Sec 4 Physics Electromagnetism topical questions

Describe the magnetic field pattern around a long straight wire carrying a current, and state the rule for its direction.

State three ways to increase the strength of an electromagnet.

State Fleming’s left-hand rule, naming what each digit represents.

A wire lies parallel to a uniform magnetic field. State the size of the force on it, with a reason.

State the purpose of the carbon brushes in a d.c. motor.

A wire carrying 4.0 A lies at right angles to a uniform field of flux density 0.50 T. The field covers a 3.0 cm length of the wire.

1. (a)Calculate the force on the wire.[2]
2. (b)The current is increased to 6.0 A. Calculate the new force.[2]
3. (c)State the direction of the force relative to the field and the current.[2]

A student sets up an electromagnet to pick up and drop iron pins on demand, using a coil, a soft-iron rod, a battery and a variable resistor.

1. (a)Explain why the core is soft iron rather than steel.[2]
2. (b)State what the variable resistor changes and what effect it has.[2]
3. (c)State what happens the instant the switch is opened, and why.[1]

In a simple d.c. motor, the coil carries current in a horizontal magnetic field. One side of the coil experiences an upward force.

1. (a)Explain why the coil rotates rather than translating as a whole.[3]
2. (b)Explain what the commutator does when the coil passes the vertical position.[2]
3. (c)The battery is reversed. State and explain the effect on the rotation.[1]

Two long parallel wires hang freely side by side. Their currents are switched on so that both flow in the same direction.

1. (a)State what is observed and explain it using magnetic fields.[3]
2. (b)State what is observed if one current is reversed, and name the rule used for the directions.[2]

A d.c. motor drives a small fan. Each side of the coil is 4.0 cm long and lies in a field of flux density 0.25 T. The current in the coil is 0.60 A.

1. (a)Calculate the force on one side of the coil. (Use the wire length of one side only.)[3]
2. (b)Explain why the total turning effect involves both sides of the coil.[2]
3. (c)The fan is fitted with a stronger magnet of flux density 0.50 T at the same current. Calculate the new force on one side and state the effect on the speed.[2]

A burglar alarm uses a relay: a 6 V control circuit with a switch by the door energises a coil whose contacts complete a siren circuit powered by its own 12 V battery.

1. (a)Explain how opening the door (and its switch) leads to the siren sounding or silencing, depending on the design.[3]
2. (b)Explain why the 12 V siren circuit cannot feed back into the 6 V control circuit.[2]
3. (c)The designer wants the relay to respond to a smaller control current. Suggest one change to the coil and one to the core.[2]

A student builds a d.c. motor but connects one brush to the wrong half of the split-ring commutator, so the current is reversed every quarter turn instead of every half turn.

1. (a)Explain why the correct timing (a reversal every half turn) keeps the coil spinning.[3]
2. (b)Predict and explain the motion with reversals every quarter turn.[3]
3. (c)State one other modification, apart from the commutator, that would keep the coil turning in one direction.[1]

A laboratory balance carries a rigid horizontal wire of length 5.0 cm between the poles of a magnet. The wire is connected to a variable supply. When the current is 3.0 A the balance reading increases by a force equivalent to 0.090 N.

1. (a)Calculate the flux density between the poles, assuming the wire is perpendicular to the field.[3]
2. (b)The current is now reduced to 1.0 A. Calculate the new force and the new balance reading change.[3]
3. (c)State and explain what happens to the reading if the magnet is turned so its field is vertical, with the wire remaining horizontal and therefore still perpendicular to the field.[2]