(PP7) ohm’s law

• use a voltmeter to measure the voltage across a metal wire and an ammeter to measure the current passing through the wire
• the temperature of the wire is kept constant using a switch and small currents
• the current and voltage are proportional for a metal wire at constant temperature, and that this is known as ohm’s law
• Independent Variable is the electric current I
• Dependent Variable is the voltage V
• Control Variables are the material, length, cross section area and temperature of the wire.
• Prediction - as the current increases, the voltage will also increase.
• Greater current will mean that more charge flows.

  This means that more energy can be converted from electrical energy to other forms of energy and so voltage increases.
• The temperature of the wire must be kept constant.

  When a current flows through a conductor there is a heating effect.

  Electrical energy is converted to heat energy.

  To ensure the temperature does not increase, switch off between readings and keep the current as small as possible.
• Read the ammeter and voltmeter accurately by reading the scale from directly above the pointer or use digital instruments.
• We can say that the voltage across the wire is directly proportional to the current flowing through it.

  As the voltage increases the current increases in direct proportion.

  This is known as ohm’s law
• Ohm’s Law state that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions, such as temperature, remain constant.
• A conductor that obeys Ohm’s Law is called an ohmic conductor.

  Copper or constantan wire are examples of ohmic conductors.
• voltage V = current I x resistance R

  V= IR

  V = voltage in V

  I = current in A

  R = resistance in Ω