Hand-Cranked Radio

Yesterday during the tsunami warning my parents found our hand-cranked radio that could run without electricity or batteries. But what really makes this radio run? Physics, of course! Recently we have been studying electromagnetism and so now I can explain how something can have power with no power source. Inside this radio (I'm assuming) there is a coil of wire attached to the radio. This wire will act as a generator and the person turning the crank will supply the necessary energy. As the mechanism is cranked energy is supplied and the small wire moves in a magnetic field. Because this wire moves in a magnetic field an emf is induced and thus a current is created. Since power equals current times voltage, the radio has power and can turn on. The power can be stronger due to more loops in the wire, turning the crank faster (greater angular velocity), a greater area within the wire, and more time. Conservation of energy applies here but some energy is lost to heat.

Flashlight Circuit

A flashlight is a good example of a simple

circuit. Inside this flashlight there are two batteries

connected in series along with a light builb and

metal connecting all of the components. The

flashlight's on-off switch completes the circuit when

it is switched on by moving a piece of metal into

place, thus completing the metal circuit relaying the

battery potential difference to the light bulb.

The amount of current running through the circuit

is found with the equation V=IR where V is the

voltage of the battery and R is the resistance across

the light bulb. Because the batteries are connected

in series the power or P=IV of the light bulb is greater than it would be with just one battery. In

order for the light bulb to stay lit, the circuit must remain completed by the on switch so that the

current is able to flow and complete the circuit.

Also, the Potential difference or voltage across the battery must be countered by the resistance in

the lightbulb so the voltage in the wire as it returns to the battery is 0V.

The Oven and Stove Top

Today as my mom and I were baking cookies to eat during the Super Bowl, I realized that our oven is a great representation of Physics concepts. A convection oven heats and cooks by convection which is heating through a fluid medium by movement of fluid. In an oven the heat that comes off of the coils rises and the surrounding heat sinks. This motion repeats and eventually cooks whatever is in the oven.

Also, the top of the oven shows the concepts of resistance and power. The coils that you place pans and pots on while cooking heat up due to electric current. The power that goes into the stove is equal to the current times the voltage. The coils then heat up due to this power. The heat that eventually comes off of the coils to heat up the food is equal to the current squared times the resistance of the coils. As the coils heat up, the resistance increases as shown in the equation P=I^2R. This heat is transfered to the food by conduction and as a side effect of the increase in temperature of the coils the resistance increases.