Current flows for half a minute through the insulated wire wound round the electromagnet nail, initially at room temperature. The wire does not glow. How does the wire's temperature change?
(a)Unchanged in temperature
(b)Warmer along its length
(c)Cooler along its length
(d)Warmer at its two ends only
Show answer
(b) Warmer along its length
The wire offers resistance to the current. Some electrical energy becomes heat along the wire, making it warmer. Ordinary connecting wires produce heat too, though the temperature rise may be small. A wire need not glow to produce heat.
Question 2
A nichrome wire and a copper wire have the same length and thickness. Each carries the same current for the same length of time. Neither wire glows. How does the heat generated in the two wires compare?
(a)Heat in both wires; more in nichrome
(b)No heat in either wire
(c)Heat in both wires; more in copper
(d)Heat in nichrome only
Show answer
(a) Heat in both wires; more in nichrome
Both wires generate heat even without glowing. For the same length and thickness, nichrome has higher resistance than copper. With the same current flowing for the same time, nichrome generates more heat.
The magnetic effect of electric current
Question 3
A compass deflects while current flows through a copper coil wound on a paper cylinder for a few seconds. The switch is moved to OFF; the wire and compass stay in place. What are the needle’s response and its cause?
(a)Deflected; magnetism stored in the copper wire
(b)Usual direction; disappearance of the wire’s magnetic field
(c)Usual direction; magnetic fields restricted to magnets
(d)Deflected; attraction by nearby copper itself
Show answer
(b) Usual direction; disappearance of the wire’s magnetic field
A current in the copper wire produces a magnetic field. With current flowing, this field deflects the nearby compass. When current stops, this field disappears. Copper does not retain the magnetism. The compass still responds to Earth’s magnetic field.
Question 4
A wire and a compass stay in place. The needle remains deflected while current flows. It returns when the current stops. What causes this difference?
(a)The cardboard under the compass
(b)The wire's metal lying near the needle
(c)Something outside the apparatus
(d)The current in the wire
Show answer
(d) The current in the wire
The compass is a magnet, but no separate magnet is moved near it. The wire and compass stay in place. Only the current changes. Its magnetic field explains the change in the compass direction.
Where the magnetic effect is used
Question 5
A crane’s lifting electromagnet holds a stack of steel plate above the unloading area. How should the operator release the load?
(a)Increase the current
(b)Pull the load away by force
(c)Add more turns to the coil
(d)Switch off the current
Show answer
(d) Switch off the current
The crane operator controls the magnet by switching the current ON and OFF. When the current is turned on, the electromagnet lifts the iron and steel objects. When the current is switched off, the magnetic field disappears and the objects are released.
Question 6
Which effect of current makes a lifting electromagnet attract iron and makes an electric bell ring?
(a)Chemical reactions inside the devices
(b)Magnetic effect
(c)No shared effect
(d)Heating effect
Show answer
(b) Magnetic effect
Electromagnets, electric motors, fans, bells and loudspeakers use the magnetic effect of current. Heating is a different effect of current.
What makes a coil an electromagnet
Question 7
A current-carrying coil with an iron nail inside deflects the needle of a compass at one end of the coil. The nail is removed while the current, coil shape and compass positions stay the same. What happens to the compass deflection?
(a)Greater deflection
(b)Smaller but nonzero deflection
(c)No deflection
(d)Unchanged deflection
Show answer
(b) Smaller but nonzero deflection
A current-carrying coil produces a magnetic field even without an iron core. The iron core strengthens this field. Removing the nail reduces the compass deflection, but the needle still deflects.
What makes an electromagnet stronger
Question 8
A workshop needs to adjust the pull of a magnet while lifting a load heavier than the one lifted yesterday. Why is an electromagnet useful for this job?
(a)Because once it is built its pull stays fixed, even if the current and the number of turns are changed.
(b)Because its pull can be made greater or smaller by changing the current through its coil or the number of turns.
(c)Because its iron core alone settles the pull, so changing the current or the turns makes no difference.
(d)Because reversing the current makes it keep pulling after the current has been switched off.
Show answer
(b) Because its pull can be made greater or smaller by changing the current through its coil or the number of turns.
The strength of an electromagnet can be changed by changing the amount of current through its coil, the number of turns, or both. That lets its pull be adjusted for the job.
