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Electricity BHS Physical Science K Warne

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Grade 9 Revision Revision PresentationRevision PresentationRevision PresentationRevision Presentation

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Electrical Circuits A V1V1 Voltmeter Ammeter Resistor The Ammeter measures the ……………flowing in the circuit. (…….. A) The Voltmeter Measures potential difference or …………….. in volts. (V) The Resistance of the Resistor is given in …………… (Ω). V 1 = V 2 V2V2 Voltage across …………..= voltage in ……………

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Electric Current in a Conductor Conventional current - __________________ Direct current - moves in _______________. Alternating current - ___________________ continuously Maintaining a current Conductor - ___________ _____________________ _____________________ - SOURCE e < electrons “_____________” > Conventional current is the movement of from + to - in a conductor. Conventional current e-e-

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Electric Current & Voltage The VOLTAGE is how much ENERGY they have Current is the number of soldiers moving past a point. Conventional current e-e-

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Potential Difference The Voltmeter: is connected in ……………. to another component in the circuit (the current does NOT flow through it)is connected in ……………. to another component in the circuit (the current does NOT flow through it) has a …………. resistancehas a …………. resistance Is connected positive to positive - negative to negative.Is connected positive to positive - negative to negative. + _ Voltmeter V 1.50 Resistor V The DIFFERENCE in POTENTIAL (energy) per unit ………………. of the current flowing between two points in the circuit. Measured by a ………………... Volts = …………. (J) …………. (C ) R V =V =V =V = W or E (J) Q (C ) Q (C )

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? A Resistance R Example 1: Calculate the potential difference between two points if 20 J of work are required to move a charge of 2 C. Example 2: Calculate the work done in moving a charge of 5 C through a potential difference of 2 V. POTENTIAL DIFFERENCE 20J Energy lost 2 coulombs of charge 2V A Resistance R ?? Energy lost or work done?? 5 coulombs of charge

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V ex A Resistance R V Cell Emf 400V I = 0A V crt. = ….. v Emf = ….. cell Emf is the …………… amount of …………… that the cell can produce (per unit charge). Measured when the current in the circuit is ……….. EMF - Electro Motive Force Open Circuit!! V cell = EMF

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Emf = V ….. + V …… External Potential Difference ………… Circuit!! (EMF = 400V) 350V V ex = V cell V ex A Resistance R V cell I = IA Energy …………….. by battery is lost by resistance in the circuit. The EMF of the cell is equal to the sum of the ………………..cell voltage and the ………… voltage. This continues until the cells have no more energy. 400V 50V 350V 50V

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Measuring Current The Ammeter: is connected in ……….in the circuit (the current flows through it)is connected in ……….in the circuit (the current flows through it) has a …………resistancehas a …………resistance Is connected positive to positive - negative to negative.Is connected positive to positive - negative to negative. An Ammeter measures the ……………… flowing through the circuit. + _ A Ammeter A 1 The current is the number of charges passing a point in one second. I = Q / t 1A = …..C/……s

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Calculating Current Calculate the current flowing through the circuit. + _ Ammeter A 0.53 If 160 C of charge flow through the ammeter in 3s what current is flowing? I = Q / t =

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Parallel Circuits Adding resistors in parallel…decreases the total resistance.Adding resistors in parallel…decreases the total resistance RtR1 R2 RtR1 R2 RtR1 R2 RtR1 R R t …… R t …… R t R t R t = …./…. = ……. R t = …./…. = ……. Ω = + R1R1 2 2 Ω R1R1 R2R2 Total R = …. Total R = …. Ω Total R = … Total R = … Ω Total R = …… Total R = …… Ω R1R1 2 2 Ω R2R2 R2R2 = + = + +

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Parallel Circuits The voltage is EQUAL over the resistances.The voltage is EQUAL over the resistances. V T = ……………. The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow.The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow. A T = ……………….. V1V1 R1R1 VTVT V2V2 R2R2 2 2 V ….. ….. V R1R1 A R2R2 A1A1 A2A2 …. A ….. A 4 A4 A4 A4 A

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Parallel Circuits Adding resistors in parallel…decreases the total resistance.Adding resistors in parallel…decreases the total resistance RtR1 R2 RtR1 R2 RtR1 R2 RtR1 R2 The voltage is EQUAL over the resistances.The voltage is EQUAL over the resistances. V T = V 1 = V 2 The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow.The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow. = + V1V1 R1R1 A VTVT V2V2 R2R2 A1A1 A2A2 A T = A 1 + A Ω The current will divide in such a way that the potential lost by both all branches of current will be the same.

