Current, Voltage and Resistance - St Edmund's Girls'...

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Current, Voltage and Resistance


How do different components affect the flow of current?

What obstacles affect the flow of current?


What affects the size of the current?

Current can be changed by increasing or decreasing

the voltage of the circuit.

Current is a measure of the rate of flow of electric charge

through a circuit. A large current means a faster rate of flow.

Components in a circuit may

reduce the size of the current.

This is called resistance.

All wires and components

resist current.

Resistance is a measure of

how hard it is for electrons to

move in an electrical circuit.


Resistance simulation


What is a resistor?

A resistor is a component designed to reduce the current.

A variable resistor has a resistance that can be changed.

A fixed resistor has a resistance that remains the same.

Many domestic appliances use

resistance to transfer electrical

energy to heat and light energy.

As current is passed through

the metal element in this kettle,

resistance causes the metal to

get hot and so boils the water.

Resistors are also used to control

the current in electric circuits.


Set up this circuit with a resistor and a variable resistor.

Slowly adjust the variable resistor,

increasing the voltage by 0.5V at a

time and record the current for

each setting.

Plot a current-voltage graph of the results.

V

A

How can resistance be investigated?

voltage (V)

0.0

0.6

1.1

1.8

2.5

3.0

3.5

4.2

0.0

0.5

1.0

1.5

2.5

2.0

3.0

3.5

current (A)


Investigating current and voltage


Current–voltage graphs


Plotting current–voltage

results for nichrome and

copper wires on the same

graph gives straight lines

with different gradients.

At the same voltage, a

copper wire allows a larger

current than a nichrome

wire of the same size.

nichrome

copper

This means that the copper wire has a lower resistance

than the nichrome wire. What does the gradient tell you

about resistance?

cu

rren

t (A

)

voltage (V)

The steeper the gradient of a current–voltage

graph, the lower the resistance of the wire.

Is resistance the same in all wires?


Current–voltage graphs


Resistance is a measure of

how hard it is for electrons to

move in an electrical circuit.

The connection between current,

voltage and resistance was

discovered in 1827 by Georg

Ohm, a German physics and

maths teacher.

The formula V = I × R is known as

Ohm’s Law. It was such an

important discovery that the unit

of resistance is called the ohm.

It is represented by the symbol W.

The irresistible Georg Ohm


What are the units of voltage, current and resistance?

Ohm’s law is usually written as:

Voltage is measured in volts (V).

Current is measured in amps (A).

Resistance is measured in ohms (W).

This formula can also be written as:

voltage = current × resistance

V = I × R

resistance =

R =

voltage

current

What is Ohm’s law?

V

I


What does Ohm’s law show?

What do the different arrangements of Ohm’s law show

about the links between current, voltage and resistance?

V = I × R

R = V / I

I = V / R

As voltage increases, the current increases.

Voltage and current are proportional, while

the resistance remains constant.

Voltage and current are proportional, so

the resistance of a material is constant, as

long as the temperature does not change.

If voltage is constant, then current

decreases as resistance increases.


A formula triangle helps you to rearrange a formula.

The formula triangle for V = IR is shown below.

Cover up the quantity that you have to work out.

This gives the formula needed.

So to find current (I),

cover up I……which gives

the formula…

I =V

R

×

Resistance formula triangle


A filament bulb has a current

of 0.2A running through it,

with a potential difference of

5V across it.

V = IR

Calculating the resistance of a bulb

What is the resistance of

the filament in the bulb?

= 5V

0.2A

V

I

= 25 W

In calculations like this one,

the resistance of the

connecting wires is usually

small enough to be ignored.

R =


Resistance calculations


Finding the resistance of a component


The resistance of a wire depends on several factors:

material

length

thickness

temperature

What experiments could be used to test how length

and thickness affect resistance?

What affects resistance?


Resistance and length


How does length affect resistance?

When the length of the wire increases, resistance increases.

The table shows the resistance of different lengths of wire.

What general pattern do these results show?

length (cm)

5

10

20

voltage (V)

current (A)

resistance (W)

2.6

2.9

3.0

5.0

4.3

2.1

0.52

0.67

1.48

Resistance is caused by electrons colliding with metal ions.

When the length of the wire is increased, the electrons

have to travel further, so more collisions will occur.


Resistance and thickness


How does thickness affect resistance?

When the thickness of wire increases, resistance decreases.

The table shows the resistance of different thicknesses

of wire. What general pattern do these results show?

thickness (mm)

1

2

4

voltage (V)

current (A)

resistance (W)

3.0

2.9

2.6

2.1

4.3

5.0

1.48

0.67

0.52

Increasing the thickness of a wire increases the surface

area that the electrons can flow through. This decreases

the chance of collisions with metal ions.


How does temperature affect resistance?

If a component obeys Ohm’s law, then decreasing the

temperature decreases the resistance of the material.

A superconductor is a

material that conducts

electricity with very little

resistance at very low

temperatures.

This is because the positively charged ions in the metal do

not vibrate as fast, and the electrons can flow more easily.

Superconductors can be

used to make very fast

circuits and to make

magnets levitate.


Resistance – true or false?


Glossary


Multiple-choice quiz

Current, Voltage and Resistance - St Edmund's Girls'...

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