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Week 2 Electricity and circuits For centuries people have used the energy of flowing water to help them do work. What are the limitations of using water for energy? Would it work to put a water wheel in this pond? Dams allow better control over the flow of water There are some similarities between using water for useful work and using electricity for useful work. Electric current is the flow of electrons The amount of flow or current (I) is measured in amperes (A or amps) Current must flow through something going somewhere. Current is measured using an ammeter. Voltage (E) is the Electromotive force that makes the electrons move. Voltage is measured in volts (V) and the . measurement is made by comparing one point to another reference point. It helps to understand electricity if we compare it to flow of water. Energy can be stored in water by pumping it up to a higher elevation. Gravity is the force exerted on the water trying to pull it back down to its original level. The higher the water is held above its original level the more potential energy that is stored in the water. Voltage is the difference between the 2 points. Voltage is sometimes referred to as the potential difference. The battery serves as the pump. The push comes from the negative end (pole). A voltmeter is used to measure the voltage in a circuit. All materials exhibit some resistance to the flow of its electrons. This property is called resistance (R) and is measured in ohms (W) Materials with little resistance are conductors (metals) Materials that prevent the flow of electrons are insulators (glass, plastics, ceramics, nonmetals) What is the name for the flow of electrons in an electric circuit? Current In what unit is electrical current measured? Amperes What is the basic unit of resistance? The Ohm What do we call materials that offer little resistance to the flow of electrons? Conductors What instrument is used to measure the flow of current in an electrical circuit? Ammeter What term is used to describe opposition to current flow in ordinary conductors such as wires? Resistance Give an example of a material that is a good electrical insulator. Glass/ plastic/ non-metals What instrument is used to measure Electromotive Force (EMF) between two points such as the poles of a battery? Voltmeter George Ohm discovered the relationship between voltage, current and resistance in a circuit. Ohm’s Law The more resistance in a material, then the less current will flow in response to the voltage. This diagram can help you Remember Ohm’s Law. V I R What formula is used to calculate current in a circuit? Current (I) = Voltage (E) / Resistance (R) What is the resistance of a circuit when a current of 2 amperes flows through a resistor connected to 50 volts? 25 ohms What is the current flow in a circuit with an applied voltage of 110 volts and a resistance of 75 ohms? 1.5 amps What is the voltage across the resistor if a current of 1 amp flows through a 5 ohm resistor? 5 volts The rate at which electrical energy is used is called power. Power = current x voltage The unit for power is the watt. What is the formula used to calculate electrical power in a DC circuit? Power (P) = voltage(E) multiplied by current(I) How much power is being used in a circuit when the applied voltage is 13.8 volts DC and the current is 10 amperes? 138 watts How much power is being used in a circuit when the applied voltage is 12 volts DC and the current is 2.5 amperes? 30 watts How many amperes are flowing in a circuit when the applied voltage is 12 volts DC and the load is 120 watts? 10 amperes Sometimes it is necessary to convert from small units to larger units or larger units to smaller units. This is when an understanding of the metric prefixes comes in handy. 12 milliamps = ___ amps 24,000 watts = ___ kilowatts 70 millivolts = ___ centivolts 15 volts = ___ millivolts ___ microvolts ___ kilovolts ___ megavolts Which of the following is equivalent to 500 milliwatts? A. 0.02 watts B. 0.5 watts C. 5 watts D. 50 watts How many volts are equal to one kilovolt? One thousand volts How many milliamperes is 1.5 amperes? 1500 millamperes How many volts are equal to one microvolt? 1 millionth of a volt If an ammeter calibrated in amperes is used to measure a 3000-milliampere current, what reading would it show? A. 0.003 amperes B. 0.3 amperes C. 3 amperes D. 3,000,000 amperes