The poles of an electromagnet
Question 9
The north pole of a compass needle is attracted towards end A of a current-carrying coil. What is the polarity of end A?
(a)Polarity cannot be determined
(b)north pole
(c)No magnetic pole
(d)south pole
Show answer
(d) south pole
Unlike magnetic poles attract each other. Since the north pole of the compass needle is attracted towards end A, that end is a south pole.
Appliances that use the heating effect
Question 10
Which industrial process uses the same heating effect of current as an electric kettle?
(a)No industrial process
(b)Melting scrap steel with heat produced by electric current in a furnace
(c)Melting scrap steel by burning fuel inside an electric furnace
(d)Lifting scrap steel with an electromagnet
Show answer
(b) Melting scrap steel with heat produced by electric current in a furnace
The heating effect of current is used in industry as well as in homes. Specially designed electric furnaces produce enough heat to melt scrap steel for recycling. Electric current produces this heat.
When the heat is a problem
Question 11
A plug at a wall socket becomes very hot while current flows. What damage can this cause?
(a)Damage to every appliance in the street
(b)Melting of plastic parts in plugs or sockets, and possibly a fire
(c)Melting of metal parts only
(d)Damage to the appliance only
Show answer
(b) Melting of plastic parts in plugs or sockets, and possibly a fire
Overheating can damage plugs and sockets as well as the appliance. Their plastic parts may melt. Overheating may also start a fire.
What the heat produced depends on
Question 12
A nichrome wire is being chosen for the element of a heater. On what does the heat generated in that wire depend?
(a)On the material of the wire, on its thickness and its length, and on the size of the current, but not on how long the current flows.
(b)On the shape into which the wire has been bent, and on nothing else about either the wire or the circuit around it.
(c)On the material of the wire alone, since once a wire is nichrome the heat that it gives out is settled.
(d)On the material of the wire, on its thickness and its length, on the size of the current, and on how long the current flows.
Show answer
(d) On the material of the wire, on its thickness and its length, on the size of the current, and on how long the current flows.
The chapter's list is complete, and none of its terms may be dropped. The heat generated in a wire depends on the material, the thickness and the length of the wire. It also depends on how long the current flows. And the amount of heat depends on the magnitude of the electric current.
How a cell generates electricity
Question 13
A Voltaic cell has gone dead. It is left disconnected for a week, then joined again to the working lamp it used to light. What does the lamp do?
(a)Flashes on and off
(b)Does not glow
(c)Glows more brightly than before
(d)Glows again
Show answer
(b) Does not glow
A dead cell has used up the chemicals that produce its electricity. Leaving it unused does not bring them back, so it supplies no current and the lamp does not glow.
Dry cells and rechargeable batteries
Question 14
In a zinc-carbon dry cell, which terminal does the carbon rod at the centre act as?
(a)Positive and negative terminals in turn
(b)No terminal
(c)The positive terminal
(d)The negative terminal
Show answer
(c) The positive terminal
The zinc container of a zinc-carbon dry cell acts as the negative terminal. The carbon rod at the centre connects to the metal cap, which acts as the positive terminal.
Oersted and how the link was found
Question 15
The needle of a compass lying near a circuit deflected each time Oersted opened or closed that circuit during a demonstration. What did he do between that moment and his findings appearing in print?
(a)He wrote it up straight away, since a needle moving in front of a room full of people needs no further testing.
(b)He worked at it until he was certain that an electric current really does produce a magnetic field.
(c)He set out the plan of the search that had brought him to that demonstration in the first place.
(d)He asked other scientists to try it for him, and printed only the results that they sent back to him.
Show answer
(b) He worked at it until he was certain that an electric current really does produce a magnetic field.
The chapter names three steps, and the middle one is easy to skip. First he noticed the deflection. Then he investigated it. He published his findings only when he was certain that an electric current produced a magnetic field. Other scientists repeated the experiment later, after he had published his findings.
Common misconception in Electricity: Magnetic and Heating Effects
Why it feels right. The coating prevents electrical contact, so it seems to stop other effects too.
Right
Current inside an insulated wire still produces a magnetic field.
Worked example
Insulated turns carry current around the nail. The nail attracts steel clips while the coil is powered.
Wind insulated wire around an iron nail and pass a suitable current briefly in a supervised demonstration.
The nail attracts small steel clips.
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