Electric current and potential difference
Water in a tank on the terrace flows down the pipe to the tap because it is at a higher level. Electricity also flows only when one end is 'higher' than the other. A cell works like a pump that keeps this difference alive.
Current electricity (प्रवाही वीज) is the flow of electrons in a conductor. Potential difference (विभवांतर) is the difference of electric potential between two points, measured in volt (V).
In the figure : In the figure charges flow from a point of higher potential to a point of lower potential, as water flows from the higher level A to the lower level B. The difference of potential is the potential difference and its unit is the volt.
Rule 1: Potential difference — Positive charge tends to flow from higher electrostatic potential to lower potential. The difference between two points is the potential difference, SI unit volt. With no cell there is no potential difference and so no current.
- Easy : A torch cell gives a potential difference that lights the bulb.
- Medium : Water flows from a terrace tank to the tap because of the height difference, just as current flows because of the potential difference.
- Exam-type : A bulb with no cell does not glow, because there is no potential difference.
In the figure : In the figure electrons move through a wire from the negative end to the positive end. The conventional current is taken in the opposite direction.
Rule 2: Direction of current — Electrons flow from the negative to the positive terminal of the cell. By old convention the conventional current (परंपरागत विद्युत प्रवाह) is taken to flow from positive to negative, the opposite way.
- Easy : Electrons move from the negative terminal toward the positive terminal.
- Medium : Conventional current is shown in the opposite direction to the electron flow.
- Exam-type : In circuit diagrams, arrows for current usually point from the positive terminal.
In the figure : In the figure 1 ampere is 1 coulomb per second. A current of 2.5 A carries 2.5 C of charge across any section of the wire every second, so 2.5 × 20 = 50 C in 20 s.
Rule 3: Electric current — Current is the charge flowing per second: I = Q ÷ t. 1 ampere = 1 coulomb per second. Current is a scalar quantity.
- Easy : A current of 3 A for 10 s carries 3 × 10 = 30 C.
- Medium : 120 C passes in 40 s: current = 120 ÷ 40 = 3 A.
- Exam-type : A current of 0.5 A flowing for 1 minute (60 s) carries 0.5 × 60 = 30 C.
Electric current and potential difference
In the app you can move this figure with your finger.
Open in the appPotential difference in volt. Current in ampere: 1 A = 1 C/s. Electrons flow negative to positive.
Saying that the conventional current and electrons flow the same way. They are opposite.
Memory clue: "Q = I × t": Charge = Current × time. Think "I Take Quantity".
Spot the difference
| Quantity | Unit | Symbol |
|---|---|---|
| Potential difference | Volt | V |
| Electric current | Ampere | A |
| Charge | Coulomb | C |
Practice questions
- 1What is the unit of potential difference?
- 2Define 1 ampere.
- 3Is current a scalar or a vector?
- 4In which direction do electrons flow in a wire?
- 5Why does a bulb not glow without a cell?
- 6What charge flows when 4 A flows for 15 s?
- 7In 50 s a charge of 200 C passes through a wire. Find the current.
- 8How long will 2 A take to carry 90 C?
Show answers (try them yourself first)
- 1Volt
- 21 coulomb of charge flowing per second.
- 3Scalar
- 4From the negative terminal to the positive terminal.
- 5There is no potential difference, so no current flows.
- 6Q = 4 × 15 = 60 C
- 7I = 200 ÷ 50 = 4 A
- 8t = 90 ÷ 2 = 45 s
Electric cells
A wall clock, a torch and a mobile each need a different cell. Cells come in many shapes because each is built for a special job: small current for a long time, or big current that can be recharged.
An electric cell (विद्युत घट) keeps a constant potential difference between its two terminals.
In the figure : In the figure of a dry cell: A is a metal cap, B the graphite rod (positive terminal), C the paste of manganese dioxide, D the zinc cover (negative terminal) and E the wet pulp of zinc chloride and ammonium chloride.
Rule 1: Dry cell — It has a zinc cover (negative) and a graphite rod (positive) with wet pulp of zinc chloride and ammonium chloride and manganese dioxide paste. It has a long shelf life but cannot give a large current. It is used in radios, clocks and torches.
