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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.

- 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.

- Blue = 6
- Red = 2
- Multiplier = 2 (Number of zeros is 2)
- 6200 - Resistor