Page 151 - Physics Coursebook 2015 (A level)

Chapter 9: Electric current, potential difference and resistance
Summary
■■ Electric current is the rate of flow of charge. In a metal this is due to the flow of electrons. In an electrolyte, the flow of positive and negative ions produces the current.
■■ The direction of conventional current is from positive to negative; the direction of electron flow is from negative to positive.
■■ The SI unit of charge is the coulomb (C). One coulomb is the charge which passes a point when a current of 1A flows for 1s.
charge = current × time (∆Q = I∆t)
■■ The elementary charge e = 1.6 × 10−19 C.
■■ The current I in a conductor of cross-sectional area A depends on the mean drift velocity v of the charge carriers and their number density n.
I=nAvq
■■ The term potential difference (p.d.) is used when charges lose energy in a component. It is defined as the energy transferred per unit charge.
V=WorW=V∆Q ∆Q
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The term electromotive force (e.m.f.) is used when charges gain electrical energy from a battery or similar device. It is also defined as the energy transferred per unit charge.
E= W or W=E∆Q ∆Q
A volt is a joule per coulomb. That is, 1V = 1JC−1.
Power is the rate of energy transfer. In electrical terms, power is the product of voltage and current. That is, P = VI.
Resistance is defined as the ratio of voltage to current. That is:
resistance = voltage current
(R = V ) I
The resistance of a component is 1 ohm when a potential difference of 1 volt is produced per ampere.
For a resistance R, the power dissipated is given by: P=I2R or P=V2
R
Energy transferred in a circuit in a time ∆t is given by:
W=IV∆t
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