Magnetic effect of electric current
A security guard checks whether a hidden wire in the wall is "live" by holding a small compass near it; if the needle swings, current is flowing. You cannot see current, but you can see what it does to a compass.
When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This is the magnetic effect of electric current. The region where a compass needle is deflected is a magnetic field.
Key idea 1: Oersted's discovery — In 1820 Hans Christian Oersted noticed that a compass needle near a wire deflected whenever the circuit was closed or opened. He investigated, made sure, and published it; other scientists repeated the experiment.
- Easy: A compass needle is a tiny magnet; it deflects when current flows nearby.
- Medium: When the switch is turned OFF, the magnetic effect disappears and the needle returns to its original direction.
- Exam-type: The effect acts through non-magnetic materials such as cardboard, so a compass placed under a wire on cardboard still deflects.
In the figure: On the left no current flows and the needle points north. On the right the current flows and the needle turns away.
Key idea 2: Uses and a bigger magnet — Bells, motors, fans, loudspeakers and electromagnets work because of this effect. Earth itself acts like a giant magnet because moving liquid iron in its core creates electric currents; birds and fish use this field to find direction, and it shields us from harmful particles from space.
- Easy: No complete circuit means no current and no magnetic field.
- Medium: To find out whether current flows in a circuit without a lamp, place a compass near the wire.
- Exam-type: Electricity and magnetism are linked: current can produce a magnetic field, and (in higher classes) a moving magnet can produce current.
A deflection of the needle shows that the wire is carrying current.
Thinking the wire itself is a permanent magnet. It is a magnet only while current flows.
Watch the needle with the switch. Memory clue: ON = needle Off-course; OFF = needle back Original.
Spot the difference
| Switch | Current | Compass needle |
|---|---|---|
| ON | flows | deflects |
| OFF | stops | returns to original direction |
Practice questions
- 1What is a compass needle?
- 2Who discovered the link between electricity and magnetism?
- 3What happens to the needle when the switch is turned OFF?
- 4Why does the needle deflect only when current flows?
- 5Name two devices that work on the magnetic effect of current.
- 6Why does a compass under a wire on cardboard still deflect?
- 7Anaya closes the switch but the needle does not move. Give a likely reason.
- 8You have a cell, a switch and a wire but no lamp. How can you check for current?
Show answers (try them yourself first)
- 1A tiny magnet.
- 2Hans Christian Oersted.
- 3It returns to its original direction.
- 4The current produces a magnetic field around the wire; with no current there is no field.
- 5Electric bell and loudspeaker (also motor, fan).
- 6The magnetic effect acts through non-magnetic materials like cardboard.
- 7The circuit is not complete, so no current flows.
- 8Place a compass near the wire; a deflecting needle shows current.
Electromagnets
In a scrap yard, a crane carries a big round magnet over a heap of old iron. When the driver presses ON, the iron sticks to it; when he presses OFF over the truck, the whole load drops. An ordinary magnet could never let go like that.
A current-carrying coil of wire that behaves like a magnet is an electromagnet. Most electromagnets have an iron core inside the coil to make them stronger.
Key idea 1: How to make and strengthen one — Wind insulated wire tightly around an iron nail and join its ends to a cell. The nail now picks up paper clips; stop the current and the clips fall.
- Easy: Without current, the coil is not a magnet.
- Medium: The strength increases with more cells (more current) and with more turns of the coil.
- Exam-type: Inserting an iron nail into a paper tube coil makes the compass deflect much more; connect the coil to the cell only for a few seconds, or the cell weakens quickly.
In the figure: The coil is wound around the iron core and joined to a cell; paper clips cling to the end of the nail.
Electromagnets
In the app you can move this figure with your finger.
Open in the appKey idea 2: Poles and lifting magnets — An electromagnet has a north and a south pole, and the poles swap if the current direction is reversed. A lifting electromagnet on a crane is switched ON to lift iron and steel objects and OFF to release them.
- Easy: An electromagnet has two poles.
- Medium: If the north pole of a compass is attracted to end A, then end A is a south pole.
- Exam-type: To make a model crane magnet lift more without changing the cell, add more turns and keep an iron core.
Strength depends on the current and the number of turns; the poles depend on the direction of the current.
Thinking an electromagnet stays magnetic after the switch is OFF. It loses its magnetism when current stops.
Three ways to be stronger. Memory clue: C-T-I = Cells, Turns, Iron core; reverse the current to reverse the poles.