- Easy : A TV remote uses small dry cells.
- Medium : A wall clock runs for months on one dry cell because it needs only a small current.
- Exam-type : Never cut or heat a dry cell or an old cell: the chemicals can leak and harm skin and eyes. Hand used cells to a collection point.
In the figure : In the figure of a lead-acid cell a lead plate (Pb, negative) and a lead dioxide plate (PbO2, positive) dip in dilute sulphuric acid. It gives about 2 V and can be recharged.
Rule 2: Rechargeable cells — The lead-acid cell has Pb (negative) and PbO2 (positive) in dilute sulphuric acid, gives about 2 V, can be recharged and gives a large current (cars, trucks, UPS). The Ni-Cd cell gives 1.2 V and is rechargeable. Lithium-ion cells are rechargeable and store more energy (smart phones, laptops).
- Easy : A mobile phone uses a rechargeable lithium-ion cell.
- Medium : Five Ni-Cd cells in series give 5 × 1.2 = 6 V.
- Exam-type : An inverter (UPS) keeps a lead-acid battery that gives a large current when power fails.
In the figure : In the figure a car battery is a battery of six lead-acid cells joined in series: 6 × 2 V = 12 V.
Rule 3: Battery of cells — Six lead-acid cells, each 2 V, joined in series, give 6 × 2 = 12 V in a car battery.
- Easy : Two 2 V cells in series give 4 V.
- Medium : Three lead-acid cells give 3 × 2 = 6 V.
- Exam-type : A 12 V battery needs 12 ÷ 2 = 6 lead-acid cells.
Dry cell: not rechargeable, small current. Lead-acid: rechargeable, 2 V, big current.
Thinking all cells can be recharged. A dry cell cannot.
Memory clue: "Dry = Day-to-day small things (clock, torch); Lead = Large load (car, UPS)".
Spot the difference
| Cell | Negative / positive | Voltage | Use |
|---|---|---|---|
| Dry cell | Zinc / graphite | About 1.5 V | Torch, clock |
| Lead-acid | Pb / PbO2 | About 2 V | Car, UPS |
| Ni-Cd | Rechargeable | 1.2 V | Small devices |
Practice questions
- 1What does an electric cell do?
- 2Which terminal of a dry cell is the zinc cover?
- 3What is the positive terminal of a dry cell made of?
- 4Name two uses of the lead-acid cell.
- 5What is the voltage of a Ni-Cd cell?
- 6Why can a dry cell not be used to start a car?
- 7How many 2 V lead-acid cells make a 6 V battery?
- 8Why are smart phones and laptops given lithium-ion cells? Mention two points.
Show answers (try them yourself first)
- 1It keeps a constant potential difference between its terminals.
- 2Negative
- 3Graphite rod
- 4Cars and trucks (or UPS).
- 51.2 V
- 6It cannot give a large current.
- 76 ÷ 2 = 3
- 8They are rechargeable and store more energy.
Electric circuit and connecting cells
A switch in your home opens or closes the way for electricity. The fan runs only when this path is a complete loop from the supply and back. This loop is a circuit.
An electric circuit (विद्युत मंडल) is a closed path of cell, wires, bulb and key through which current flows. A battery (घटमाला) is a group of cells joined together.
In the figure : In the figure the circuit symbols are: P a cell (long plate positive), Q a bulb, R a plug key (open) and S a battery of two cells.
Rule 1: Circuit symbols — Each part has a standard symbol: a cell (long plate positive), a bulb, a plug key (open or closed) and a battery. Drawing symbols makes it easy to show circuits.
- Easy : The long line of the cell symbol is the positive terminal.
- Medium : A battery of 3 cells is drawn as three cell symbols in a row.
- Exam-type : Drawing symbols is faster than drawing real objects and everyone understands it.
In the figure : In the figure the bulb glows only in a complete circuit: with the plug key closed (Q) a current flows, with the key open (P) it does not.
Rule 2: Closed and open circuit — A cell holder, bulb and plug key joined by wires form a circuit. The bulb glows when the key is closed (complete path). With the key open the path is broken and no current flows.
