The Magnetic, Heating and Chemical Effects of a Current – Electric Current and Ohm's Law (Senior 1 Physics) | Ganzaa

Electric Current and Ohm's Law

Lessons

Senior 1 · Physics

Electric Current and Ohm's Law

5 lessons

Lesson 5 of 5

The Magnetic, Heating and Chemical Effects of a Current

25 min

What can a current do?

We cannot see a current in a wire; we know it is there only from what it does. It heats the element of a kettle and the filament of a lamp. It turns the motor of a hair clipper in a barber's kiosk and rings an electric bell at the school gate. It charges the battery of a motorbike and coats a cheap bracelet with a thin shine of silver. These are the three effects of an electric current: magnetic, heating and chemical.

The magnetic effect: Oersted's discovery

In 1820 the Danish physicist Hans Christian Ørsted noticed that a compass needle near a wire moved each time he switched a current on. Repeat his experiment: lay a wire north–south directly over a compass, then close the switch for a moment.

Figure 5.1: (a), (b) Oersted's experiment seen from above. (c) The magnetic field round a straight wire: circles centred on the wire.

With no current, the needle points north, along the wire.
With a current, the needle swings round towards east–west, and it swings back when the current stops.
Reverse the current and the needle swings the OTHER way. A bigger current gives a bigger swing.

A current produces a magnetic field around it. The field lines round a straight wire are circles centred on the wire. Right-hand grip rule: grip the wire with your right hand, thumb pointing along the conventional current; your curled fingers show the direction of the field lines.

The electromagnet

Figure 5.2: An electromagnet. The iron core becomes a strong magnet only while current flows in the coil.

Winding the wire into a coil adds the fields of all the turns together, and an iron core inside the coil makes the field hundreds of times stronger. When the switch is opened, soft iron loses its magnetism at once and the clips fall. That is the great advantage of an electromagnet: it can be switched on and off.

To make an electromagnet stronger	Why it works
increase the current	each turn makes a stronger field
add more turns of wire	more turns add their fields together
use a soft-iron core	the iron is magnetised and adds its own strong field

Definition

An electromagnet is a coil of insulated wire, usually wound on a soft-iron core, that becomes a magnet only while a current flows through it. Uses: electric bells, relays and door locks, the lifting magnets on scrap-metal cranes, loudspeakers and electric motors.

The heating effect

As electrons drift through a conductor they keep colliding with its atoms, giving them energy, so the conductor warms up. The bigger the resistance and the current, the more heat. For a resistor of resistance $R$ carrying a current $I$ for a time $t$ , the heat produced (Joule's law) is

H = I^{2} R t

with $H$ in joules, $I$ in amperes, $R$ in ohms and $t$ in seconds. Because $I$ is squared, doubling the current gives FOUR times the heat.

Definition

The heating effect of a current is the production of heat when a current flows through a resistance: $H = I^{2} R t$ . It is used in kettles, irons, cookers, water heaters, hair dryers, filament lamps and fuses.

Worked example 1: heat from a resistor

A current of $2.0\,\mathrm{A}$ flows through a $30\,\Omega$ resistor for 5.0 minutes. How much heat is produced?

H = I^2 R t = (2.0)^2 \times 30 \times 300 = 4.0 \times 30 \times 300 = 36\,000\,\mathrm{J}

That is $36\,\mathrm{kJ}$ .

Worked example 2: a kettle on the mains

A kettle element has a resistance of $26\,\Omega$ and is connected to the $230\,\mathrm{V}$ mains. Find the current, and the heat it produces in 2.0 minutes.

I = \frac{V}{R} = \frac{230}{26} = 8.8\,\mathrm{A}

H = I^2 R t = (8.85)^2 \times 26 \times 120 = 2.4 \times 10^{5}\,\mathrm{J}

(Keep the unrounded current, $8.846\,\mathrm{A}$ , in the second step.)

A cartridge fuse. Look for the thin wire inside the glass tube: if the current becomes too large, the heating effect melts this wire and breaks the circuit. Photo: André Karwath (Aka), CC BY-SA 2.5, via Wikimedia Commons

The chemical effect

Figure 5.3: Copper plating a key. Copper dissolves from the anode and is deposited on the key, the cathode.

Pure water hardly conducts, but a solution of a salt, an acid or an alkali does, because it contains ions that can move. When a current passes through such a liquid, chemical changes happen at the two plates where the current enters and leaves. In Figure 5.3, positive copper ions $\mathrm{Cu^{2+}}$ in the copper(II) sulfate solution move to the negative key and are deposited on it as a thin, even layer of copper, while copper from the positive plate dissolves to replace them.

