Section 15.5 Electrical Power Section 15.5 Electrical Power

Section 15.5 Electrical power • Heating effect of current • Electrical power

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15.5 Electrical power (SB p. 67)

Heating effect of current current passing through → temperature ↑ Reason

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Discussion 5

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charges lose energy to atoms by collision → atoms vibrate more vigorously → heats up electrical energy → heat

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15.5 Electrical power (SB p. 67)

Electrical power

Electrical Electrical energy transferred = Time taken power

E P= t unit: watt (W)

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Discussion 6 © Manhattan Press (H.K.) Ltd.

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15.5 Electrical power (SB p. 67)

Electrical power each second consumes 100 J

220 V © Manhattan Press (H.K.) Ltd.

each second consumes 60 J

220 V 4

15.5 Electrical power (SB p. 68)

Electrical power (1) Useful equations of electrical power

E P= t QV ( E = QV ) = t Q  =V  t  ∴ P = VI © Manhattan Press (H.K.) Ltd.

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15.5 Electrical power (SB p. 68)

Electrical power (2) Useful equations of electrical power

P = VI = IR × I

( V = IR )

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∴P = I R

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15.5 Electrical power (SB p. 68)

Electrical power (3) Useful equations of electrical power

P = VI V  =V  R

V   I =  R 

V2 ∴P = R © Manhattan Press (H.K.) Ltd.

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15.5 Electrical power (SB p. 68)

Electrical power

P = VI or 2

P =I R or 2

V P= R Thinking 5 © Manhattan Press (H.K.) Ltd.

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To section 15.6

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15.5 Electrical power (SB p. 66)

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Text In each of the following electrical appliances, what kind of energy will the electrical energy be converted to? Electric cooker, fluorescent tube, electric iron, vacuum cleaner, radio, washing machine, toaster Discussion 5:

Ans wer

Electric cooker – heat, fluorescent tube – light, electric iron – heat, vacuum cleaner – mechanical energy, radio – sound, washing machine – mechanical energy and toaster – heat. © Manhattan Press (H.K.) Ltd.

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15.5 Electrical power (SB p. 67)

Discussion 6:

In Chapter 6, we have learnt that power is defined as Energy transferred P=

Time Taken

Discuss with your classmates how to derive an equation in terms of the electrical power (P), voltage (V ) and current (I ) from their definitions.

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E Q V= , I= Q t E E Q ∴ P = = × = VI t Q t

Ans wer Return to

Text 11

15.5 Electrical power (SB p. 68)

Thinking 5 In the circuit, L1, L2 and L3 are three identical light bulbs. L1 L3 L2

1. Is the power dissipated in L3 the same as the sum of power dissipated in L1 and L2? © Manhattan Press (H.K.) Ltd.

Ans wer

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15.5 Electrical power (SB p. 68)

Thinking 5 (Cont)

The current passing through L1 and L2 is only half of the current passing through L3. Power 2 2 dissipated in L3 = I2R. The Isum of power  I

  R+  R 2 2     dissipated in L and L = 1 2 R

= I2

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≠

power dissipated in L3.

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15.5 Electrical power (SB p. 68)

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Text

Thinking 5 (Cont)

2. Arrange the light bulbs in descending order of Ans brightness. wer

L1 L3 L2

L1 and L2 have the same resistance and current passing through. By P = I2R, their brightness is the same. By P = I2R, L3 is brighter than L1 and L2. Therefore the order of the brightness is L3 > L1 = L2.

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