26.8
Electrical Power
Electric power is the product of current and voltage, represented in units of joules per second, or watts. For example, cars often have one or more au…
The product of the current flowing through a conductor and the voltage across it is known as electric power. The SI unit of electric power is the watt.
Consider a potential difference across a conductor. The electrical field in a conductor is directed from higher to lower potentials. Since the change in the potential difference is negative, the electric field can be determined.
If the charge is positive, the electrical field exerts a force on the charge and moves it through the conductor.
The work done on the charge is equal to the product of the electric force and the length at which the force is applied.
The power dissipated by the material in the form of heat and light is equal to the time rate of change of the work.
In a circuit with a resistor, the potential drop across the resistor is dissipated as heat. By recalling Ohm's law, the power dissipated by the resistor can be calculated.
A relationship can be established between power, current, voltage, and resistance for all ohmic devices.
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Q1: What is electrical power and how is it calculated?
Electrical power is the product of current flowing through a conductor and the voltage across it, measured in watts (joules per second). The formula is P = IV, where I is current and V is voltage. For example, a 20-amp, 12-volt power outlet delivers 240 watts maximum. This relationship applies to all ohmic devices in a circuit.
Q2: How does a resistor dissipate electrical power?
When current flows through a resistor, the potential drop across it is dissipated as heat and light. The kinetic energy lost by electrons in collisions is converted into internal energy of the conductor and radiation. Using Ohm's law, power dissipated can be calculated as P = I²R or P = V²/R, establishing relationships between power, current, voltage, and resistance.
Q3: Why does a 60-watt bulb glow brighter than a 25-watt bulb?
A 60-watt bulb dissipates more electrical energy per unit time than a 25-watt bulb. The higher power rating means more electrical energy is converted into heat and light. Consequently, the 60-watt bulb produces greater brightness and warmth due to this increased energy conversion rate.
Q4: What role does a fuse play in protecting a circuit?
A fuse is a short piece of wire between two contacts designed to protect circuits from sudden power surges. When current exceeds the rated value, kinetic energy of charge carriers converts to thermal energy, heating the wire until it melts and breaks. This stops current flow, protecting the rest of the circuit. Once destroyed, the fuse must be replaced.
Q5: How does electrical energy convert to heat in a conductor?
When electrical current flows through a conductor, charge carriers lose kinetic energy through collisions. This lost kinetic energy is converted into thermal energy and radiation within the conductor material. The rate of this energy conversion is the electrical power dissipated, which depends on the current, voltage, and resistance of the conductor.
Q6: What is the difference between a fuse and a circuit breaker?
Both fuses and circuit breakers are rated for maximum current and protect circuits from power surges. However, fuses are one-time devices that must be replaced after melting, while circuit breakers can be reset and reused. Circuit breakers remain open to protect the circuit but offer the advantage of reusability without replacement.
Q7: How does the electric field move charge through a conductor?
The electric field in a conductor is directed from higher to lower potential. When a potential difference exists across a conductor, the electric field exerts a force on positive charges, moving them through the material. The work done on the charge equals the product of electric force and the distance traveled, which determines the power dissipated.