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Electricity

Electricity. BHS Physical Science K Warne. Grade 9 Revision. Revision Presentation. V 1. Electrical Circuits. The Ammeter measures the ……………flowing in the circuit. (…….. A) The Voltmeter Measures potential difference or …………….. in volts. (V)

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Electricity

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  1. Electricity BHS Physical Science K Warne

  2. Grade 9 Revision • Revision Presentation

  3. V1 Electrical Circuits 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). V1 = V2 Voltmeter A Ammeter Resistor V2 Voltage across …………..= voltage in ……………

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

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

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

  7. POTENTIAL DIFFERENCE A ? 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. 2 coulombs of charge Resistance R 20J Energy lost A 2V 5 coulombs of charge Resistance R ?? Energy lost or work done??

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

  9. Vcell I = IA A Vex Resistance R External Potential Difference ………… Circuit!! 50V 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. (EMF = 400V) 350V 50V Vex= Vcell 400V 350V Emf = V….. + V……

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

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

  12. Parallel Circuits • Adding resistors in parallel…decreases the total resistance. 111 Rt R1 R2 111 Rt … … 1111 Rt 2 2 2 Rt = …./…. = ……. Ω Total R = …. Ω R1 2 Ω = + R1 2 Ω = + R2 2 Ω Total R = … Ω = + + R1 2 Ω R2 2 Ω 2 Ω R2 Total R = …… Ω

  13. Parallel Circuits • The voltage is EQUAL over the resistances . VT = ……………. • The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow. 2 V VT V1 ….. V R1 V2 ….. V R2 4 A A R1 AT= ……………….. A1 …. A A2 ….. A R2

  14. Parallel Circuits • Adding resistors in parallel…decreases the total resistance. 111 Rt R1 R2 • The voltage is EQUAL over the resistances . VT = V1 = V2 • The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow. VT A = + V1 2 Ω R1 A1 V2 A2 2 Ω R2 The current will divide in such a way that the potential lost by both all branches of current will be the same. AT= A1 + A2

  15. VT VT A V1 v2 A2 Series Circuits 12v • Adding resistors in series…increases the total resistance - because all the current flows through all the resistors. Rt = R1 + R2 • 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. Vt = V1 + V2 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 - A = A2 = A3 A3 1Ω 3Ω

  16. 12v A 6 Ω 4 Ω 12 Ω 2 Ω 8 Ω Worked Example 4.1 Calculate I

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

  18. Ohm’s Law - Practical AIM: • Investigate the relationship between the …………………….. across a resistor and the ………………..flowing through it. • Determine the …………………..of a resistor. rheostat METHOD: 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>> A V Resistance R

  19. Ohm’s Law - Results Analysis - Graph Results Draw a graph of your results.

  20. 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. Rise run Slope = DY/ DX =

  21. Current, voltage & resistance - + 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: We define the unit of resistance; one …….() is one voltper ampere. R = V/I 1 = …V/…A R V Calculate the voltage drop across a 2  resistor when a current of 1.5 A is flowing.. Rx ..

  22. 12v A 6 Ω 4 Ω 12 Ω 2 Ω 8 Ω Worked Example 4.1 Calculate I = V/R = 12/6 = 2 A

  23. - + A V - + Ohm’s Law • Factors affecting Resistance • Material • Length • Temperature

  24. _ + _ + 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 generatefar more heat. As a conductor heats up the RESISTANCE INCREASES.

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

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