Understanding Systems ire doors are doors that, when closed, can slow the spread of fire from room to room. In some buildings, fire doors are held open by electromagnets. Explain why electromagnets rather than permanent magnets are used.

The magnetic field inside a long straight solenoid carrying current
A)is zero
B)increases along its radius
C)increases as we move towards its ends
D)is the same at all points

Which of the following units is used to express frequency?
Hertz
Watt
Newton
Pascal

What does tangential force produce?

How Many Different $f$ Orbitals Are There?

What is $\frac{e}{m}$ value for electrons ?

Which of the following correctly describes the magnetic field near a long straight wire?
A. The field consists of straight lines perpendicular to the wire
B. The field consists of straight lines parallel to the wire
C. The field consists of radial lines originating from the wire
D. The field consists of concentric circles centered on the wire

Is energy directly proportional to frequency?

What is a compass? How is a compass used to find directions?

Magnetic field lines never intersect each other because
There will be two directions of the field at the same point.
Feild lines repel each other
Field lines follow discrete paths only
If field lines intersect they create a new magnetic field within the existing field

Two parallel wires carry currents of 20 A and 40 A in opposite directions. Another wire carrying a current antiparallel to 20 A is placed midway between the two wires. The magnetic force on it will be
Towards 20 A
Towards 40 A
Zero
Perpendicular to the plane of the current

What is the wavelength of the light emitted when an electron in a hydrogen atom undergoes transition from the energy level n = 4 to energy level n = 2? What is the colour corresponding to their wavelengths? (Given $R_H=109677c{m}^{-<!--\; -\; -->1}$)
A) 486 nm, Blue
B) 576 nm, Blue
C) 650, Blue
D) 450 nm, Blue

Two identical conducting spheres A and B are separated by a distance greater than their diameters the spheres carry equal charges and electrostatic force between them is F a third identical uncharged sphere C is first brought in contact with A, then with B and finally removed as a result, the electrostatic force between A and B becomes

Why is alternating current used in homes?

Which of the following radiation has the shortest wavelength.
X-ray
Infra red
microwave
ultraviolet

How to find the local extrema for ${\displaystyle f\left(x\right)=5x-x^2}$?