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Electricity BHS Physical Science K Warne Grade 9 Revision • Revision Presentation 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 V1 Voltmeter A Ammeter Resistor V2 Voltage across …………..= voltage in …………… Electric Current in a Conductor Conventional current - __________________ Direct current moves in _______________. Alternating current ___________________ continuously Maintaining a current Conductor - ___________ _____________________ _____________________ SOURCE + - Conventional current + e- - ee- ee- 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. Electric Current & Voltage + The VOLTAGE is how much ENERGY they have. - Conventional current + Current is the number of soldiers moving past a point. e- - Potential Difference The DIFFERENCE in POTENTIAL (energy) per unit ………………. of the current flowing between two points in the circuit. Measured by a ………………... W or E (J) ………….(J) V= 1.50 V Volts = Q (C ) ………….(C ) Voltmeter _ + 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 V negative. R POTENTIAL DIFFERENCE Example 1: Calculate the potential difference between two points if 20 J of work are required to move a charge of 2 C. A ? 2 coulombs of charge Resistance R 20J Energy lost Example 2: Calculate the work done in moving a charge of 5 C through a potential difference of 2 V. 2V A 5 coulombs of charge Resistance R ?? Energy lost or work done?? 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 ……….. Emf 400V VCell Open Circuit!! Vcell = EMF A I = 0A Vex Vcrt.= ….. v Resistance R Emf = …..cell External Potential Difference ………… Circuit!! 50V Vcell (EMF = 400V) 350V A I = IA 50V Vex= Vcell Vex 400V 350V Resistance R 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 = V….. + V…… Measuring Current An Ammeter measures the ……………… flowing through the circuit. _ 1 A Ammeter + 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. A Calculating Current Calculate the current flowing through the circuit. _ 0.53 A Ammeter + If 160 C of charge flow through the ammeter in 3s what current is flowing? I = Q/t = Parallel Circuits • Adding resistors in parallel…decreases the total resistance. 1 = 1 + 1 Rt R1 R2 1 = 1 + 1 Rt … … 1 1 = + Rt 2 1 1 + 2 2 Rt = …./…. = ……. Ω Total R = …. Ω R1 R1 2Ω 2Ω R2 2 Ω Total R = … Ω R1 2Ω R2 2Ω R2 2 Ω Total R = …… Ω Parallel Circuits • The voltage is EQUAL over the resistances . VT V1 VT = ……………. R1 2V ….. V V2 R2 • The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow. 4A A AT= ……………….. R1 A1 …. A A2 ….. A R2 ….. V Parallel Circuits • Adding resistors in parallel…decreases the total resistance. 1 1 1 = + Rt R1 R2 • The voltage is EQUAL over the resistances . VT = V1 = 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. AT= A1 + A2 VT A V1 A1 A2 R1 2Ω V2 R2 2Ω The current will divide in such a way that the potential lost by both all branches of current will be the same. 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 = 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 - A = A2 = A3 VT V VTT A A vV22 V11 V R1 1Ω A3 A2 R2 3Ω Worked Example 4.1 12v A 6Ω 4Ω 12 Ω 8Ω Calculate I 2Ω 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. 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: 1. Set up the circuit as shown. 2. Using the rheostat vary the current in the circuit, obtain a range of readings for the potential difference across R for different currents. 3. A V RESULTS>> Resistance R Ohm’s Law - Results Analysis - Graph Results I (A) V (V) Draw a graph of your results. Ohm’s Law - Analysis I (A) 0.80 0.81 0.85 0.90 0.95 V (V) 3.40 3.50 3.70 3.90 4.20 V/I 4.3 4.3 4.4 4.3 4.4 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) Current vs Voltage Voltage (V) 4.40 The SLOPE of the graph gives the RESISTANCE. 4.20 4.00 3.80 3.60 3.40 3.20 3.00 0.75 Slope Rise run 0.80 0.85 0.90 Current (A) 0.95 1.00 = DY/ DX = Current, voltage & resistance The relationship between the …………… through a resistor, the …………... drop across the resistor and the resistance of the resistor is expressed by the following equation: - + A V R - + V Rx .. Calculate the voltage drop across a 2 resistor when a current of 1.5 A is flowing.. We define the unit of resistance; one …….() is one volt per ampere. R = V/I 1 = …V/…A Worked Example 4.1 12v A 6Ω 4Ω 12 Ω 8Ω Calculate I = V/R = 12/6 = 2 A 2Ω Ohm’s Law Factors affecting Resistance - + 1. Material 2. Length 3. Temperature A + V - 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. 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.