Answer to Question #101802 in Electric Circuits for Laura

Question

A.
Your refrigerator is located a bit too far away from the nearest electrical power point, so you use your 10m extension lead to connect the appliance. You leave the extension lead wound up in a coil. After 4 hours you notice that the extension lead if very warm but the appliance wire is not. Explain.

B.
A wire carrying 2 A DC will experience a pull to one side when the wire is carrying current in a strong external magnetic field. However, if that same wire were carrying 2 A AC instead, the wire does not experience any sideways pull. Explain.

C.
A standard household circuit that is rated for 10 A, how many 2000 W devices could be plugged in and operating at the same time without overloading the circuit?

Answer 1

a) When transmitting electrical energy, heat is released in the wires. During long-term, stationary operation of the electric circuit, a thermal balance is established between the release of heat as a result of electric heating and the removal of heat to the environment (air). In the coil, the heat removal condition is bad (the outer surface area of a coil is much smaller than the surface area of the deployed wire). Therefore, the coiled wires are always cooled worse and their temperature in stationary mode is higher than the same wire deployed in any way.

b) The force of interaction of a direct current with a magnetic field is constant (does not change in time) so the wire bends in the direction of the action of this force and we notice this bend. In the case of alternating current, the force of the wire's interaction with the magnetic field is variable and we may not notice the bending and oscillation of the wires.

с) The maximum power in the AC circuit can be written in the form $W=U\cdot I$ in which $U$ and $I$ - the Root Mean Square (RMS) values. For the case of a normal circuit of 220 V and a current limit of 10 A, the maximum power of the circuit is 2200 W. Therefore, you can turn on no more than one device with the 2000 W power. No devices can be connected to a circuit with a voltage of 127 V.