Problem solving tips

## Moving Charges And Magnetism

- Want to score better marks in exams? Have a look at some problem solving tips.
1
Tip 1
According to Biot Savart's Law, the field due to a current element carrying current I at a distance is given by
How do we approach further to find the field due to an arbitrarily shaped wire using Biot Savart,s Law?
Follow the given problem solving strategy.
A current flows in a infinitely long wire with cross section in the form of a semicircular ring of radius R. The magnitude of the magnetic induction at its axis is
A
B
C
D
2
Tip 2
Still figuring out how to apply Ampere's Law on a certain problem. Let's make it easy for you.
Always remember, the integral is easy to evaluate when the magnetic field is symmetric. So, always look for symmetries in the magnetic field around a source.
The path along which the integral is evaluated is called the Amperian Loop. But how do we select a most suitable Amperian Loop for a given problem?
Now, lets demystify the correct approach on how to Apply Ampere's Circuital law on a given current distribution.
A long straight wire of radius carries a steady current i. The current is uniformly distributed across its cross section. The ratio of the magnetic field at and is:
A
B
C
D
A long straight wire of radius R carries a steady current , uniformly distributed throughout the cross-section of the wire. The magnetic field at a radial distance r from the centre of the wire, in the region , is?
A
B
C
D
E
3

## Tip 3: Galvanometer to Voltmeter

Voltmeters are devices which are used to measure voltage across a section of a circuit.Always remember,
• Voltmeters are high resistance devices. For an ideal voltmeter the resistance is INFINITE
• Voltmeters are connected in parallel to the circuit and ideally, should not draw any current from the circuit
Now, how will you convert a galvanometer into such a device. Follow the given step-by-step strategy.
A galvanometer has a coil of resistance Omhs and gives a full-scale deflection for mA current. If it is to work as a voltmeter of range, the resistance required to be added will be
A
B
C
D
An galvanometer deflects full-scale for a potential of 20 mV. A voltmeter deflecting full-scale of 5 V is to be made using this galvanometer. We must connect
A
a resistance of parallel to the galvanometer
B
a resistance of in series with the galvometer
C
a resistance of k parallel to the galvanometer
D
a resistance of k in series with the galvanometer
4

## Tip 4: Galvanometer to Ammeter

Ammeters are devices used for measuring the current flowing in a circuit. Ammeters are
• Low Resistance devices. Resistance of an Ideal ammeter is ZERO
• Always connected in series in circuit. The ammeters must experience the same current as is flowing in the current without altering the potential drop across the circuit
Now, how will you convert a galvanometer into such a device. Follow the given step-by-step strategy.
A galvanometer coil has a resistance of gives full scale deflection for a current of . To convert it to an ammeter of range to
A
resistance is to be connected in parallel to the galvanometer
B
resistance is to be connected in series with the galvanometer
C
resistance is to be connected in parallel to the galvanometer
D
resistance is to be connected in series with the galvanometer
A galvanometer of 100 resistance gives full scale deflection with 0.01A current. How much resistance should be connected to convert it into an ammeter of range 10A?
A
in parallel
B
in series
C
in parallel
D
in series