Chapter 4.3 Notes Resistance in Electricity

Charges can easily flow through conductors because they contain many free electrons. Charges can easily flow through conductors because they contain many free electrons. conductors Even conductors have resistance though. Even conductors have resistance though. Some conductors can be cooled to very low temperatures to reduce the friction. These are called superconductors. 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. 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. Examples of insulators include wood, plastic, glass, and rubber.

Some substances are intermediate in their ability to conduct charge and are called semiconductors. Some substances are intermediate in their ability to conduct charge and are called semiconductors. Insulators are used for computer chips and microprocessors. Insulators are used for computer chips and microprocessors.

In wire, free electrons move throughout the wire, but not in straight lines. In wire, free electrons move throughout the wire, but not in straight lines. The electrons continuously bump into other electrons and atoms. The electrons continuously bump into other electrons and atoms. Each collision causes the electron to change direction. 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. 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. 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. 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. 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 Resistance = voltage / current R = V / I R = V / I Unit for resistance is the ohm. Unit for resistance is the ohm. Symbol for resistance is Ω 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? 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 = V / I R = 110 V / 2 A R = 110 V / 2 A R = 55 Ω R = 55 Ω

Physicist George Simon Ohm discovered that the ratio of voltage to current is constant for most conductors. Physicist George Simon Ohm discovered that the ratio of voltage to current is constant for most conductors. Ohm’s Law says if its resistance is constant, than change in Voltage will equal current times resistance. Ohm’s Law says if its resistance is constant, than change in Voltage will equal current times resistance. Ohm’s Law Ohm’s Law

The amount of resistance in a wire depends on 3 things: The amount of resistance in a wire depends on 3 things: a. Length of the wire – longer increases resistance a. Length of the wire – longer increases resistance b. The radius of the wire – the smaller the wire, the less resistance b. The radius of the wire – the smaller the wire, the less resistance c. The material of which the wire is made c. The material of which the wire is made

Resistivity is a measure of the capacity of a material to resist electric charge flow. Resistivity is a measure of the capacity of a material to resist electric charge flow. Resistivity In a series circuit their is only 1 path for electricity to flow. In a series circuit their is only 1 path for electricity to flow.

3 important rules for Series circuits: 3 important rules for Series circuits: Since there is only path for charges to flow, the current is the same everywhere. 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 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. 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, 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? 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: = 280 Total resistance in series equals sum of each resistance: = 280 Total resistance in series Total resistance in series

In a parallel circuit, the current from the battery flows through one lamp and part of the current flows through the other lamp equally. 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. 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: 3 rules for Parallel: 3 rules for Parallel 3 rules for Parallel Add the current through the bulbs to get the total current of the circuit. Add the current through the bulbs to get the total current of the circuit. To calculate resistance, use this equation: To calculate resistance, use this equation: _1_ _1_ 1_ _1_ _1_ 1_ R total = R 1 + R 2 R total = R 1 + R 2 The voltage drop across the light bulbs is equal to the voltage of the battery. 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? 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_ _1_ _1_ 1_ R total = R 1 + R 2 R total = R 1 + R 2 1 / R total = 1/90 +1/70 (Time out!) 1 / R total = 1/90 +1/70 (Time out!)(Time out!)(Time out!) 1 / R total = / R total = / R total = / R total =.0254 R total = 1/.0254 = R total = 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. 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.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 colorStarting 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.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) Resistor