Transcript Resistance - MrRibeyron

Resistance



An electric current is the movement of electrons ( negative
charges ).
All conductors however, have resistance, making it harder for the
electrons to move.
For example:
1. copper wire has a low resistance (used in flexes)
2. nichrome wire has a high resistance (used in toasters)

The unit of resistance is the Ohm ( Ω ).

The better the conductor, the lower the resistance.
The Effect of Resistance

Nichrome wire is connected to a circuit.
nichrome wire

When a current flows through a high resistance wire like nichrome, an
energy change takes place.
Electrical

Heat
Appliances which use this effect are toasters, hair-driers and kettles.
Voltage, Current & Resistance

The resistance of a resistor
can be measured using circuit 1.
Ω
Circuit 1

+
The current through a resistor
and the voltage across it can be
measured using circuit 2.
R
V
-
A
Circuit 2
R (Ω)
V (V)
I (A)
V/I
Can you spot a relationship between resistance, voltage and current?
Use this relationship to complete the last column of the table…
V/I is equal to the Resistance of the resistor.
V/I remains constant for different values of voltage and current.
V
÷
I
R
x
Quantity
Unit
Voltage ( V )
Volts ( V )
Current ( I )
Amps ( A )
Resistance ( R )
Ohms ( Ω )
Example
A 100  resistor is connected to a 12 V battery. What size of current
will flow?
R  100 Ω
I
I  ???
V  12 V

V
R
12
100
I  0.12 A
The Variable Resistor
+
A
 The Resistance of a variable resistor can be changed:
1. High resistance means a small current so the lamp is dim.
2. Low resistance means a high current so the lamp is bright.
 Variable Resistors can be used for
• Dimmer switches for lights
• Volume controls for radios and CD players
How It Works
 Variable resistors are usually made of a length of wire.
wire
x
As contact x is moved to the right,
 the length of wire in the circuit
 the resistance of the circuit
 the current in the circuit
 the brightness of the bulb
increases
increases
decreases
decreases
Power, Current & Voltage
Appliance
P (W)
V (V)
I (A)
Drill
690
230
3
Food Mixer
200
230
Hair Dryer
1150
230
Heater
2300
230
Tumble Dryer
2990
230
R ()
264.5
5
23
13
Calculate the required value for each of the appliances showing ALL your
working.
Can you spot a relationship between, power, current & voltage?
What is the relationship?
P
÷
I
V
Quantity
Unit
Power ( P )
Watts ( W )
Current ( I )
Amps ( A )
Voltage ( V )
Volts ( V )
x
Example
A 1 kW electric saw is connected to the mains supply. Calculate the
size of current it will take.
I  ???
V  230 V
P  1 kW
 1000 W
I

P
V
1000
230
I  4.35 A
Energy, Power and Time
+
-
Joule Meter
Power (W)
Time (s)
24
60
36
60
48
60
Energy (J)
Power is the rate at which energy is used up (transferred), so we can
define power as:
One watt is one Joule per second
E
÷
P
t
x
Quantity
Unit
Energy ( E )
Joules ( J )
Power ( P )
Watts ( W )
Time ( t )
Seconds ( s )
Example
A 100 W television is switched on for 30 minutes.
How much energy will it use?
P  100 W
E  ???
t  30 mins
 1,800 s
E  Pt
 100  1,800
E  180,000 J
Power, Current and Resistance
Consider the equations:
V  IR
and
P  I V
Start with:
P  I V
But
P  I  I R
V=IxR
So what we get is
P  I2  R
P
÷
I2
R
Quantity
Unit
Power ( P )
Watts ( W )
Current ( I )
Amps ( A )
Resistance ( R )
Ohms ( Ω )
x
Example
An electric drill has a power rating of 460 W. Whilst in
operation, the drill has a resistance of 115 Ω. What is the
current passing through the drill?
P
I 
R
460

115
I2  4
I 4
I 2A
2
P  460 W
R  115 Ω
I  ???
Power, Voltage and Resistance
Consider the equations:
V
I
R
P  I V
P  I V
Start with:
But since
and
I
And so we get
V
R
V
P  V
R
V2
P
R
V2
÷
P
R
Quantity
Unit
Voltage ( V )
Volts ( V )
Power ( P )
Watts ( W )
Resistance ( R )
Ohms ( Ω )
x
Example
A lamp has a power of 36 W and a resistance of 4Ω.
What voltage should it be connected to?
P  36 W
R4Ω
V  ???
V2  P  R
 36 4
V 2  144
V  144
V  12 V