# Physics hw =p need formula help

1. Jan 31, 2004

### tzvu

Physics hw =p need formula help plz

4. [Serway.20.2.10] A coil of radius 20 cm is placed in an external magnetic field of strength 0.20 T so that the plane of the coil is perpendicular to the field. The coil is pulled out of the field in 0.30 s. Find the average induced emf during this interval.
V

5. [Serway.20.2.18] The plane of a rectangular coil, 4.0 cm by 9.0 cm, is perpendicular to the direction of a magnetic field, B. If the coil has 75 turns and a total resistance of 8.0 , at what rate must the magnitude of B change to induce a current of 0.10 A in the windings of the coil?
T/s

6. [Serway.20.2.20] A 500-turn solenoid with a length of 20 cm and a radius of 1.5 cm carries a current of 3.0 A. A second coil of four turns is wrapped tightly about this solenoid so that it can be considered to have the same radius as the solenoid. Find
(a) the change in the magnetic flux through the coil and when the current in the solenoid increases to 7.0 A in a period of 0.9 s
T · m2
(b) the magnitude of the average induced emf in the coil when the current in the solenoid increases to 7.0 A in a period of 0.9 s.
V

7. [Serway.20.3.25] Over a region where the vertical component of the Earth's magnetic field is 40.0 µT directed downward, a 10.0-m length of wire is held in an east-west direction and moved horizontally to the north with a speed of 40.0 m/s. Calculate the potential difference between the ends of the wire, and determine which end is positive.
V

8. [SF5 20.P.10.] The flexible loop in Figure P20.10 has a radius of 12 cm and is in a magnetic field of strength 0.12 T. The loop is grasped at points A and B and stretched until it closes. If it takes 0.18 s to close the loop, find the magnitude of the average induced emf in it during this time.
mV

Figure P20.10.

9. [SF5 20.P.17.] Consider the arrangement shown in Figure P20.17. Assume that R = 6.0 and = 1.0 m, and that a uniform 2.5 T magnetic field is directed into the page. At what speed should the bar be moved to produce a current of 0.50 A in the resistor?
m/s

Figure P20.17.

10. [SF5 20.P.18.] Over a region where the vertical component of the Earth's magnetic field is 40.0 µT directed downward, a 100.00 m length of wire is held in an east-west direction and moved horizontally to the north with a speed of 50.0 m/s. Calculate the potential difference between the ends of the wire, and determine which end is positive.
mV (potential difference)

11. [SF5 20.P.21.] A bar magnet is held above the center of a wire loop in a horizontal plane, as shown in Figure P20.21. The south end of the magnet is toward the loop. The magnet is dropped.

12. [SF5 21.P.35.] A step-up transformer is designed to have an output voltage of 2200 V (rms) when the primary is connected across a 110 V (rms) source.
(a) If there are 72 turns on the primary winding, how many turns are required on the secondary?

(b) If a load resistor across the secondary draws a current of 1.5 A, what is the current in the primary, assuming ideal conditions?
A

13. [SF5 21.P.37.] A transformer on a pole near a factory steps the voltage down from 3600 V to 120 V. The transformer is to deliver 1700 kW to the factory at 88% efficiency.
(a) Find the power delivered to the primary.
kW
(b) Find the current in the primary.
A
(c) Find the current in the secondary.
A

14. [SF5 21.P.48.] An AC adapter for a telephone answering unit uses a transformer to reduce the line voltage of 120 V to a voltage of 8.0 V. The rms current delivered to the answering system is 380 milliamps.
(a) If the primary (input) coil in the transformer in the adapter has 250 turns, how many turns are there on the secondary (output) coil?
turns
(b) What is the rms power input to the transformer? Assume an ideal transformer.
V

15. A force F is applied to a conducting rod so that the rod slides with constant speed v=4.64 m/s over a frictionless pair of parallel conducting rails that are separated by a distance l= 0.27 m. The rod and railsahve negligible resistance, but the rails are connected by a resistance R= 16.2 ohms. There is a uniform magnetic field B= 0.124 T perpendicular to and directed out of the plane of the paper.

What is the direction of the induced current in the resistor R.

a). Going up through the resistor

b). Going down through the resistor

15 c. Determine the electric field in the rod. Answer in units of V/m.

15 d. Determine the magnitude of the induced current in the resistor R. Answer in units of A.

15 e. Determine the power dissipated in the resistor as the rod moves in the magnetic field. Answer in units of W.

16 f. Dtermine the magnitude of the external force F applied to the rod to keep it moving with a constant speed v= 3.12 m/s

I know this is alot, but all I need are just the formulas.. I cant find them in my stupid book or anywhere online!!!

Any help would be appreciated!

2. Jan 31, 2004

### Hurkyl

Staff Emeritus
I don't know what formulas you need; all I see are a bunch of totally unworked homework problems.

3. Jan 31, 2004

### tzvu

<just read the sticky post> Is this for more theoretical stuff or what? Sorry, I'm just having problems figuring these out =P sorry for the bother.

4. Feb 1, 2004

### HallsofIvy

Staff Emeritus
You have listed 16 problems all of which can be solved by a little thought and applying a formula from your textbook. If you honestly have no idea how even to start then you need more help that we can give you here. Go immediately to your teacher and throw yourself on his/her mercy!

5. Feb 1, 2004

### Hurkyl

Staff Emeritus
Or to put it more nicely!

We're here to help you solve problems, not to solve problems for you. In nearly every circumstance, the most effective form of help is for you to present your thoughts on a problem and where you get stuck, and we can help you through it so the next time you won't get stuck.

You mentioned you just need formula; let me demonstrate "good" help vs "bad" help with a sample problem:

You throw a ball straight up, and it takes 5 seconds before it hits the ground. At what velocity did you throw it?

You: "What formula do I use?"
Us: "Use $\Delta x = v_0 \Delta t + (1/2) a \Delta t^2$"

Good help:

You: "I know that the net displacement of the ball is 0, and the acceleration is $-9.8 m/s^2$, and the problem gives me the change in time... I know there's a formula that lets me solve for initial velocity given these, but I can't find it!"
Us: "Use $\Delta x = v_0 \Delta t + (1/2) a \Delta t^2$"

6. Feb 1, 2004

### himanshu121

This should be added to Sticky