Electricity 3

Resistance and electric circuits

Explain why high voltages are used in transmission of electrical energy

High voltage imply smaller currents. Smaller currents imply the enrgy loss will be reduced

What are the charge carriers when an electric current passes through a (i)electrolyte (ii) metals (iii) gases (iv) semiconductors

(i) ions (ii) electrons (iii) electrons and ions (iv) electrons and holes

why is it more economical to transmit electrical energy at high-voltage

high voltages imply smaller currents. smaller currents imply the loss will be reduced

show a graph between resistance and temperature

define resistance

R=V/I R-resistance V-voltage across it I-current

power is generated in a resistor of specific resistance when a specific potential difference is applied across it. Write power in terms of resistance and potential difference

P=VI and V=IR so P=V²/R

define power

P=W/t p-power W-work t-time

define resistivity

p= RA/l P-resistivity R-resistance A-area l-length

draw a graph to show the relationship between current and voltage for (i) a metal at constant temperature (ii) an ionic solution with inactive electrodes (iii) a gas

hw would a graph showing the relationship between current and voltage differ for a metal if its temperature was increasing

it would become non-linear. the resistance increases as temperature increases and the temperature increases as the current increases

how would a current vs voltage graph for ionic solution differ if its concentration was reduced

it would remain linear but the slope would be less

explain why the resistance of a bulb is different when it is not connectd to the mains

A cold filament has lower resistance than a hot filament

what happens to the resistance of a wire as its (i) temperature falls below 0 (ii) length is increased (iii) if its diameter is increased

(i) resistance decreases (ii) resistance increases) (iii) resistance decreases

name another device apart from a metre bridge that can be used to measure resistance give an advantage and disadvantage of the device

ohmmeter -a:compact,faster da:less accurate,fragile

Wheatstone bridge a:compact more accurate da:expensive blackbox difficult to comprehend

describe what happens (i)when switch A is closed and the rheostat is adjusted (ii) when switch A and B are closed

(i) when switch A is closed and the rheostat is adjusted the fan operates and its speed of rotation changes

(ii) Current flows through the coil , heating the coil so that the fan blows hot air

what happens as a coil gets hotter

resistance increases

two resistors of resistance R1 and R2 are connected in series. Derive an expression for the effective resistance of the two resistors in terms of R1 and R2

V_t=V₁+V₂

IR_t=IR₁+IR₂

R_t=R₁+R₂

how does a fuse operate

A fuse contains a pieceo f wire that will melt when a current of a certain size passes through it. As current increases the metal conductor in the fuse heats up. WHen the current reaches a certain value, it causes the temperature to reach the melting point of the metal, thus breaking the circuit. A fuse should always be connected to the live wire

Electricity generating companies transmit electricity over large distances at high voltage.wxplain why high voltage is used

High voltages imply smaller currents. Smaller currents imply the energy loss will be reduced

the radius of a wire is doubled what effect does this have on the resistance of the wire

The resistance decreases by a factor of four

derive an expression for the effective resistance of two resistors in parallel

I_t=I₁ + I₂

V/R_t = V/R₁ + V/R₂

1/R_t=1/R₁+1/R₂

state the relationship between resistance R and cross-sectional area A

Resistance is inversely proportional to cross-sectional Area