Spot the difference
| Point | Ordinary (permanent) magnet | Electromagnet |
|---|---|---|
| Magnetism | always present | only while current flows |
| Strength | fixed | can be changed |
| Poles | fixed | can be reversed |
Practice questions
- 1What is an electromagnet?
- 2Which core makes an electromagnet stronger?
- 3How many poles does an electromagnet have?
- 4Name two ways to make an electromagnet stronger.
- 5How can you reverse the poles of an electromagnet?
- 6Why is a lifting electromagnet useful in a scrap yard?
- 7Coil A has 20 turns and coil B has 60 turns on identical nails with the same cell. Which lifts more clips and why?
- 8A crane operator wants to drop sorted steel at a new place. What should he do?
Show answers (try them yourself first)
- 1A current-carrying coil that behaves like a magnet.
- 2An iron core (nail).
- 3Two: north and south.
- 4Use more cells (more current) or more turns of the coil.
- 5Reverse the direction of the current.
- 6It can be switched ON to lift and OFF to release the load.
- 7Coil B, because more turns make a stronger magnet.
- 8Switch the current OFF.
Heating effect of electric current
After charging a phone for a long time, the charger feels warm. A bread toaster glows orange inside, and its wires get so hot that bread browns. The same electricity that runs a lamp also makes wires hot.
When current flows through a conductor, it meets some opposition called resistance, and some electrical energy changes into heat. This is the heating effect of electric current.
Key idea 1: What decides the heat — Heat in a wire depends on the current, the material, the thickness, the length and the time for which the current flows. Nichrome has a higher resistance than copper, so it gets much hotter.
- Easy: The heating effect means a conductor gets heated when current flows.
- Medium: With a battery of 2 cells the same nichrome wire gets hotter than with 1 cell, because the current is larger.
- Exam-type: A copper connecting wire stays cool while a heater coil glows red with the same current, because the coil has much higher resistance.
In the figure: Each column is one factor; more current, a high-resistance material and more time all give more heat.
Key idea 2: Uses and dangers — Heaters, stoves, kettles, irons, immersion rods, hair dryers and incandescent lamps use a heating element. Steel factories use electric furnaces to melt scrap. But too much heat can melt plugs and sockets and cause fires, and heat is lost from long wires.
- Easy: A heating element is a rod or coil of wire that gets hot.
- Medium: Plugs, sockets and wires must be rated for the current, or they overheat.
- Exam-type: Touch a hot nichrome wire only momentarily and only with a teacher's supervision, because holding it for long can burn your fingers.
Resistance turns some electrical energy into heat.
Thinking all wires heat equally. High-resistance nichrome heats much more than copper.
Link resistance to heat. Memory clue: R-H = Resistance high, Heat high; nichrome = Nice and hot, copper = Cool.
Spot the difference
| Point | Magnetic effect | Heating effect |
|---|---|---|
| What happens | a magnetic field forms | the wire gets hot |
| Example devices | bell, motor, loudspeaker | kettle, iron, heater |
Practice questions
- 1What is the heating effect of electric current?
- 2Which wire is commonly used in heaters?
- 3What is a heating element?
- 4Why does a nichrome wire become warm?
- 5Which gets hotter, nichrome or copper of the same size?
- 6Name two appliances that use the heating effect.
- 7A new kettle's plug melts after a few minutes. Give a likely cause.
- 8Why is some electrical energy lost in long transmission wires?
Show answers (try them yourself first)
- 1A conductor gets heated when current flows through it.
- 2Nichrome wire.
- 3A rod or coil of wire that gets hot.
- 4It opposes the current (resistance), and some electrical energy changes into heat.
- 5Nichrome.
- 6Electric kettle and electric iron (any two).
- 7The plug and wire are not rated for the current, so they overheat.
- 8The heating effect warms the wires and wastes energy as heat.
Voltaic cell and lemon cell
A torch glows weakly after months in a drawer and then dies. Inside its cell, a chemical change was slowly giving electricity, and when the chemicals ran out, the cell was "dead".
A Voltaic cell has two rods of different metals (electrodes) partly dipped in a liquid (electrolyte), such as a weak acid or salt solution. A chemical reaction between them produces electricity.
Key idea 1: How a Voltaic cell works — The reaction gives a current that flows from the positive terminal through the circuit to the negative terminal. Over time the chemicals get used up and the cell is dead.
- Easy: A Voltaic cell has two different metal rods and an electrolyte.
- Medium: Common metal pairs are zinc and copper, or iron and copper.
- Exam-type: Two identical metal rods do not make a cell, because two different metals are needed.
In the figure: Two different rods dip into the liquid and are joined to a lamp by wires.
