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Chapter 4.3 Notes. Resistance in Electricity. Charges can easily flow through conductors because they contain many free electrons. Even conductors have resistance though. Some conductors can be cooled to very low temperatures to reduce the friction. These are called superconductors.

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Chapter 4.3 Notes

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Chapter 4.3 Notes

Resistance in Electricity

  • Charges can easily flow through conductors because they contain many free electrons.

  • Even conductors have resistance though.

  • Some conductors can be cooled to very low temperatures to reduce the friction. These are called superconductors.

  • In insulators, electrons are tightly bound and cannot move freely and so they do not allow charges to flow easily.

  • Examples of insulators include wood, plastic, glass, and rubber.

  • Some substances are intermediate in their ability to conduct charge and are called semiconductors.

  • Insulators are used for computer chips and microprocessors.

  • In wire, free electrons move throughout the wire, but not in straight lines.

  • The electrons continuously bump into other electrons and atoms.

  • Each collision causes the electron to change direction.

  • However, Electricity allows flows from the negative terminal to the load (light bulbs) to the positive terminal.

  • When fast moving electrons bump into atoms, they transfer energy.

  • Energy is transferred from the electron to the atom which increases the wire’s temperature.

  • These collisions between electrons and atoms is what slows down the free flow of electrons and is the cause of electrical resistance.

  • Resistance = voltage / current

  • R = V / I

  • Unit for resistance is the ohm.

  • Symbol for resistance is Ω

  • If we have a current of 2 A when the power supply is producing a voltage of 110 V, what is the resistance?

  • R = V / I

  • R = 110 V / 2 A

  • R = 55 Ω

  • Physicist George Simon Ohm discovered that the ratio of voltage to current is constant for most conductors.

  • Ohm’s Lawsays if its resistance is constant, than change in Voltage will equal current times resistance.

  • The amount of resistance in a wire depends on 3 things:

  • a. Length of the wire – longer increases resistance

  • b. The radius of the wire – the smaller the wire, the less resistance

  • c. The material of which the wire is made

  • Resistivity is a measure of the capacity of a material to resist electric charge flow.

  • In a series circuit their is only 1 path for electricity to flow.

  • 3 important rules for Series circuits:

  • Since there is only path for charges to flow, the current is the same everywhere.

  • The current is slowed down by the first light bulb and then slowed down more by a second light bulb. Therefore, the resistance is the sum of the individual resistances of each light bulb.

  • The sum of the voltage drop across each light bulb = the voltage of the battery.

  • In a series circuit, when there is a break in the circuit, the current everywhere is stopped.

  • In a series circuit, light bulb one has a resistance of 90 ohms, light bulb 2 has a resistance of 70 ohms, and light bulb 3 has a resistance of 120 ohms. What is the total resistance in the circuit?

  • Total resistance in series equals sum of each resistance: 90 + 70 + 120 = 280

  • In a parallel circuit, the current from the battery flows through one lamp and part of the current flows through the other lamp equally.

  • If one bulb is removed, the current can still flow through the second path and the other bulb will remain lit.

  • 3 rules for Parallel:

  • Add the current through the bulbs to get the total current of the circuit.

  • To calculate resistance, use this equation:

  • _1__1_ 1_

    Rtotal = R1 + R2

  • The voltage drop across the light bulbs is equal to the voltage of the battery.

  • A circuit in parallel has two light bulbs. Bulb 1 has a resistance of 90 and bulb 2 has a resistance of 70. What is the total resistance?

  • _1__1_ 1_

    Rtotal = R1 + R2

  • 1 / Rtotal = 1/90 +1/70 (Time out!)

  • 1 / Rtotal = .0111 +.0143

  • 1 / Rtotal = .0254

  • Rtotal = 1/.0254 = 39.37

  • Sometimes, we want resistance in a circuit. A resistor is an electrical device that has a specific resistance we can add to a circuit.

How to Read Resistors

  • First find the tolerance band, it will typically be gold or silver.

  • Starting from the other end, identify the first band - write down the number associated with that color

  • Now 'read' the next color, here it is red Now read 'multiplier' band and write down that number of zeros.

Read Resistors

What is our resistor?

  • Blue = 6

  • Red = 2

  • Multiplier = 2 (Number of zeros is 2)

  • 6200 - Resistor

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