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# Resistance - PowerPoint PPT Presentation

Resistance. Review of Resistors. The resis tance is a n intrinsic property of a material which impedes the flow of c harge requiring a pd to be applied so that there can be current flow. Review of Resistors.

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## PowerPoint Slideshow about ' Resistance' - winter

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Presentation Transcript

### Resistance

• The resistance is an intrinsic property of a material which impedes the flow of charge requiring a pd to be applied so that there can be current flow.

• The resistance is an intrinsic property of a material which impedes the flow of charge requiring a pd to be applied so that there can be current flow.

• From ohm’s law, the resistance of a device is the ratio of the potential difference across it to the current flowing through it.

### RC Circuits

• The current in the previous circuits are time independent once the emf of the source is time independent.

• The current in the previous circuits are time independent once the emf of the source is time independent.

• However we may have circuits which are time dependent.

• An example is an RC circuit.

• Consider charging: discharging of the capacitor through the resistor.

• Initially the capacitor is uncharged.

• Consider charging: discharging of the capacitor through the resistor.

• Initially the capacitor is uncharged.

• When in the charging position current flows and the capacitor charges.

• From Kirchoff’s law:

• Which can be written as: discharging of the capacitor through the resistor.

• Since

• We can rewrite the equation as,

• Doing some algebra,

• Which can be written as: discharging of the capacitor through the resistor.

• Since

• We can rewrite the equation as,

• Doing some algebra,

• We must separate the variables so that we can integrate and find the final charge on the capacitor.

q(t) discharging of the capacitor through the resistor.

VbatC

t

• Taking the antilog and simplifying we get,

V constant.c

Vbat

t

• The pd across the capacitor

• Which gives

I(t)

Vbat/R

t

• Since constant.

• We can write that

• Since constant.

• We can write that

• Separating variables,

• Since constant.

• We can write that

• Separating variables,

• Which in separated form is,

• Integrating, constant.

• We get

• Which after simplification is,

q

CVbat

t

t the initial charge is CV

I(t)

Vbat

t

### Power, Energy the initial charge is CV

Power the initial charge is CV

• The net rate of energy transfer from the source (battery) P is given by,

• Power is in watts(W) or joules/second

• The rate at which energy is dissipated through through the resistor is,

• The energy lost is in the form of thermal energy.

• The power supplied to the capacitor is,

Energy the initial charge is CV

• The total energy supplied by the battery in a time t is given by,

• The total energy dissipated in a time t,

• The total energy supplied to the capacitor in time t,

Energy the initial charge is CV

• From the conservation of energy,