Potential difference and electromotive force.

Starter True/false

Ω is the measure of an objects resistance

Semiconductors have a high number density

Starter True/false

Ω is the measure of an objects resistance True

False

Semiconductors have a high number density

False

True

True

Learning outcomes

• Recall key facts about circuits• Define: potential difference, volt and

electromotive force.• Describe how you would find the energy in an

electrical circuit• Apply the equation

Energy in electrical circuits

Check the relationship between potential difference, current, time and energy.

• What equipment would you need?• What reading would you take?• What formula would you use?

1. Set up the circuit shown with a 10 Ω resistor for R.

2. Set the d.c. supply to 10 V.3. Check that the joulemeter reading

increases by 10 J each second when the current is kept constant at 1 A.

4. Record the current and energy dissipated in the load resistor as a function of time for different supply p.d.s.

5. Replace the 10 Ω resistor with a 12 V, 24 W lamp. Adjust the supply to 12 V.

6. Check that the joulemeter reading increases by 24 J each second when the ammeter displays 2 A.

7. Measure the p.d. across the lamp, the current in it and the energy supplied to it for a period of 10 s for a range of supply p.d.s.

You need to work in two groups to collect these results

What’s wrong with this circuit? Draw it correctly

On the figure above add an LED with a 100 resistor in series, an ammeter and a voltmeter to complete the circuit between terminals A and B.

[3]

A

V

A

B

Learning outcomes

• Recall key facts about circuits• Define: potential difference, volt and

electromotive force.• Describe how you would find the energy in an

electrical circuit• Apply the equation

Questions

1. What is the relationship between electrical energy, p.d., current and time?

Answers

Question 1Using the formulae for power and power = rate of transfer of energy gives ΔW = VIΔt.

Definitions Potential difference is the electrical energy transferred per unit charge when electrical energy is converted into another form of energy.

𝐼=𝑄𝑡 𝑉=

𝑊𝑄

𝑃=𝑊𝑡

Potential difference is energy per unit charge. Its unit the volt, is the joule per coulomb.

1 volt = 1 joule per coulomb = 1 watt per ampere

𝑃=𝑊𝑡 =

𝑉𝑄𝑡 =𝑉𝐼

Definitions Electromotive force (e.m.f.) is the energy transferred per unit charge when one other type of energy is converted into electrical energy.

𝑒 .𝑚 . 𝑓 .=𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑒𝑑h𝑐 𝑎𝑟𝑔𝑒

This definition tells us two things:• That e.m.f. is measured in joules per coulomb• That it always deals with the supply of electrical energy

1 volt = 1 joule per coulomb

𝐸=𝑊𝑄

e.m.f. and p.d.

The voltage shown on a cell or ………… tells you its e.m.f. From this, you can tell the amount of energy given to each ………………… of …………….. Passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is …… than the e.m.f. because some voltage is …………… pushing the ………………. through the cell.

e.m.f. and p.d.

The voltage shown on a cell or battery tells you its e.m.f. From this, you can tell the amount of energy given to each ………………… of …………….. Passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is …… than the e.m.f. because some voltage is …………… pushing the ………………. through the cell.

e.m.f. and p.d.

The voltage shown on a cell or battery tells you its e.m.f. From this, you can tell the amount of energy given to each coulomb of charge passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is …… than the e.m.f. because some voltage is …………… pushing the ………………. through the cell.

e.m.f. and p.d.

The voltage shown on a cell or battery tells you its e.m.f. From this, you can tell the amount of energy given to each coulomb of charge passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is less than the e.m.f. because some voltage is …………… pushing the ………………. through the cell.

e.m.f. and p.d.

The voltage shown on a cell or battery tells you its e.m.f. From this, you can tell the amount of energy given to each coulomb of charge passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is less than the e.m.f. because some voltage is ‘lost’ pushing the ………………. through the cell.

e.m.f. and p.d.

The voltage shown on a cell or battery tells you its e.m.f. From this, you can tell the amount of energy given to each coulomb of charge passing around the circuit, including through the cell itself. In practice the useful p.d. across an appliance is less than the e.m.f. because some voltage is ‘lost’ pushing the current through the cell.

• A 1.5V cell gives 1.5 J to each coulomb• A 6V battery gives……………………………..• The 230 V mains gives ………………………

Questions 1. How much energy is transferred to each coulomb of charge by a 9V battery?2. Which types of voltage are described here?

a) a measure of energy transferred to electrical charges by a supplyb) a measure of the energy transferred from electrical charge to a

component in a circuit3. 10C of charge flows through a p.d. of 6.0V. How much energy is transferred?4. How much work is done by the 230V mains supply in pushing 1C of charge

round a circuit?5. A current of 2.5A flows through a resistor for 1 minute. It transfers 600J of

energy to the resistor. What is the p.d. across the resistor?6. How many 1.2V rechargeable batteries must be connected in series to provide

an e.m.f. of 6.0V7. What is the e.m.f. provided by the combination of cells shown in the diagram?

2.0V 1.5V 2.5V 3.0V

Plenary

Resistance Circuit symbols

e.m.f.

Energy and power