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A slab of copper of thickness b is thrust into a

parallel-plate capacitor of plate area A

and plate separation d; the

slab is exactly halfway between the plates. (a) What is the

capacitance after the slab is introduced? (b) If a potential

difference V is maintained between the plates, what is

the ratio of the stored energy before to that after the slab is

inserted? (c) How much work is done on the slab as it is

inserted? (d) Is the slab sucked in or must it be pushed in?

Copper is a kind of metal. What is its dielectric constant?

A straight wire carrying a current of 42 A lies along the axis of a 6.6 cm-diameter solenoid. The solenoid is 70 cm long and has 250 turns carrying a current of 6.0 A.

Estimate the magnitude of the magnetic field at the point P, 4.2 cm from the wire.

This is what I did:

The magnetic field at P due to the straight current is B = mu_0*current (42 A)/(2*pi*0.042 m) = 0.20 mT

Note 4.2 cm > radius 3.3 cm, P is outside the solenoid, so the magnetic field due to the solenoid is zero.

Therefore the answer is 0.20 mT. But the website says it is incorrect.

Is there anything wrong in the above answer?

The flash unit in a camera uses a special circuit to “step up” the 3.1 V from the batteries to 300 V, which charges a capacitor The capacitor is then discharged through a flashlamp. The discharge takes 12*10^-6 s, and the average power dissipated in the flashlamp is 12 W. What is the capacitance of the capacitor?

If a 2.0 nC charge was placed 1.0 cm to the right of the 4.0 nC charge, what would its acceleration be if its mass is 1 gram?

a 3rd charge of 4.0 nC is at x = 1.0 cm. What is the electric force on the 1.0 nC charge?