperature. The constant α depends only on the material being considered.
Ohmic Conductors
V
The uniform gradient shows uniform resistance
I
O
(a) Pure metal
Pure metal, carbon and copper sulphate
V
O
I (b) Copper sulphate solution
Non-Ohmic Conductors At low temperature, the tungsten wire obey Ohm’s Law but at higher temperature it is not obeyed the Law.
V Constant resistance
Higher resistance due to higher temperature
I
O filament bulb
Non-Ohmic Conductors Semiconductor diode A diode allows an electric current to pass in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a valve.
Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph). Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Bridge Rectifiers Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required.
References http://www.cartft.com/image_db/1n4001.jpg http://image.wistatutor.com/content/current-electricity/vacuum-diode-graph.gif http://cyberchalky.files.wordpress.com/2010/03/web_ohms_law_triangle.gif http://www.kpsec.freeuk.com/components/diode.htm
