Electricity (2) -pt 2

Question 1

Why does the national grid use a high pd and a low current?

Answer 1

• to transmit a lot of power you need either a high pd or a big current (P=VI)
• a big current has a lot of waste thermal energy through the wires
• it’s cheaper to boost the pd really high (400 000V) and keep the current as low as possible
• increasing the pd decreases the current, which decreases energy loss and makes it an efficient way of transferring energy

Question 2

How does the national grid change its potential difference?

Answer 2

• it requires a transformer, big pylons with huge insulators
• the potential difference is increased by a step up transformer
• it’s then decreased for domestic use by a step down transformer

Question 3

Explain how static charge is formed?

Answer 3

• when 2 insulating materials rub together, negatively charge ions are scraped off one and onto the other
• this leaves the materials electrically charge, one positively charged and the other equally negatively charged

Question 4

What happens when you rub a polythene rod with a cloth duster?

Answer 4

• electrons move from the duster to the polythene rod
• the polythene rod becomes negatively charged
• the cloth duster becomes positively charged

Question 5

What happens when you rub a acetate rod with a cloth duster?

Answer 5

• the electrons move from the rod to the duster
• the acetate becomes positively charged
• the duster becomes negatively charged

Question 6

How are sparks created from static charge?

Answer 6

• electric charge builds up, so the pd between the earth and the object increases (its usually 0V)
• if the pd is big enough electrons can jump the gap between the object and the earth, creating a spark
• they can also jump to any earthed conductor nearby
• usually quite small (but not always eg. Lightning)

Question 7

2 things with opposite electric charges are ………… to each other, while 2 things with the same electric charge will ………….. each other

Answer 7

Attracted

Repel

Question 8

What is it called when like charges repel and opposite charges attract causing an object to move

Answer 8

Electrostatic attraction/repulsion

Which is a non-contact force

Question 9

What’s an electric field?

Answer 9

• it’s created by electric charges
• the closer to the object you get the stronger the field is
• (shown by drawing field lines)

Question 10

What do you need to remember when your drawing field lines?

Answer 10

• electric field lines go from positive to negative
• they’re right angled to the surface
• the closer the lines the stronger the field - the further from the charge you go the further apart the lines si the weaker the field

Question 11

………….. objects in an electric field feel a force

Answer 11

Charges

Question 12

Explain sparking by electric fields?

Answer 12

• sparks are caused by high a pd between a charged object and the earth
• a high pd causes a strong electric field between the charged object and the earthed object
• the strong electric field causes electrons in the air particles to be removed (ionisation)
• air is normally an insulator, but when ionised it’s a lot more conductive si current can flow through it. Creating a spark