Key idea 2: History and the lemon cell — Galvani saw a dead frog's leg kick when touched with two metals and thought the electricity came from the frog. Volta proved that two metals and a liquid are enough, using paper soaked in salt water. You can make a lemon cell with a copper wire, an iron nail and lemon juice as the electrolyte; five or six lemons joined together can light an LED.
- Easy: In a lemon cell, the juice is the electrolyte.
- Medium: If the LED does not glow, reverse its connections, because the LED passes current only one way.
- Exam-type: One lemon gives only a small current, so several are joined to get a larger current.
A glowing LED shows that the cell is working and current is flowing.
Believing the electricity in a Voltaic cell comes from the lemon itself or from a frog. It comes from the chemical reaction of two metals with the electrolyte.
Count the ingredients. Memory clue: 2 different metals + 1 liquid = Volta; Galvani = frog legs kicking.
Spot the difference
| Scientist | Believed | Showed or saw |
|---|---|---|
| Galvani | electricity came from the frog | dead frog's leg kicked |
| Volta | electricity came from the metals | two metals and wet paper gave current |
Practice questions
- 1What are the metal rods of a Voltaic cell called?
- 2What is the liquid in a Voltaic cell called?
- 3What is a 'dead' cell?
- 4What produces electricity in a Voltaic cell?
- 5Which two metals can you use in a lemon cell?
- 6What should you try if the LED does not glow?
- 7A student uses two identical iron nails in a lemon. Why does nothing happen?
- 8A Voltaic cell lights a lamp for some hours and then the lamp goes out. Why?
Show answers (try them yourself first)
- 1Electrodes.
- 2Electrolyte.
- 3A cell whose chemicals are used up and which cannot supply electricity.
- 4A chemical reaction between the rods and the electrolyte.
- 5Copper and iron.
- 6Reverse the LED connections.
- 7A cell needs two different metals as electrodes.
- 8The chemicals are used up and the cell is dead.
Dry cells and rechargeable batteries
A TV remote runs on two small cells that you throw away after a few months. A mobile phone, on the other hand, is plugged in every night and used again. Both are batteries, but they behave differently.
A dry cell has a thick moist paste as its electrolyte and is used once. A rechargeable battery can be charged and reused many times.
Key idea 1: The dry cell — A zinc container is the negative terminal and a carbon rod with a metal cap at the centre is the positive terminal; the paste lies between them. It is called dry because there is no free liquid to spill, which makes it portable.
- Easy: A dry cell is a single-use cell.
- Medium: The zinc container is negative; the carbon rod with its cap is positive.
- Exam-type: A dry cell is more convenient than a Voltaic cell because it has no liquid to spill and is easy to carry.
In the figure: Each column is one kind of cell with how it is used.
Key idea 2: Rechargeable and future batteries — Rechargeable batteries save money and reduce waste, but they slowly wear out after many charges, so a phone needs charging more often after a year or two. The lithium-ion battery is the most common type. Lithium and cobalt are limited, so countries recycle old batteries and work on safer solid-state batteries.
- Easy: A mobile phone battery is rechargeable.
- Medium: Solid-state batteries replace the liquid or paste electrolyte with a solid.
- Exam-type: A family that uses its phone every day should prefer a rechargeable battery, because it can be reused many times.
Rechargeable batteries are not everlasting; they wear out slowly.
Trying to recharge an ordinary dry cell. It is single-use and must be disposed of.
Two words. Memory clue: Dry = Discard; Rechargeable = Reuse.
Spot the difference
| Point | Dry cell | Rechargeable battery |
|---|---|---|
| Use | once | many times |
| Electrolyte | moist paste | liquid, paste or solid (solid-state) |
| Example | torch, remote | phone, laptop, inverter |
Practice questions
- 1Why is a dry cell called 'dry'?
- 2Which part of a dry cell is the negative terminal?
- 3Name one rechargeable battery.
- 4Give two advantages of rechargeable batteries.
- 5Which is the most common rechargeable battery today?
- 6What replaces the liquid or paste in a solid-state battery?
- 7Why do countries now recycle old batteries?
- 8A dead dry cell and a flat phone battery both stop working. How do they differ?
Show answers (try them yourself first)
- 1Its electrolyte is a thick moist paste, not a liquid.
- 2The zinc container.
- 3A mobile phone battery (or laptop battery).
- 4They can be reused many times, save money and reduce waste.
- 5The lithium-ion battery.
- 6A solid material.
- 7Lithium and cobalt are limited and mined in only a few places.
- 8The phone battery can be recharged; the dry cell must be disposed of.