2 Unit Three: Electricity and Magnetism Chapter 7 Measuring Electricity7.1 Voltage7.2 Current7.3 Resistance
3 Chapter 7 Learning Goals Measure volts with an electrical meter.Describe the role of a battery in a circuit.Describe the transfer of energy in a circuit.Explain the relationship between voltage and energy in a circuit.Describe current as a flow of electric charge.Measure amperes with an electrical meter.Classify materials as conductors, semiconductors, or insulators.Differentiate between electrical conductivity and resistance.Explain why metals are good electrical conductors.Measure ohms with an electrical meter.
5 7.1 VoltageKey Question:Why do charges move through a circuit?
6 7.1 What does a battery do?A battery uses chemical energy to move charges.If you connect a circuit with a battery the charges flow out of the battery carrying energy.
7 7.1 What does a battery do?We measure the energy level of any place in a circuit in volts.A fully charged battery adds energy proportional to its voltage.The positive end of a 1.5 volt battery is 1.5 volts higher in energy than the negative end.
8 7.1 Voltage and PotentialVoltage is related to potential energy, just like height is related to pressure in water flow.A greater difference in height means that the water has more potential energy.Differences in electrical energy are measured in volts.If there is a difference in volts, current will flow from the higher voltage to the lower voltage.
10 Measuring voltage of a cell Set the meter to DC volts.Touch the red (+) lead of the meter to the (+) battery terminal.Touch the black (-) lead of the meter to the (-) battery terminal.Adjust the meter dial as necessary.
11 Measuring voltage in a circuit Measure the voltage across the battery exactly as before.DO NOT DISCONNECT THE CIRCUIT.NOTE: Since voltage is measured from one point to another, we usually assign the negative terminal of a battery to be zero volts (0 V).
12 7.1 Measuring VoltageEvery point in a circuit connected to the same wire is at the same voltage
13 7.1 Voltage DropsIf we connect anything that uses energy, like a light bulb, we reduce the voltage.Anything that uses energy (motors, bulbs, resistors) lowers the voltage since it takes energy away.
14 7.1 How do these batteries differ? Some are smaller and don't store as much energy.Other batteries made with Ni and Cd can be recharged.Which battery above has the greatest voltage capacity?
15 7.1 Mini Quiz Describe voltage? How are electrical differences measured ?What can you say about every point in a circuit connected by the same wire?What is the voltage of the negative terminal of a battery?What happens to the voltage when you connect a battery to a circuit?
16 7.2 CurrentKey Question:How do charges move through a circuit?
17 7.2 Current Current is the flow of electric charges. One ampere is a flow of one coulomb per second.
20 7.2 Which way does current flow? Either positive or negative charges can flow.It depends on the materials making up the circuit. We label circuits from high to low current.
21 Measuring CurrentIn practical electricity, we still label current flowing from plus to minus or HIGH voltage to LOW voltage.Current can't be measured unless the charges flow through the meter.
22 7.2 Electricity in your house These devices protect you from short circuits and fires.
23 7.2 AC or DCThe electricity in your house uses alternating current, also called AC current. This means the direction of the current goes back and forth.The current reverses direction 60 times per second.The hot wire carries 120 volts AC.A battery only makes current that flows in one direction. This is called direct current, or DC.
24 7.2 Mini Quiz What is current? What are the units of current? What is the sign of the charges that move in a wire?What supplies the energy to make the charges flow?What type of current is used in homes? What type is used in batteries?
25 7.3 ResistanceKey Question:How well does current travel through different materials and objects?
26 Measuring Resistance Set the meter to measure resistance (W). Set the black and red leads on opposite ends of the objects.
30 7.3 The ohm Resistance is measured in ohms (W). One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.
31 7.3 Why does a bulb light?A tungsten filament has a high resistance and glows white when it reaches 2,500 oC.Argon gas inside the bulb is "inert", it doesn't chemically react with the tungsten so it protects it from air for use over and over again.
32 7.3 Mini Quiz What is a conductor? What is a semi-conductor? What is an insulator?What makes a material a conductor or an insulator?What is resistance ?