ELECTRICAL RESISTANCE 11.4

Every time electrons flow in a circuit, there is electrical resistance, or a force that tries to slow them down. When electrons (or electricity) meet resistance in an electrical circuit, the electrical energy is transformed into one of the following forms of energy: heat, light or sound.

Conductors A conductor is an object that lowers the amount of energy lost when electrons flow through a circuit. Conductors lower resistance Copper (Cu) is a very good conductor and is used to make most electrical wires.

Ohm’s Law In 1827, Georg Ohm, a German physicist, discovered a relationship among voltage, current and resistance. Ohm’s Law: Voltage drop between two points in a circuit is directly related (proportional) to the current flowing through a conductor.

Where: V is the voltage drop (V), I is the electrical current (A), and R is the Resistance ()

This equation can also be written in the following formats: I = V and V = I x R R

The larger the resistance in a particular circuit is, the smaller the current is. For example, if voltage remains constant, current will decrease as resistance increases. The unit for resistance is volt per ampere (V/A) or ohm

USING OHM’S LAW

Example 1 What is the voltage drop across the filament in a 60W light bulb if the resistance of the filament is 100  and a current of 0.60A is flowing through it? Given: V = ? I = 0.60A R = 100 Formula: V = I x R

Calculations: V = 0.60A x 100 V = 60V The voltage drop across a 60W light bulb is 60V.

Example 2 What is the current through an alarm clock that has a voltage drop of 80V and a resistance of 60 ? Given: Formula: Calculations: V = 80V I = V I = 80V = 1.3A I = ? R 60 R = 60 The current through the alarm clock is 1.3A.

RESISTANCE AT WORK Most electrical devices have built-in resistors. The resistor controls how much current flows through the load to make sure the device doesn’t become overloaded with electricity (which can cause a fire).

WHAT IS THE PATTERN? It is important to understand how circuits work. If voltage drop stays constant (therefore does not change), current will always increase when resistance decreases. The opposite is also true: if voltage drop stays constant, current will always decrease when resistance increases.

Factors that Affect Resistance of Wires Length – the longer the resistor, the more resistance there is Cross sectional area – the greater the cross sectional area, the lower the resistance Material – same materials have more resistance than other materials Temperature – the greater the temperature, the greater the resistance

Ohm’s Law Worksheet