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# Current Electricity (NA)

## on May 02, 2010

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Key Concepts Learnt:

Key Concepts Learnt:
- conventional/electron flow
- electric circuit
- current
- voltage - potential difference, electromotive force
- resistance

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## Current Electricity (NA)Presentation Transcript

• Electric current
• Electromotive force
• & Potential Difference
• Resistance
Chapter 16: Current Electricity Part I
• state that current is a rate of flow of charge and that it is measured in amperes
• recall the relationship charge = current x time
• apply the relationship to new situations or to solve related problems
Chapter 14 At the end of the chapter, you should be able to: 1. Electric Current
• Electric Current
• Current flow
• Actual
• electrons flowing from
• -ve to +ve terminal.
Chapter 14 Pg 241 Definition: Current is a rate of flow of charge.
• Conventional
• Charges flowing from
• +ve to –ve terminal.
Actual Conventional
• Ampere (A) Coulomb (C) second (s) Measurement of Current Definition: Current is a rate of flow of charge. The amount of charge passing thru a given pt in 1 sec. Chapter 14 Pg 241 Time (t) Charge (Q) Current (I) SI Unit Quantity Q _______ I t Formula: I = or Q = I t Q t
• Example 1: A current of 10 A flows through an electric heater for 10 minutes. What is the total charge circulated through the heater? [Solution] t = 10 min x 60 = 600 s I = 10 A Q = I t = 10 A x 600 s = 6000 C The total charge is 6000 C Measurement of Current Chapter 14 Q _______ I t
• Example 2: In an electrical circuit, a charge of 60C flows past a point in 10s. What is the current in the circuit? Measurement of Current Chapter 14 [Solution] t = 10 s ; Q = 60 C Q = I t I = = 60 / 10 = 6 A The current is 6 A Q _______ I t Q t
• Example 3: A lightning flash carries 25 C of charge and lasts for 0.01 s. What is the current? [Solution] Q = 25 C ; t = 0.01 s Q = I t 25 C = I x 0.01s 25 / 0.01 = I I = 2500A Current is 2500A Measurement of Current Chapter 14
• Example 4: A current of 2 A is flowing through a conductor. How long does it take for 10 C of charge to pass any point? [Solution] I = 2 A ; Q = 10 C Q = I t 10C = 2A x t 10 / 2 = t t = 5 s Time taken is 5 s Measurement of Current Chapter 14
• Ammeter
• measures the current in a circuit
• connects in series
• measures in A or m A
• has very low resistance
There must be a closed path in order for current to flow. Ammeter Chapter 14 Pg 241 A A A +  + 
• More common symbols can be found on pg 243 Chapter 14 Pg 243 Electric Symbols
• Circuit Diagram Variable resistor Bulb Ammeter Voltmeter Battery Fixed resistor Switch Chapter 14 Pg 243
• Electric current
• Electromotive force & Potential Difference
• Resistance
Chapter 14: Current Electricity Chapter 14 Pg 245 Part II
• define electromotive force (e.m.f.) as the work done by a source in driving a unit charge around a complete circuit
• state that the potential difference (p.d.) across a circuit component is measured in volts
At the end of the chapter, you should be able to: 2. Electromotive Force & Potential Difference Chapter 14 Pg 245
• Electromotive Force (e.m.f) Chapter 14 Pg 245 Definition: Electromotive force is defined as the total work done by a source in driving a unit charge around a complete circuit 1 Unit charge = 1 coulomb of charge
• Electromotive Force (e.m.f) Chapter 14 Pg 245
• Sources of e.m.f are:
• Electrical cells (i.e. batteries)
• Thermocouples
• Generators
• etc
• 2V Electromotive Force (e.m.f) Hi I’m Mr Coulomb (1 C) 2J of energy 2J of energy 2 J of work is done when 1 C of charge moves round the circuit Mr Coulomb goes back to the source for energy Note: 2J of electrical energy 2J of light and heat energy 2 J of energy is supplied by the cell in moving 1 C of charge round
• Electromotive Force (e.m.f) Chapter 14 Pg 245
• Cell
• Source of energy
• Produces e.m.f that pushes the charges round the circuit.
Work done/energy is used to light up the bulb. Direction of current travel
• Analogy The pump pushes the water to flow flow of water Work done/ energy is used to move the mill
• Potential Difference (p.d.) Chapter 14 Pg 246 Definition: The p.d. between two points is the energy required to move 1 C of charge between them. Potential Difference (p.d.) OR Voltage (V) SI Unit : V (volts)
