P5 Electric Circuits

Electric charge – objects become charged when electrons are transferred to or from them, for example, by rubbing

Two types of charge are positive and negative (these names are just labels)

Two objects with the same charge repel each otherTwo objects with different charges attract each other

+ metal ions

electrons

The electrons experience resistance when they flow through the metal.

The potential difference (voltage) provides energy which makes the electrons move through the metal ie it generates a current.

potential difference

The symbol for voltage is V

The symbol for current is I

The symbol for resistance is R

Metal wire Normally the free electrons in a metal move around slowly at random.

current =voltage

resistance

V

v1 v2

I

i1

i2

I = i1 = i2 The current is the same everywhere

V = v1 + v2

SERIES

The sum of the voltages across each component equals the supply voltage

R = r1 + r2Resistance

I

i1

i2

PARALLEL Current

I3

i4

i5

I = I3 I1 = I4 I2 = I5 I = I1 + I2

Current does not get used up

Total current = the sum of the currents through each component

V

v1

v2

V = v1 = v2 The voltage across each component is the same as the supply voltage.

PARALLEL

Voltage = energy per unit of charge

If more bulbs are added in parallel to a circuit then they will all be as bright as normal and more current is drawn from the power supply

The potential difference is largest across the component with the greatest resistance, because more energy is transferred by the charge flowing through a large resistance than through a small one

The current is smallest through the component with the largest resistance, because the same battery voltage causes more current through a smaller resistance than a bigger one

12 V

4 V 5 V

2A

i3

SERIES

v3

1 ohm 3 ohm r3

i3 = 2 A v3 = 3 V r3 = 2 ohm

12 V

v2

PARALLEL

1 A

1 A

i3

4 A

r3

i3 = 2 A

v2 = 12 V

12 ohm

12 ohm

Current is a flow of electrons

Electrons have charge (negative)

So current is a flow of charge

How do we quantify current ?

Current is the amount of charge flowing in a particular amount of time

Voltage provides energy to the electrons

Electrons have charge (negative)

So Voltage provides energy to the charge

How do we quantify voltage ?

Voltage is the amount of energy a particular amount of charge has

What about resistance ?

All components will offer resistance to a flow of electrons

How do we quantify resistance ?

If a current of 1A flows through a component when the voltage across it is 1V then the component is said to have a resistance of 1 ohm [ 1 W ]

IV

R=

Multiply both sides by R IV

R=R x x R

R I = V Or V = I R

Take V = I R and divide both sides by I V I R

I I=

V

I= R or

V

I=R

IV

R= V = I R

V

I=R

V

I R

Q. A current of 4 A flows through a circuit with resistance 3 . W What is the voltage ?

use V = I R V = 4 x 3 Voltage = 12

current = voltage / resistance voltage = current x resistance resistance = voltage / current

V

Q. A current of 5 A flows through a circuit with voltage 10 V. What is the resistance ?

V

I R

V

I=Ruse R =

10

5resistance = 2

Q. A circuit with voltage of 6 V has a resistance of 2 W . What current should flow ?

useV

R=I I =

6

2current = 3

W

A

Q. A current of 4 A flows through a circuit with voltage 12 V. What is the resistance ?

V

I R

V

I=Ruse R =

12

4resistance = 3

Q. A circuit with voltage of 8 V has a resistance of 2 W . What current should flow ?

useV

R=I I =

8

2current = 4

W

A

Q. A current of 60 A flows through a circuit with resistance 4 . W What is the voltage ?

use V = I R V = 60 x 4 Voltage = 240 V

IV

R= V = I R

V

I=R

Q. A current of 2 A flows through a circuit with voltage 16 V. What is the resistance ?

V

I R

V

I=Ruse R =

16

2resistance = 8

Q. A circuit with voltage of 230 V has a resistance of 5 W . What current should flow ?

useV

R=I I =

230

5current = 46

W

A

Q. A current of 25 A flows through a circuit with resistance 3 . W What is the voltage ?

use V = I R V = 25 x 3 Voltage = 75 V

IV

R= V = I R

V

I=R

The higher the temperature the lower the resistance

The greater the light intensity the lower the resistance

Variable resistorsResistors are used in circuits to control the sizeof the current

Two resistors in series have a larger resistance thanone on its own.Connecting two resistors in parallel makes a smallertotal resistance

Two resistors in series make a potential divider

Voltage (V)

Cur

rent

(A

)

Current through a filament bulb

Current is less here due to the extra resistance of the heating effect

Power = current X voltage(watt,W) (ampere, A) (volt, V)

If you know the power, it is easy to calculate how muchwork is done (or how much energy is transferred) in a given period of time:

Work done (or energy transferred) = power x time (joule, J) (watt, W) (second, s)

AC generator

DC generatorThe size of the induced voltage can be increased by:• increasing the speed of rotation of the magnet or electromagnet or coil;• increasing the strength of its magnetic field;• increasing the number of turns on the coil;• placing an iron core inside the coil

Generators produce a voltage by a process called electromagnetic induction

AC = alternating current

If a magnet is moving out of the coil, or the other pole of the magnet is moving into it, there is a voltage induced in the opposite direction

Vp / Vs = Np / Ns

Voltage across primary coil

Voltage across secondary coil

Number of turns primary coil

Number of turns secondary coil

=

8 turns 4 turns

Transformer

Rate/speed of rotation

Strength of magnet/ magnetic field

Number of turns/coils of wire

a.c / alternating current

Energy = power x time Power = energy / time

£0.78

2990

30 ohm