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Water vs. Electric Current There are many analogous properties between water and electric current Characteristics such as •Pressure •Volume •Flow Voltage • Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. • Voltage is measured in volts (V). Using a voltmeter • A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second. Voltage source VOLTAGE causes current VOLTAGE causes current Charges do NOT flow unless there is potential difference A voltage source is needed to provide a sustained potential difference i.e. batteries or generators The battery or source is represented by an escalator which raises charges to a higher level of energy. Current is a flow of charge Current • Electric current is measured in units called amperes, or amps (A) for short. • One amp is a flow of a certain quantity of electricity in one second. • The amount of electric current entering a circuit always equals the amount exiting the circuit. Current Electricity Example • The continuous flow of charge in a complete circuit. Electrical resistance • Resistance measures how difficult it is for current to flow. The ohm • Resistance is measured in (W). • One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp. Ohm’s Law The formulae Ohm Quiz Ohm plate Ohm on the range Ohm sweet Ohm Broken Ohm Ice cream Ohm Ohm Alone Practice Quiz 1. If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12 volt battery? = 12 V / 100000 = 0.00012 A 2. If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24 volt battery, how much current would you draw? See = 0.024 A slide = 24 V / 1000 18 Power Power (watts) P = VI Voltage (volts) Current (amps) Electric Power The work done by an electric current moving through a circuit is given by W=VIt Power = Volts x Amps 1 watt = 1 volt x 1 amp Power Equations Power (P) Rate at which work is performed Measured in watts (W) • Power • P = V I P = (I R) I = I 2 R Example problem • A light bulb with a resistance of 1.5Ω is connected to a 1.5-V battery in the circuit shown at left. • Calculate the power used by the light bulb. • 1. Find the IT = V / R • = .5 A • 2. Find the P = I V • = .75 watts Power Dissipated in an Electricity Distribution System 150 miles 120 Watt Light bulb Power Plant on Colorado River 12 Volt Connection Box • Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 W/m 2105 m = 20 Ohms • We can figure out the current required by a single bulb using P = VI so I = P/V =120 Watts/12 Volts = 10 Amps Power Dissipated in an Electricity Distribution System 150 miles 120 Watt Light bulb Power Plant on Colorado River 12 Volt Connection Box • Power in transmission line is P = I 2R = 102 20 = 2,000 Watts!! • “Efficiency” is έ = 120 Watts/4120 Watts = 0.3% Series Circuit ……… Charges can move having one SINGLE PATH for the charges to flow • SERIES circuits, current can only take one path. If one of the items in the circuit is broken then NO charge will be able to flow Adding resistances in series The total resistance of the circuit a.k.a effective resistance is equal to the sum of the individual resistances Rtotal = R1 + R2 + R3... Individual resistances Ω Total resistance ( ohms Ω ) CHARACTERISTICS OF A SERIES CIRCUIT • The current is numerically equal to the voltage supplied – Ohm’s Law Because there is only ONE possible path Voltage in a series circuit • Each separate resistance creates a VOLTAGE DROP as the current passes through. The voltage drop across each device depends on its resistance Total voltage divides among the devices • As current flows along a series circuit, each type of resistor has an effect Voltage applied to Series Circuits The sum of the potential drops equals the potential rise of the source. The total voltage is the sum of the voltage on each component Ohm’s Law & Circuits An important caveat to Ohm's Law All quantities (voltage, current, resistance, and power) must relate to each other in terms of the same two points in a circuit. Solve the Problem What is the voltage drop across each resistor? Step 1 Calculate RT RT = R1 + R2 + R3 = 18 k Ω Step 2 Calculate IT IT = VT / RT = 5 x 10-4 amps Step 3 Calculate V for each resistor V1 = 1.5 V V2 = 5 V V3 = 2.5 V Parallel circuit ….. There are Multiple pathways for the current to flow through If one of the items in the circuit is broken then charge will move through other paths & will continue to have charges flow through them Parallel Circuits • In parallel circuits the current can take more than one path. • Because there are multiple branches, the current is not the same at all points in a parallel circuit. The inverse of the total resistance of the circuit (also called effective resistance) is equal to the sum of the inverses of the individual resistances Example problem for resistance in parallel circuits • A circuit contains a 2 ohm resistor and a 4 ohm resistor in parallel. • Calculate the total resistance of the circuit. • 1.33 ohms Voltage applied to Parallel Circuits The potential drops of each branch equals the potential rise of the source Current applied to Parallel Circuits The total current is equal to the sum of the currents in the branches Parallel Circuit Rules Important thing to notice the more branches you add to a parallel circuit (the more things you plug in) the lower the total resistance becomes As the total resistance decreases, the total current increases. Why are Parallel circuits found in most household electrical wiring? Advantages of parallel circuits Parallel circuits have two big advantages over series circuits: 1. Each device in the circuit sees the full battery voltage. 2. Each device in the circuit may be turned off independently without stopping the current flowing to other devices in the circuit. Practice Problems What is the total resistance? 5.5 27.5 V What is the total voltage? What is the voltage and current on A, B, and C? A.) Voltage is constant B.) C.)