 Potential difference and electromotive force.

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Potential difference and electromotive force.

Starter True/false Ω is the measure of an objects resistance 𝑉= 𝐼 𝑅
𝑉= 𝐼 𝑅 Semiconductors have a high number density 𝐼=𝐴𝑛𝑒𝑣 𝑃= 𝐼 2 𝑅

Starter True/false Ω is the measure of an objects resistance True
𝑉= 𝐼 𝑅 False Semiconductors have a high number density 𝐼=𝐴𝑛𝑒𝑣 𝑃= 𝐼 2 𝑅

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 𝑊=𝑉𝑄

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?

Set up the circuit shown with a 10 Ω resistor for R.
You need to work in two groups to collect these results Set up the circuit shown with a 10 Ω resistor for R. Set the d.c. supply to 10 V. Check that the joulemeter reading increases by 10 J each second when the current is kept constant at 1 A. Record the current and energy dissipated in the load resistor as a function of time for different supply p.d.s. Replace the 10 Ω resistor with a 12 V, 24 W lamp. Adjust the supply to 12 V. Check that the joulemeter reading increases by 24 J each second when the ammeter displays 2 A. 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.

What’s wrong with this circuit? Draw it correctly
V A B A

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 𝑊=𝑉𝑄

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

Answers Question 1 Using 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. Potential difference is energy per unit charge. Its unit the volt, is the joule per coulomb. 𝐼= 𝑄 𝑡 𝑃= 𝑊 𝑡 𝑉= 𝑊 𝑄 𝑃= 𝑊 𝑡 = 𝑉𝑄 𝑡 =𝑉𝐼 1 volt = 1 joule per coulomb = 1 watt per ampere

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 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑒𝑑 𝑐ℎ𝑎𝑟𝑔𝑒 𝐸= 𝑊 𝑄 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.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 How much energy is transferred to each coulomb of charge by a 9V battery? Which types of voltage are described here? a) a measure of energy transferred to electrical charges by a supply b) a measure of the energy transferred from electrical charge to a component in a circuit 10C of charge flows through a p.d. of 6.0V. How much energy is transferred? How much work is done by the 230V mains supply in pushing 1C of charge round a circuit? 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? How many 1.2V rechargeable batteries must be connected in series to provide an e.m.f. of 6.0V What is the e.m.f. provided by the combination of cells shown in the diagram? 2.0V V V V

Plenary Circuit symbols Resistance Energy and power e.m.f.