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Electrical Resistance and Ohm’s Law. The Electric Current. Electric current is a measure of the rate at which electric charges move past a given point in a circuit. The SI Unit used to measure electric current is the ampere. The symbol for ampere is A. It is measured using an instrument
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The Electric Current • Electric current is a measure of the rate at which electric charges move past a given point in a circuit. • The SI Unit used to measure electric current is the ampere. • The symbol for ampere is A. • It is measured using an instrument called an ammeter that would be connected to the circuit in the series. Analogy : Water wheel run by water - if only one scoop of water each second – wheel would turn relatively slowly – not much work could be done -if have 20 scoops of water each second – wheel will turn more quickly – more work could be done
Electrical Potential • As the electric charges are released into the circuit from the power source they carry a certain amount of energy. • The amount of energy each one has will determine how much work they can do for us. The amount if energy it has is the electrical potential. • The SI Unit used to measure electrical potential is called the volt. • The symbol for the volt is V. • It is measured using an instrument called a voltmeter that would be connected to the circuit in parallel. Analogy -if each scoop of water is raised only 6 cm and then released it will hit the water wheel with very little force – moves the wheel very little and results in very little work being done. -if each scoop of water is raised 60 cm and then released it will hit the water wheel with much more force – moves the wheel much more resulting in much more work being done.
Electrical Resistance • When electric charges move through various parts of a circuit they can lose some of their energy (voltage). • This is due to friction between the charges and the molecules that make up the conductors in the circuit and loads. • The ability of a conductor to slow down or impede the flow of electricity is referred to as resistance. • The loss of electric potential difference due to resistance is called the potential difference. • The SI Unit used to measure resistance is called the ohm (Ω). • The symbol for resistance is R. Analogy -if one person is running on a flat empty road – relatively easy then they will not be losing energy as quickly. -if the same person is running on the same road which is crowded with lots of people – makes it more difficult as the person has to fight to get through crowds so they will use up more energy to push through the crowds and therefore will get tired faster.
Ohm’s Law • The potential difference between two points on a conductor is proportional (directly related) to the electric current flowing through the conductor. • This very simple law is used to calculate the resistance of the load when designing many different electrical devices. • Although potential difference is the correct term, the term voltage drop is commonly used instead.
Ohm’s Law • Potential Difference = Electric Current x Electrical Resistance V = I x R Where potential difference (voltage drop) (V) is measured in volts (V), electric current (I) is measured in amperes (A), and resistance (R) is measured in ohms (Ω).
Examples • What is the voltage drop across the tungsten filament in a 100-W light bulb? The resistance of the filament is 144 Ω and a current of 0.883 A is flowing through it. • LIST THE VARIABLES • I = 0.883 A • R = 144 Ω • V = ? EQUATION V = I x R SUBSTITUTE V = (0.883 A) (144 Ω) = 120 V ANSWER The voltage drop across a 100 W light bulb is 120 V.
You TRY!!! • 1. An electric toaster is connected to a 120-V outlet in the kitchen. If the heating element in the toaster has a resistance of 14 Ω, calculate the current flowing through it. • 2. The current required to operate an electric can opener is 1.5 A. What is its resistance of the supply voltage is 120 V?