- Easy : A light switch 'on' = closed circuit; 'off' = open circuit.
- Medium : A loose bulb in the holder breaks the path, so the lamp does not glow.
- Exam-type : If a torch does not light, first check whether a cell is placed wrongly or the contact is loose (a broken circuit).
In the figure : In the figure of a series connection the positive terminal of one cell is joined to the negative terminal of the next, and the potential differences add up: 1.5 + 1.5 + 1.5 = 4.5 V.
Rule 3: Series connection — In series the positive terminal of one cell is joined to the negative terminal of the next. The potential differences add up. A group of cells connected this way is a battery.
- Easy : Two 1.5 V cells in series give 3 V.
- Medium : A TV remote with 2 cells of 1.5 V works at 2 × 1.5 = 3 V.
- Exam-type : Four cells of 1.5 V in series: 4 × 1.5 = 6 V. If one of them is put the wrong way round, it cancels one cell and the total becomes 3 × 1.5 − 1.5 = 3 V.
Closed circuit = current flows. Series: add voltages. Long plate is positive.
Joining positive to positive in series. In series join positive to negative.
Memory clue: "Series = Stack like a Staircase; + to −, voltages add".
Spot the difference
| Point | Closed circuit | Open circuit |
|---|---|---|
| Key | Closed | Open |
| Current | Flows | Does not flow |
| Bulb | Glows | Does not glow |
Practice questions
- 1What is an electric circuit?
- 2What is a battery?
- 3In the cell symbol, which line is positive?
- 4When does the bulb glow?
- 5How are cells joined in series?
- 6Three cells of 1.5 V are in series. What is the total?
- 7A torch has 4 cells of 1.5 V in series. Find the voltage. If the bulb needs 9 V, how many cells are needed?
- 8A torch will not light even though the cells are fresh. Give two possible reasons using the idea of a circuit.
Show answers (try them yourself first)
- 1A closed path through which electric current flows.
- 2A group of cells connected together.
- 3The long line
- 4When the plug key is closed and the circuit is complete.
- 5Positive terminal of one to negative terminal of the next.
- 63 × 1.5 = 4.5 V
- 74 × 1.5 = 6 V; 9 ÷ 1.5 = 6 cells.
- 8The switch contact is loose (open circuit) or a cell is placed the wrong way.
Magnetic effect of electric current
A compass needle always points north, but if you bring a wire carrying current near, the needle turns. Electricity and magnetism are cousins.
The magnetic effect of electric current (विद्युत प्रवाहाचा चुंबकीय परिणाम) is the magnetic field produced around a wire carrying current.
In the figure : In the figure of Oersted's observation, with the key open (P) the needle is undisturbed. When a current flows (Q) the needle is deflected, so a current-carrying wire has a magnetic field around it.
Rule 1: Oersted's discovery — A magnetic needle placed near a wire is deflected when current flows, so a current produces a magnetic field around the wire. Hans Christian Oersted first observed this.
- Easy : When the key is open, the compass needle does not move.
- Medium : A bigger current deflects the needle more, since the field is stronger.
- Exam-type : Never test a wire by connecting it directly across a cell for long: it heats up and drains the cell.
In the figure : In the figure a nail with a coil of insulated wire is an electromagnet while a current flows (P: the pins stick). When the key is opened the magnetism vanishes and the pins fall (Q).
Rule 2: Electromagnet — A coil of insulated wire wound on an iron nail or screw becomes an electromagnet (विद्युत चुंबक) when current flows and attracts iron pins. The magnetism vanishes when the current stops.
- Easy : A coil on an iron nail picks up pins while the key is on.
- Medium : Scrap-yard cranes use big electromagnets, which can drop the iron when the current is switched off.
- Exam-type : More turns of wire and more current make a stronger electromagnet.
Current makes a magnetic field (Oersted). Electromagnet works only while current flows.
Thinking an electromagnet stays magnetic after switching off. It loses magnetism.
Memory clue: "Oersted = Oh, current deflects the needle!"
