# Electricity and magnetism ( word problems )

1. Jan 25, 2013

### chipM

1. The problem statement, all variables and given/known data

thankyou !
(1)if you were to expend 10 J of work to push a 1 C charge against an electric field , what would be its change of Voltage ?

voltage = electric potential energy / charge

10/6.25x10^18

=1.6x10^-18 V

(2)(why is it safe to remain inside a car during a lightning storms?)

since like charges repel the electrons on the metal surface repel electrons of the lightining bolt hence its safe .

(3)your elcetronics friend needs a 20 ohm resistor and only has 40 ohm resistors , how can he combine them to produce a 20 ohm resistor ?

by putting them in parallel

(4)when preparing to undergo an MRI scan, why are patients advised to remove eyeglass watches jewlarry and other metal objects ?

(seriosuly i got no clue for this , maybe a hint? or two ? )

2. Relevant equations

3. The attempt at a solution

2. Jan 25, 2013

### Staff: Mentor

on #4 -- How do MRI scans work? They don't use X-rays; what do they use instead?

3. Jan 25, 2013

### chipM

cool i am on it

is 1 and 3 correct though ?

4. Jan 25, 2013

### chipM

for 1 is it

If the vehicle is struck the charge would be distributed across the conductive surface so contact should not be made with the inside of the vehicle until it has been discharged?

5. Jan 25, 2013

### chipM

for 4)

they are told to remove their jewlarry because of the magnetic field which can attract metals and as a result can pose risk to the patient or anyone in the room as the object is forced to move to the centre of the Mr system ?

6. Jan 25, 2013

### Staff: Mentor

Not quite. There are two issues at work helping to protect the occupants. Read about Faraday Cage at wikipedia to see if it helps with one issue. The other has to do with how currents flow. What path does current seek out?

Sounds reasonable.

7. Jan 25, 2013

### Staff: Mentor

#1 looks right, but I didn't check the math. on #3, I'd be explicit on how many resistors he should put in parallel. The wording of the problem makes it sound like he could have more than 2 resistors...

8. Jan 25, 2013

### chipM

Regarding 2)

is it because the car spreads out the charge around the cars exterior , which cancels out the charge in the cars exterior ?

9. Jan 25, 2013

### Staff: Mentor

Nope. Think more in terms of currents, rather than charges...

10. Jan 25, 2013

### Dick

For 1 a volt is a Joule/Coulomb. The units you are giving the potential in are Joule/e where e is the charge of a single electron. Not volts.

11. Jan 25, 2013

### chipM

is it because of the change in magnetic field causes in induced voltage, this voltage opposes the supply voltage to slow the build of current. when current become constant here is no voltage . is this it ?

12. Jan 25, 2013

### Staff: Mentor

Nope. Let's say you are driving your car, and a power pole falls over next to you and the energized power wires lay across your car. Should you get out of your car and run away?

13. Jan 25, 2013

### chipM

no because there is current outside .