Definition

Electrolysis is the chemical change produced when a current passes through a liquid that conducts (an electrolyte). The current enters and leaves through two conducting plates, the electrodes: the anode is joined to the + terminal and the cathode to the − terminal.

Electroplating uses electrolysis to coat an object (made the cathode) with a thin layer of another metal, for decoration or to stop rusting.

Worked example 3: planning to silver-plate a spoon

A jeweller wants to coat a brass spoon with silver. Which object goes where, and what liquid is used?

The spoon is to receive the metal, so it is the cathode, joined to the − terminal.
The anode is a bar of silver, joined to the + terminal; it dissolves and keeps the solution topped up.
The electrolyte must contain silver ions, for example a solution of a silver salt.
A small, steady current for a long time gives a smooth, even coat.

Effect	What happens	Where it is used
magnetic	a magnetic field forms around the current	electromagnets, bells, relays, motors, loudspeakers
heating	the conductor gets hot: $H = I^{2} R t$	kettles, irons, cookers, filament lamps, fuses
chemical	an electrolyte is decomposed at the electrodes	electroplating, refining copper, charging batteries

Warning

Common mistakes. (1) Making the object to be plated the ANODE: it then dissolves instead of being coated. It must be the cathode (−). (2) Thinking that heat is proportional to the current: it is proportional to $I^{2}$ . (3) Forgetting that a wire has a magnetic field only while a current flows in it.

Tip

Three effects, three words: a magnet (Oersted's compass), a heater ( $H = I^{2} R t$ ), a platter (the object to be plated goes on the negative side: "cathode coated").

The effects at work in Rwanda

The heating effect boils water in homes and hotels, and irons clothes in every tailor's workshop. Electric motors, which depend on the magnetic effect, run maize mills, water pumps and the machines in Kigali's factories. The chemical effect recharges the batteries of phones, solar lamps and electric motorbikes, which are becoming common on Kigali's roads.

What Oersted discovered with his compass (National MagLab). Watch the compass needle jump when the current is switched on, and how the needle's direction reveals the circular field round the wire.

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Practice

Name the three effects of an electric current and give one device that uses each.
Describe Oersted's experiment, and state what it shows.
Give three ways of making an electromagnet stronger.
Why is the core of an electromagnet made of soft iron, not steel?
Find the heat produced by a current of $3.0\,\mathrm{A}$ in a $20\,\Omega$ resistor in $10\,\mathrm{s}$ .
A heater takes $5.0\,\mathrm{A}$ and has a resistance of $46\,\Omega$ . How much heat does it give out in one minute?
A student tries to copper-plate an iron nail, but the nail gets thinner and nothing is deposited on it. Suggest what she did wrong.
The current through a heater is doubled while its resistance stays the same. Kalisa says the heat produced each second doubles. Evaluate his claim.

Show answersHide answers

Magnetic (electric bell or electromagnet); heating (kettle, iron, fuse); chemical (electroplating, charging a battery).
A wire is laid north–south over a compass. When a current flows the needle turns towards east–west; reversing the current reverses the swing. A current produces a magnetic field.
Increase the current; add more turns; use a soft-iron core.
Soft iron is magnetised strongly but loses its magnetism as soon as the current stops, so the electromagnet can be switched off; steel would stay magnetised.
$H = (3.0)^2 \times 20 \times 10 = 1800\,\mathrm{J}$
$H = (5.0)^2 \times 46 \times 60 = 69\,000\,\mathrm{J}$
She connected the nail to the + terminal, making it the anode, so it dissolved. The nail must be the cathode (−) and a copper plate the anode (+).
Wrong. $H = I^{2} R t$ : with $R$ and $t$ unchanged, doubling $I$ multiplies the heat by $2^{2} = 4$ .

Key points

Magnetic effect: a current produces a magnetic field (Oersted, 1820); coils on soft iron make switchable electromagnets.
Heating effect: $H = I^{2} R t$ ; used in heaters, lamps and fuses.
Chemical effect: a current through an electrolyte causes electrolysis; in electroplating the object is the cathode (−).

1What can a current do?

2The magnetic effect: Oersted's discovery

3The electromagnet

4The heating effect

5The chemical effect

6The effects at work in Rwanda

What can a current do?

The magnetic effect: Oersted's discovery

The electromagnet

The heating effect

The chemical effect

The effects at work in Rwanda