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Series Circuits Adding resistors in series…increases the total resistance - because all the current flows through all the resistors.Adding resistors in series…increases the total resistance - because all the current flows through all the resistors. R t = R 1 + R 2 The total potential difference (voltage) is the sum of the potential differences of the resistors – the total potential loss must equal the all the potential lost along the way.The total potential difference (voltage) is the sum of the potential differences of the resistors – the total potential loss must equal the all the potential lost along the way. V t = V 1 + V 2 The potential differences will be proportional to the resistances. The current flowing is the same all over the circuit and would decrease as more resistances are added -The current flowing is the same all over the circuit and would decrease as more resistances are added - A = A 2 = A 3 VTVT V1V1 v2v2 A VTVT A2A2A2A2 A3A3A3A3 3Ω3Ω3Ω3Ω 1Ω1Ω1Ω1Ω 12v

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Worked Example A 4 Ω 2 Ω 8 Ω 12 Ω 6 Ω 12v 4.1 Calculate I

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Current & Resistance RESISTANCE Electrical charge experiences ……………………as it moves through a conductor.Electrical charge experiences ……………………as it moves through a conductor. The resistance is due to ……………….. with particles in the metal atoms and ions.The resistance is due to ……………….. with particles in the metal atoms and ions. The moving charges lose ……………………in the collisions which …………….up the conductor.The moving charges lose ……………………in the collisions which …………….up the conductor. + _ CURRENT: An electrical current is a movement of ……………. through a conducting material from positive to negative. (?!)

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Ohm’s Law - Practical AIM: –Investigate the relationship between the …………………….. across a resistor and the ………………..flowing through it. –Determine the …………………..of a resistor. V A METHOD: 1. 1.Set up the circuit as shown Using the rheostat vary the current in the circuit, obtain a range of readings for the potential difference across R for different currents RESULTS>>RESULTS>> Resistance R rheostat

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Ohm’s Law - Results Draw a graph of your results. Results Analysis - Graph I (A)V (V)

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Ohm’s Law - Analysis The ratio V / I produces a constant value - for any resistor This is the Resistance of the resistor. The Unit of measurement for resistance is the Ohm - symbol (Ω) The SLOPE of the graph gives the RESISTANCE. Riserun Slope = Y/ X =

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Current, voltage & resistance We define the unit of resistance; one …….() is one volt per ampere. R = V / I 1 = …V / …A A V …………… The relationship between the …………… through a resistor, the …………... drop across the resistor and the resistance of the resistor is expressed by the following equation: R V Rx.. Calculate the voltage drop across a 2 resistor when a current of 1.5 A is flowing..

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Worked Example A 4 Ω 2 Ω 8 Ω 12 Ω 6 Ω 12v 4.1 Calculate I = V/R = 12/6 = 2 A

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Ohm’s Law A V Factors affecting Resistance 1. 1.Material 2. 2.Length 3. 3.Temperature

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Effects of Current Electric current generates heat in a conductor. + _ A small current (0.1A) would have only a few charges flowing. + _ A large current (15A) would have a large number of charges flowing and generate far more heat. As a conductor heats up the RESISTANCE INCREASES.

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Effects of electric current An electric current that flows in a conductor has a number of effects: 1. HEATING The friction caused by the current causes the conductor to heat up. The greater the current the more heat is generated. 2. MAGNETIC EFFECT - A magnetic field is generated around any conductor when an electric current flows through it.

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