Grade 8 Science Practice Test: Electric and Magnetic Forces, Electromagnets
- Electric and Magnetic Forces
- 10 questions
- 13 minutes
Ten questions on forces that act without touching and on how electricity and magnetism are connected. Tip: a current makes a magnetic field, and a moving magnet can make a current.
Tap an option to mark your answer. You can change your mind, or tap the same option again to leave the question blank. The correct answers and explanations appear when you finish the test.Tap an option to answer; tap it again to leave the question blank.
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Question 1
Students build an electromagnet by wrapping wire around an iron nail and connecting it to a battery.
Which change would make the electromagnet stronger?
Correct answer: B
More turns of current-carrying wire make a stronger magnetic field. Removing the iron core or breaking the circuit makes it weaker or turns it off.
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Question 2
Two charged balloons hang near each other. They are slowly moved farther apart.
What happens to the electric force between them?
Correct answer: A
Electric forces, like magnetic and gravitational forces, get weaker as the distance between objects increases. The type of force does not flip just because of distance.
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Question 3
Outside a bar magnet, in which direction do magnetic field lines point?
Correct answer: D
By convention, field lines leave the north pole, loop around and enter the south pole. A compass needle near the magnet lines up along them.
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Question 4
A compass is placed next to a straight wire. When a current flows through the wire, the compass needle turns.
What does this show?
Correct answer: C
The needle moves because the current creates a magnetic field around the wire. When the current stops, the field disappears and the needle swings back.
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Question 5
In a hand-crank flashlight, turning the crank spins a magnet inside a coil of wire.
What does this produce?
Correct answer: B
A moving magnet near a coil of wire produces an electric current. This is how a generator changes kinetic energy into electrical energy.
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Question 6
An electric motor is used in a blender. What energy change does the motor make?
Correct answer: D
A motor uses current and magnets to make something spin, turning electrical energy into motion. A generator does the opposite change.
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Question 7
Why does a crane at a scrapyard use an electromagnet instead of a permanent magnet?
Correct answer: A
Turning off the current turns off the magnet, so the crane can release the scrap. A permanent magnet would hold onto the metal.
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Question 8
Which force can act on an object without touching it?
Correct answer: C
Magnetic, electric and gravitational forces act at a distance through fields. Friction, air resistance and tension all need contact.
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Question 9
A class tests an electromagnet with different numbers of coils.
Number of coils Paper clips picked up 10 5 20 10 30 15 If the pattern continues, how many paper clips will 40 coils pick up?
Correct answer: A
Every 10 extra coils add 5 paper clips, so 40 coils pick up 20. Choosing 40 assumes one clip per coil, which does not fit the data.
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Question 10
A student reverses the battery in an electromagnet circuit so the current flows the other way.
What happens?
Correct answer: C
The direction of the magnetic field depends on the direction of the current. Reversing the current swaps the poles but keeps the strength about the same.
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