Spot the difference
| Point | Permanent magnet | Electromagnet |
|---|---|---|
| Magnetism | Always | Only while current flows |
| Can be switched off | No | Yes |
| Use | Compass needle | Bell, crane |
Practice questions
- 1Who first observed the magnetic effect of current?
- 2What happens to a compass needle near a current-carrying wire?
- 3What is an electromagnet?
- 4What happens to the pins when the key is opened?
- 5Why must the wire on the nail be insulated?
- 6Name two ways to make an electromagnet stronger.
- 7A compass needle shows a small deflection near a wire. How can you make the deflection bigger?
- 8Why is an electromagnet better than a permanent magnet for lifting scrap iron?
Show answers (try them yourself first)
- 1Hans Christian Oersted
- 2It is deflected.
- 3An iron piece with a current-carrying coil that works as a magnet.
- 4They fall, because the magnetism vanishes.
- 5So that the current goes through all the turns and does not short through the iron.
- 6More turns of wire and more current.
- 7Increase the current (add a cell in series) or bring the needle closer.
- 8It can be switched on to pick up and switched off to release.
Electromagnet and electric bell
You press the doorbell button and it rings as long as you press. Inside, a tiny magnet switches on and off many times a second. That fast switching is what you hear as ringing.
An electric bell (विद्युत घंटा) uses an electromagnet to make a striker hit a gong again and again.
In the figure : In the figure of an electric bell: A is the gong, B the striker, C the coil on an iron piece (electromagnet), D the iron strip and E the contact screw. At rest the strip touches the screw, so current flows when the key is closed.
Rule 1: Parts of the bell — It has an electromagnet, an iron strip with a striker, a gong and a contact screw. At rest the strip touches the screw, so the circuit is complete.
- Easy : The gong is the metal cup that produces the sound.
- Medium : The striker is fixed to the iron strip.
- Exam-type : The electric bell works on the magnetic effect of current.
In the figure : In the figure the magnetised coil pulls the strip D and the striker B hits the gong A. The strip leaves the screw E, the current stops, the coil loses its magnetism and the strip springs back. This repeats, so the bell keeps ringing.
Rule 2: How the bell rings — Current magnetises the coil, which pulls the strip, so the striker hits the gong. This breaks contact with the screw, so the current stops and the coil loses its magnetism. The strip springs back, contact is made again and the process repeats while the key is pressed.
- Easy : When the key is pressed, the coil becomes a magnet.
- Medium : The striker hits the gong, then the contact breaks and the strip springs back: this goes on many times a second.
- Exam-type : If the contact screw is too loose, the circuit never closes and the bell does not ring.
Bell: press key, coil magnetises, strip pulled, striker hits gong, contact breaks, repeat.
Thinking the bell rings by a motor. It rings by an electromagnet switching on and off.
Memory clue: "On → Pull → Hit → Off → Back".
Spot the difference
| Part | Job |
|---|---|
| Electromagnet | Pulls the iron strip |
| Striker | Hits the gong |
| Contact screw | Makes and breaks the circuit |
Practice questions
- 1What is the main working part of an electric bell?
- 2What does the striker hit?
- 3What is the role of the contact screw?
- 4Why does the strip spring back?
- 5When does the bell ring?
- 6Why does the bell ring continuously and not just once?
- 7If the iron strip is replaced by a plastic strip, what happens and why?
- 8Arrange in order: contact breaks, coil magnetises, striker hits gong, strip springs back.
Show answers (try them yourself first)
- 1The electromagnet
- 2The gong
- 3It makes and breaks the circuit.
- 4The coil loses its magnetism when the current stops.
- 5As long as the key is pressed.
- 6Contact breaks and re-forms again and again.
- 7The magnet cannot attract plastic, so the striker will not hit the gong.
- 8Coil magnetises → striker hits gong → contact breaks → strip springs back.
Quick revision
- 1 A = 1 C/s; unit of potential difference = volt.
- Dry cell: Zn negative, graphite positive.
- Current produces a magnetic field (Oersted); electromagnet used in bell.
Quick revision
In the app you can move this figure with your finger.
Open in the app