Electricity and Magnetism
ICSE Class 7 · Physics · Chapter 7
What this chapter covers
- The law of magnetism
- Testing for a magnet: why repulsion is the true test
- Electromagnetism
- Electromagnets and their uses
- The electric bell
- Electric current as a flow of charges
- The electric cell and the battery
- Resistors
- Symbols for circuit components
- Series and parallel circuits
- Conductors and insulators of electricity
Key definitions
Repulsion is the sure test for a magnet, because only two magnets can repel each other.
Electromagnetism is the magnetic effect produced by an electric current flowing through a conductor.
Electric current is the flow of electric charges through a conductor.
Conductors are materials that allow electric current to flow through them easily.
Insulators are materials that do not allow electric current to flow through them.
Key points
- The law of magnetism: like poles repel and unlike poles attract.
- Repulsion is the sure test for a magnet, because attraction also happens between a magnet and plain iron.
- A current in a wire produces a magnetic effect; this is electromagnetism.
- An electromagnet is a coil wound on a soft iron core that is a magnet only while current flows.
- More turns, more current and a soft iron core make an electromagnet stronger.
- Electromagnets are used in electric bells, cranes, motors and loudspeakers.
- An electric bell rings continuously because its armature makes and breaks the circuit many times a second.
- Electric current is the flow of electric charges; current = charge ÷ time, measured in amperes.
- An electric cell changes chemical energy into electrical energy; a dry cell gives about 1.5 V.
- A battery is two or more cells joined in series, and their voltages add up.
- A resistor opposes the flow of current; the heat it produces is used in irons and heaters.
- Circuit diagrams use standard symbols for the cell, battery, key, bulb, wire and resistor.
- In a series circuit there is one path, and one fused bulb puts out all the others.
- In a parallel circuit each component has its own path, and one fused bulb leaves the others glowing.
- Conductors, such as metals and graphite, let current pass easily; insulators, such as rubber and plastic, do not.
Questions answered
State the law of magnetism.
Like poles of magnets repel one another and unlike poles attract one another.
Why is attraction not a reliable test for a magnet?
A magnet attracts a piece of plain iron as well as the opposite pole of another magnet, so attraction can happen even when only one of the two objects is a magnet. Only repulsion proves that both objects are magnets.
What is an electromagnet? Why is its core made of soft iron and not steel?
An electromagnet is a coil of insulated wire wound around a soft iron core that acts as a magnet only while current flows. Soft iron loses its magnetism as soon as the current stops, while steel would stay magnetised.
Give three ways of making an electromagnet stronger.
Increase the number of turns of wire, increase the current (for example, by using more cells) and use a soft iron core.
In an electric bell, what breaks the circuit each time the hammer strikes the gong?
When the armature is pulled towards the electromagnet, its spring moves away from the contact screw. This breaks the circuit, the electromagnet loses its magnetism and the armature springs back.
A charge of 12 C passes through a wire in 4 s. Find the current.
Current = (12 C)/(4 s) = 3 A.
What is a battery? What is the voltage of a battery of four 1.5 V cells joined in series?
A battery is a combination of two or more cells connected in series. Its voltage is 4 × 1.5 V = 6 V.
What does a resistor do in a circuit? Name one appliance that makes use of the heating in a resistor.
A resistor opposes the flow of current and so reduces the current in the circuit. An electric iron, a room heater or an immersion rod uses this heating.