• The p.d. between 2 points is the energy required to move 1 C of charge between the two points. energy E p.d. = --------------- , V = ------ or E = VQ charge Q e.g. 2V = 2 J/C Potential Difference (p.d.) Formula: E _______ V Q
• V
• Voltmeter
• measures the p.d. / voltage between 2 points
• connects in parallel across 2 points
• measures in V or mV
• has very high resistance
+  +  Voltmeter Chapter 14 Pg 247 2J of energy 2J of energy
• The diagram shows a battery with an electromotive force of 6 V in a circuit. How much energy is needed to drive 30C of charge round the circuit? E = VQ = 6V x 30C = 180 J or [Solution] Example 1 6 V
• An electrical quantity is defined by “the energy converted by a source in driving unit charge round a complete circuit”. What is this quantity called?
• Current B. Electromotive force
• C. Potential difference D. Power
Example 2 B
• When a current of 0.5 A flows for 10 minutes through an electrical heater, 2400 J of energy is transformed.
• Calculate the total charge moving through the heater.
• (b) Calculate the potential difference across the heater.
Q = I t = 0.5A x (10 x 60)s = 300 C Total charge is 300 C E = V Q 2400J = V x 300C V = 2400 / 300 = 8 V The p.d. is 8 V Example 3
• Electric current
• Electromotive force & Potential Difference
• Resistance
Chapter 14: Current Electricity Chapter 14 Pg 247 Part III
• Chapter 14 Pg 247 The resistance is a measure of how difficult it is for an electric current to pass through a substance. Resistance
• Chapter 14 Pg 247 Definition: The resistance of a conductor is defined as the ratio of the potential difference across the conductor to the current flowing in it. Resistance Formula: R = SI Unit : Ohms (  ) V I where R = resistance V = p.d / voltage I = current or V = IR
• The size of the current depends on the resistance in the circuit. A A 2  5  10  With the same cell used (i.e. voltage is the same), as resistance, R increases, current, I ____________ Resistance 20 V 20 V 20 V decreases I = 10 A I = 4 A I = 2 A A
• Resistance resists the flow of current
• Resistance is low in conductors and very high in insulators .
Flow of current Resistance Resistance Chapter 14 Pg 248
• V = I R 6 = I x 4 6 / 4 = I I = 1.5 A Reading on the ammeter is 1.5 A A 4  resistor is connected in series with an ammeter and a 6 V battery, as shown. What is the reading shown on the ammeter. Example 1 R I _______ V
• Chapter 14 Pg 249 The resistance R (= V / I) of a metallic conductor is CONSTANT under steady physical conditions Ohm's Law
• For Ohmic conductors (Conductors that obeys Ohm’s law) e.g. pure metal Chapter 14 Pg 248 For non-Ohmic conductor e.g. filament lamp bulb I /A V/V I /A V/V Metal A Metal B I /A V/V
• an electrical component designed to reduce the flow of current.
• converts electrical energy to heat energy.
• (e.g. resistors used in electric fire and filament bulb
• convert electrical to heat and light energy)
• represented by the symbol
Resistor
• Rheostat
• a variable resistor that controls the size of a current in a circuit represented by
Resistor
• Procedure:
• Set up the apparatus as shown above.
• Adjust the variable resistor to allow the smallest possible current to flow in the circuit
• Adjust the variable resistor in steps to increase current flow in the circuit and
• note the values of I and V for at least five sets of readings.
• Plot a graph of V against I. The graph plotted must be a best straight line passing
• through the origin.
• The gradient of the best straight line obtained gives the resistance of the resistor, R.
To determine the unknown resistance, R of a fixed resistor Pg 253 Fixed resistor A V Variable Resistor/ Rheostat
• The unknown resistance of the resistor is found by obtaining the gradient of the straight line graph.
• Precaution :
• To prevent a rise in the temperature of the resistor, which may change its resistance,
• open the circuit between readings
• use small amount of current
Experiment to find Resistance Chapter 14 Pg 253 I /A V/V
• Example 2 C
• Example 3 C
• Example 4 B
• Example 5 B
• Resistivity
• Besides physical conditions (e.g. temperature), the resistance R of a given conductor also depends on:
• its length l
• its cross-sectional area A
• the type of material
• Resistivity Formula: where R = resistance ρ = resistivity l = length A = cross-sectional area
• Example 6
• Example 7
• Resistors in Series Resistors in Parallel Simulation from Crocodile Physics
• Example 8
• Example 9