Question

1. Wireless charging seems like an incredible new technology - instead of having to plug in your phone or electric toothbrush, you can just put it on top of a base (which is plugged into the wall) and it will charge wirelessly. But in fact the principle behind this technology has been known for over 100 years. Identify which big law that we learned in PHY 212 explains how wireless charging works, and speculate about what you would find if you opened up a wireless charger and looked inside.

2. A generator is running, providing power to a few different appliances at the moment, when someone flips a light switch so that an additional light is now being powered by the generator. The generator's rpms don't change even though it is now powering an additional load. If not the rpms, then what has actually changed about the generator? Where is that extra power coming from?

3. Consider this circuit. In case there is a problem, here is the description: resistor B and a switch are in parallel, and this parallel combination is connected in series with a battery and with resistor A. The switch is now open.

When the switch is closed, please describe what happens to each of the three quantities and why. The choices are increase, decrease, or stay the same, but be sure also to explain why each happens.

a) current through A

b) current through B

c) total power output by the battery

4.  A wire coil that encloses an area of 180 cm² and has 250 turns spins within a uniform 0.33 Tesla magnetic field at a rate of 600 rpm. What is the amplitude of the emf, in volts, induced in this loop?

5. An electron, starting at rest, is accelerated through a potential difference equal to 953 V. What is its final speed, in m/s?

Answer 1

1. Wireless charging works on the principle of electromagnetic induction. If we open a wireless charger we will mind a electromagnet inside it.

2. As we turn on more device, induced emf remains same. But as more resitance is put in parallel(by switching on more device), R decreases(adding resistance in parallel decreases R). Hence, P=V2/R increases. This additional power comes from increase in the current flowing through the coil.

3. The circuit will be:

  

As the switch is closed, resitance B will be inactive as current will find an alternative path

a) Current through Resistance A increases, as the net resistance decreases

b) No current will flow through resistance A

c) Total power output by battery, P=VI, where V is potential differece due to battery, which is constant.

As I increases, power output through the battery increases.

4. Area of the loop,A=180cm2,

No. of turns, N=250

Magnetic field B=0.33T

Frequency, f=600rpm=10rps

Amplitude of emf induced,

5. Potential energy lost = Kinetic energy gained

  

  

where V is potential difference, m is mass of electron and e is charge of electron